**Abstract:** Lower limb explosive strength has been widely used to evaluate physical fitness and general health in children. A plethora of studies have scoped the practicality of the standing broad jump (SBJ), though without accounting for body dimensions, which are tremendously affected by growth. This study aimed at modeling SBJ-specific allometric equations, underlying an objectively predictive approach while controlling for maturity offset (MO). A total of 7317 children (8–11 years) were tested for their SBJs; demographics and anthropometrics data were also collected. The multiplicative model with allometric body size components, MO, and categorial differences were implemented with SBJ performance. The log-multiplicative model suggested that the optimal body shape associated with SBJs is ectomorphic (H = −0.435; M = 1.152). Likewise, age, sex, and age–sex interactions were revealed to be significant (*p* < 0.001). Our results confirmed the efficacy of the allometric approach to identify the most appropriate body size and shape in children. Males, as they mature, did not significantly augment their performances, whereas females did, outperforming their peers. The model successfully fit the equation for SBJ performance, adjusted for age, sex, and MO. Predictive equations modeled on developmental factors are needed to interpret appropriately the performances that are used to evaluate physical fitness.

**Keywords:** allometry; standing broad jump; children; growth; maturity offset

### **1. Introduction**

The relevance of muscular strength and power is well recognized in human performance [1,2] and contributes to bone health across different age groups [3]. Several reviews highlighted health outcomes associated with muscular strength and showed that poor muscular strength in children and adolescents is associated with cardiovascular disease, metabolic profiles, skeletal health, and adiposity [4,5]. Other studies have shown that high muscular strength in youths is associated with better lipid metabolic profiles, independent of cardiorespiratory fitness [3]. Ideally, muscular power (or explosive

strength, as it is known in practical context) should be measured in lab settings [6], yielding valid values of muscular outcomes. Nevertheless, sometimes this may lack feasibility, and therefore field tests like the standing broad jump (SBJ) are preferable, since they are recognized as acceptable alternatives to lab assessments [7]. In addition, musculoskeletal fitness evaluation may enrich individual feedback to students and athletes, providing comparisons to international references on performance and health. For this reason, several youth fitness batteries [7–9] incorporated SBJ testing into their evaluation panel. Normative values for children and adolescents were created from these validation studies, proposing 50th percentiles as a proper medium level of performance that one could refer to. However, this procedure seems to have some limitations. For example, in Tomkinson et al. [10], the SBJ showed a mean improvement of 9 cm between 9–10 and 10–11 years (both males and females), while no information was obtained about changes in body mass and height, which differ according to age group and sex.

This stratification is useful for primarily surveillance purposes [11], but not for accurately monitoring training or developmental aspects. Moreover, childhood is a crucial period for detecting sensitive increases in performance (+10–15% per year) [12,13]. In children 7–8 years of age, SBJ performance is strictly dependent on body mass and height [14]. Although Body Mass Index (BMI) is the parameter summarizing the two variables, it does not represent a proper predictor of authentic performance [15,16]. Indeed, growth does not reflect parallel increases in this developmental pattern; while height increases by 3–4% per year, body mass increases by 8–10% [17]. Furthermore, a commonly used ratio scaling—the power-to-body mass ratio—may be a misleading index [18]. An alternative could be represented by the stratification of performance according to social condition [19] or BMI categories [16]. However, as mentioned earlier, this could result in a partial analysis. To convey these conceptual bottlenecks to a convincing ground of discussion, it is necessary to scale for maturity differences and determine the role of body size on performance [20], which is the allometric model. The objective of scaling is to normalize the performance for anthropometric characteristics [21]. In this framework, it is crucial to consider an adjunct polynomial model to analyze data and appropriately interpret them, discerning the contribution of developmental growth and maturation, among other factors, considering the exponential trend of human growth [22,23]. In the performance evaluation, another critical assessment item during the child growing phase could be the maturity offset (MO) at the peak of maturity. To this end, the peak high velocity (PHV) is capable of capturing a peculiar significance of speed and power results throughout growth, as well as the MO in strength (explosive strength) [16] or endurance outcomes [15]. In particular, the strength outcome depends on the developmental stage of the physiological determinants, whereas the peak of strength corresponds to the PHV [24]. Indeed, absolute values of specific strength actions, such as jumping or changing directions, are differently positioned in peak performance velocity curves as to PHV [25]. As such, controlling maturity is of paramount importance [26].

This study aims to model strength test-specific allometric equations, considering multiple individual factors such as height, body mass, sex, and age. Predictive equations will enforce a robust reliable indication of fitness per the structure and maturation levels of children (8–11 years old).

### **2. Materials and Methods**

### *2.1. Experimental Approach to the Problem*

During childhood, manifold factors could ameliorate performance, especially age and sex, according to the MO. In particular, the strength outcome depends on the developmental stage of the physiological determinants, whereas the peak of strength corresponds to PHV, where absolute values of specific strength actions are differently positioned in the peak performance velocity curves as compared with PHV. In this study, we decided to investigate the muscular power related to actual children's maturity, taking into account multiple individual proxies such as height, body mass, sex, and age. Children performed an SBJ as per the field test guidelines. The biological maturation (MO) was estimated using the somatic maturation method proposed by Werneck et al. [27]. Then, the age of PHV was determined by subtracting the MO from the chronological age. Participants were classified as late, early, or on time through the one-SD method derived from the current sample.

### *2.2. Subjects*

This cross-sectional study involved 7317 children (3627 girls) aged 8–11 years, recruited from 119 Northeast Italian primary schools (third, fourth, and fifth grade). Children with known chronic cardiac, respiratory, neurological, or musculoskeletal disorders were excluded. All the described measures were taken during physical education classes as scheduled in the morning activities (8:00–12:00 a.m.). The study protocol, including each feature of the experimental design, was approved by the ethical boards of the enrolled schools in accordance with the World Medical Association Declaration of Helsinki, as revised in 1983. All participants were free to withdraw their participation at any time. Written informed consent was obtained from the parents or legal guardians, while verbal assent was obtained from the children prior to participation.

### *2.3. Procedures*

Data collection included demographic and anthropometric information (sex, age, mass, and height) measured before the test sessions using standardized techniques (Table 1). Height was measured using a portable stadiometer with a precision of ± 1 mm (Stadiometer Seca 213, Intermed S.r.l. Milan, Italy) with children in an upright position, with bare feet placed slightly apart, arms extended, and head positioned in the Frankfort plane. Body mass was assessed using a beam scale with a precision of ±100 g (Seca 813, Intermed S.r.l. Milan, Italy), with children in light clothing, without shoes, and stood upright at the center of the platform of the mass scale. We calculated the age of the children from the birth date and subsequently rounded down values.

**Table 1.** Demographic and anthropometric information by sex. Numerosity following age: 8–9 y (1175 males, 1074 females), 9–10 y (1240 males, 1200 females), and 10–11 y (1275 males, 1353 females). All values are showed as mean ± DS.

The explosive strength of the children was measured by the SBJ test (systematic error nearly to 0) [5], a practical, time-efficient, and low-cost field test widely adopted in school or gym contexts [7,11,16,28]. Every child jumped for distance from a standstill. During the performance of the jumps, the children were asked to bend their knees with their arms in front of them, parallel to the ground, then swing both arms, push off vigorously, and jump as far as possible. The test was performed three times, scored in centimeters, and the best value was recorded. The score was obtained by measuring the distance between the last heel mark and the take off line [29].

All tests were conducted by a team of three students of the sport science degree course during scheduled physical education classes in the morning. Previous training and calibration of the operators were performed to ensure the accuracy and repeatability of the procedure. The presence and collaboration of the curricular teachers were guaranteed at any time to meet the confidence of the students [30].

### *2.4. Statistical Analysis*

The biological maturation (MO) was estimated through the somatic maturation method proposed by Werneck et al. [27].

This method estimates the MO from stature and chronological age using an algorithm, providing the result offset in years to peak height velocity (PHV). In particular, this easy approach was adopted because, briefly, involving only one anthropometric measure reduces operator-dependent errors, especially because it meets the full compliance of children and parents and avoids other maturity scales (i.e., Tanner; Marshall & Tanner 1969, 1970). In brief, the MO was determined using a specific formula for girls and boys:

$$\text{Maturity offset for girls (years)} = -7.709133 + (0.0042232 \times \text{(age} \times \text{height))}$$

$$\text{Maturity offset for boys (years)} = -7.999994 + (0.0036124 \times \text{(age} \times \text{height)})$$

Then, the age of PHV was determined by subtracting the maturity offset from the chronological age. Participants were classified as late, early, or on time through the one-SD method derived from the current sample.

The multiplicative model with allometric body size components of Body Mass (M) and Height (H) was used to identify the most appropriate body size and shape characteristics (Equation (1)) associated with, as well as detect any maturity (using MO) and categorical differences (e.g., sex, age) in, the physical performance variable (SBJ). The model is an extension of the one used to predict the physical performance variables of Greek children [23]:

$$\mathbf{Y} = \mathbf{a} \cdot \mathbf{M} \mathbf{k}^1 \cdot \mathbf{H} \mathbf{k}^2 \cdot \exp(\mathbf{c} \cdot \mathbf{M} \mathbf{O}) \cdot \varepsilon \tag{1}$$

where k1 and k2 are the ontogenetic allometric coefficients; a, b, and c are allowed to vary randomly from child to child; and Log (ε), is assumed to have a constant error variance. The constant a is also allowed to vary for different populations, in this case the fixed factor: sex.

The model has the advantage of having proportional body size components and a multiplicative error that assumes they will increase proportionally with the physical performance variable Y (e.g., see Figure 1a,b).

(**a**)

**Figure 1.** *Cont*.

**Figure 1.** (**a**) The association between SBJ and height in 8–11-year-old male children, and (**b**) the association between SBJ and height in 8–11-year-old female children.

The model (Equation (1)) can be linearized with a log transformation (Equation (2)). A linear regression analysis or analysis of covariance (ANCOVA) on log(Y) can then be used to estimate the unknown parameters of the log-transformed model:

$$\text{Log(Y)} = \log(\text{a}) + \text{k1} \cdot \log(\text{M}) + \text{k2} \cdot \log(\text{H}) + \text{c} \cdot \text{MO} + \log(\varepsilon) \tag{2}$$

Further categorical differences within the population (e.g., sex and age) can be explored by allowing the constant intercept parameter log(a) to vary for each group by introducing them as fixed factors (plus possible interactions) within an ANCOVA. The significance level was set at *p* < 0.05.

### **3. Results**

The explosive strength expressed by the SBJ revealed a mean value (SD) of 128.29 cm (22.76) and 121.17 cm (21.49), while the MO was, on average, 3.36 y (0.72) and 6.35 y (0.44) for boys and girls, respectively. A significant difference between sex was found (*p* < 0.001). Figure 2 shows the results between sexes (with the covariates factored in), suggesting that females outperformed males in the SBJ; females had greater performances than males by up to 20 cm (on average). Furthermore, the parameter Log a that results from interaction between height, mass, MO, sex, and age was 0.973, 0.985, 0.982, and 0.995 for males and 1.199, 1.211, 1.208, and 1.211 for females, respectively, for 8, 9, 10, and 11-year-olds.

The estimated parameters from the multiplicative model relating the SBJ distance to the body size components in Equation (1), incorporating the MO, are given in Table 2.

**Figure 2.** Estimated marginal means of SBJ by sex. Covariates appearing in the model are evaluated as follows: Ln Height = 4.9147, LN Mass = 3.4928, and MO = −4.8206.

**Table 2.** Parameters of Equation (1) about SBJ performance. The parameter Log a for males is 0.973, 0.985, 0.982, and 0.995, and 1.199, 1.211, 1.208, and 1.211 for females, respectively, for 8, 9, 10, and 11-year-olds. Note that the baseline group is the 11-year-old boys from which all other sex and age groups are compared.

The model (Equation (1)) relating the SBJ distance to the body size components was

$$\text{[SB] distance (cm)} = \text{a} \cdot \text{M}^{-0.435} \cdot \text{H}^{1.152} \tag{3}$$

With a positive height (H) and negative mass (M), the model suggested that the optimal body shape associated with the SBJ is to be taller, but lighter (less body mass).

In particular, the parameter Log a appears in the footnote of Table 2. Note that the maturity offset (MO) made significant positive contributions to predicting the log-transformed SBJ distance (*p* < 0.001).

**Figure 3.** Estimated marginal means of the SBJ according to sex and age group. Covariates appearing in the model are evaluated as follows: LN height = 4.9147, LN weight = 3.4928, and MO = −4.8206.

### **4. Discussion**

Since childhood, musculoskeletal fitness evaluation has been a crude indicator of both child performance and general health. To evaluate muscular power, the SBJ was included in all field test batteries. Researchers created reference values to characterize growth curves, not only in terms of anthropometrics (body mass and height) but also regarding physical performance. A close relation does exist, in fact, between the BMI and SBJ performance. In contrast, recent studies found that the BMI has limitations for measuring growth during childhood. From the age of seven to eight, body mass and height did not follow the same growth trends, making the BMI an improper predictor of SBJ performance [11,31]. In this light, this study aimed to investigate SBJ performance according to a robust scaling in which height and body mass are objectively valued, considering their actual trends owing to PHV assessment as a crucial individual factor of all physical performances. The data of this study confirmed the efficacy of the allometric approach to identify the most appropriate body size and shape characteristics associated with SBJ performance through the PHV. For boys and girls 8–11 years old, it is the ectomorph one (M = −0.435; H = 1.1152). In fact, in explosive strength, the MO contributed to predicting log-transformed SBJ distance since childhood. Without an allometric approach, consistent with other studies [13,14], child SBJ performance differs between boys and girls, with boys outperforming in a constant growth trend. Instead, the introduction of the size approach through allometric modeling to investigate SBJ performance with MO evaluation allowed us to refine the analysis, releasing different results. Males throughout the studied time frame (8–11 years) did not significantly augment their performances, whereas females showed a significant increase with a better SBJ performance (Figure 2). In line with other results [31,32], males did not outperform females in SBJ performance until reaching 11 years old. Furthermore, Newton's second law suggests that the force acting on a body is directly proportional to acceleration and shares its direction and orientation, with a proportionality constant given by the mass of the body. With that being said, Martin et al. [33] also showed that during maturation, female peak power is most likely to be determined by the quantitative properties of the muscle. Even in our study, the influence of body mass and related MO on strength performance in females was supported by the significant relationship that emerged between the SBJ and PHV offset. Moreover, Armstrong et al. [18] identified that, during the growth period, an index of lower limb power (assessed through the Wingate test) is strongly linked with both body mass and

fat-free mass. This is probably explained by a lower body mass of the participants, which allows them to perform more efficiently. However, the possibility of a genuine comparison is hard due to the lack of data investigated with allometric approaches in the same age group. A small number of studies investigated such a performance during the early stages of maturity. Dos Santos et al. [34] suggested a better SBJ performance in 10–15-year-old boys controlled by body and shape with respect to their female peers. On the other hand, Meylan et al. [21] suggested that, when maturity was adjusted, the horizontal jump length results were slightly better (*p* = 0.04) for males than females.

Overall, the results of this study highlighted that females outperformed males in SBJ performance. A putative explanation may reside in the lower mass and higher coordination level of the females, with respect to males. Although our study did not investigate coordination, D'Hondt et al. [31] found an inverse correlation between motor coordination and lower mass in children of 5–10 years, and Stodden et al. [35] reported that children with a higher level of motor coordination exhibit greater performance (explosive strength). In this sense, sex differences in lower limb power performance have been related to the expression of significant differences in fat-free mass [36]. Subsequently, changes in force production at the age of 13–14 years have been attributed to the dramatic increase in steroid concentrations in males [37]. From a body shape view, the explosive strength in the lower limbs is facilitated through an ectomorphic body shape [23]. This is of relevance since these values are similar to those found by Lovecchio et al. [11] in middle school students (11–14 years old, Log(M) = −0.357, Log(H) = 1.302). Interestingly, in the two studies the trend was comparable, as though the explosive force paralleled the growth factors. Furthermore, the values of the a parameter for males and females (Table 2) are very practical, as they effectively complete the predictive equation for a field test widely used to evaluate physical fitness and, possibly, related general health beyond the performance extent by itself. Future studies may concern other field tests very commonly employed in children, such as speed and agility tests.

### **5. Conclusions**

This study suggested some practical applications for young people in the early stages of maturation. First, coaches of young athletes must be aware of the sex differences because of maturity issues; therefore, females with lower body masses than males may be favored in lower limb performance. Second, coaches should be adopting tests and sample-specific scaling modeled after body mass, rather than using theoretical models based on the assumption of body dimension similarity. From this perspective, the use of the parameter a (Table 2) would be of meaningful assistance. Third, sex-specific training may also be needed, given the early PHV in girls. This should make coaches and physical education teachers aware that females can train strength earlier than males. Furthermore, this approach was aligned with the bio-banding rationale that employed a stratification per maturity offset, within a competitive context (and not only assessment procedure or in-training evaluation) [38,39]. Finally, age, maturity offset, body shape, and body mass should be carefully considered when interpreting children's lower limb performance, as they are relevant indicators to the growth curves.

**Author Contributions:** Conceptualization, N.L., M.G., and A.M.N.; methodology, R.C., G.C., and N.L.; validation, M.G., M.V., and A.M.N.; investigation, R.C. and M.V.; data curation, M.G. and M.V.; writing—original draft preparation, R.C. and G.C.; writing—review and editing, N.L. and M.G.; supervision, N.L. and A.M.N.; project administration, V.C.P., R.C., and M.V. All authors have read and agreed to the published version of the manuscript.

#### **Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to acknowledge all the students, teachers, and staff of the schools, university students, and graduates. All authors disclose professional relationships with companies or manufacturers who will benefit from the results of the present study.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## **Contributions of Anthropometric and Strength Determinants to Estimate 2000 m Ergometer Performance in Traditional Rowing**

### **Sergio Sebastia-Amat, Alfonso Penichet-Tomas \*, Jose M. Jimenez-Olmedo and Basilio Pueo**

Department of General and Specific Didactics, University of Alicante, 03690 Alicante, Spain;

<EMAIL_ADDRESS>(S.S.-A<EMAIL_ADDRESS>(J.M.J.-O<EMAIL_ADDRESS>(B.P.)

**\*** Correspondence<EMAIL_ADDRESS>
Received: 29 August 2020; Accepted: 18 September 2020; Published: 20 September 2020

**Abstract:** The purpose of this study was to analyze the contribution of anthropometric and strength determinants of 2000 m ergometer performance in traditional rowing. Nineteen rowers competing at national level participated in this study. Anthropometric characteristics, vertical jumps and bench pull tests were assessed to determine conditional factors, whereas the 2000 m test was used to set rowing performance. Pearson correlation coefficient, linear stepwise and allometric regression analyses were used to predict rowing performance (R<sup>2</sup> > 50%). Height, body mass and body muscle correlated with rowing performance in male and female rowers. Similarly, power output for squat jump and countermovement jump power correlated with performance. Finally, mean propulsive velocity, mean power and maximum power in bench pull also correlated with the test. Stepwise multiple regression analysis identified body mass (R<sup>2</sup> = 0.69, *p* < 0.001) and mean propulsive velocity in bench pull (R2 = 0.76, *p* < 0.001) for male rowers and body muscle (R2 = 0.89, *p* = 0.002) and maximum power in bench pull (R<sup>2</sup> = 0.62, *p* = 0.036) for female rowers as the best predictors of rowing performance. These results determine the relevance of anthropometric characteristics and, in contrast to Olympic rowing, support the greatest importance of upper body power in traditional rowing training.

**Keywords:** bench pull; vertical jump; power; talent detection; training

### **1. Introduction**

Rowing is a cyclic, strength-endurance sport that requires high levels of aerobic and anaerobic capacity to displace a boat through the water [1,2]. Rowers use the whole body to perform the rowing stroke for a distance that differs according to the modality [3]. There are two different rowing modalities, traditional—or fixed—rowing and Olympic rowing, with different performance indicators: distance, race time, mean force, total number of strokes and power per stroke and velocity of the boat [4]. Traditional rowing is a non-Olympic modality that demands high physical condition to carry out between 35–40 strokes per minute throughout a 19–20-min race, slightly longer than in Olympic rowing [4]. Traditional rowers show a 250–350 W average force applied for an optimal stroke length [4,5]. The power-capacity at each stroke has been identified as a key factor of rowing performance [4,6], together with other factors like large body size, relatively large limbs, high muscular strength, high muscular and cardiovascular endurance and proper balance [6–9].

Whereas approximately 46% of the power produced in Olympic modalities is generated by legs, the remaining is produced by trunk and arms [10]. In traditional rowing, the contribution of legs is slightly lower (40%) and the role of trunk and arms are slightly higher (60%) [11]. This fact may be due, among other factors, to the semi-flexed position of legs during the recovery of the traditional rowing cycle [12]. This position must be adopted as a consequence of the fixed seat, which entails an increase in the degree of body extension [13].

Traditional rowing is experiencing a significant increase in athletes worldwide and has attracted the attention of sports scientists in the same way was as in Olympic rowing. International research interest has enhanced due to physiological, performance and championships differences [4], the professionalization of this modality and the rise of worldwide championships in Europe (Spain, Italy, United Kingdom, etc.), America (Canada, USA, etc.) and other regions like Saudi Arabia [13]. Traditional rowing studies has increased in the field of sports profile [14], championships performance [13], supplementation [15], physiological factors [4] and different training characteristics and methodologies [4,16].

Most studies have mainly focused on describing performance factors in Olympic rowing [11], mainly those related with physiological and anthropometric variables and, to a lesser extent, in traditional rowing [4,13,16,17]. In the same way, research conducted to date relates the importance of some anthropometric characteristics like height, body mass, muscle mass or body fat in Olympic rowing performance [1]. Most of these studies have investigated the determinants of both Olympic and indoor rowing using correlation and linear regression techniques, assuming a linear relationship between rowing performance and determinants [8,18,19]. For curvilinear relationships between various measures of power output, a proportional allometric model can also be used [20,21]. However, the contribution of these determinants to rowing ergometer performance in traditional rowing has not been widely demonstrated. Several studies showed the relationship between 1500 m or 2000 m rowing ergometer performance in Olympic modality and lower body power obtained from different types of jump [8,22,23]. Similarly, a link has been shown between rowing ergometer performance and power produced by the upper body during different protocols of bench pull (BP) tests [4,24,25]. Even peak power output sustained during maximal incremental testing is an overall index of physiological rowing capacity and rowing efficiency and allows predicting rowing ergometer performance [26]. However, to the knowledge of the authors, there are no studies that relate the rowing ergometer performance in traditional rowing, considering that the contribution of the upper body is slightly higher in traditional rowing than in Olympic rowing [11,27]. Likewise, no available studies in the literature have considered the use of proportional allometric modeling to predict 2000 m ergometer rowing performance in traditional rowing.

Therefore, the purpose of this study was to analyze the contribution of anthropometric and strength determinants of rowing ergometer performance in traditional rowing. To that end, the relation of anthropometric characteristics and upper/lower body contribution with 2000 m rowing ergometer performance test was carried out using correlation, linear and allometric regression techniques.

### **2. Materials and Methods**

### *2.1. Participants*

Nineteen sweep (board) rowers competing at the national level participated in this study: 12 males (7 port and 5 starboard, 10 heavyweight and 2 lightweight, age: 24.6 ± 3.9 years, height: 178.4 ± 8.9 cm, body mass: 77.3 ± 7.9 kg) and 7 females (3 port and 4 starboard, 2 heavyweight and 5 lightweight, age: 25.7 ± 4.4 years, height: 166.3 ± 7.5 cm, body mass: 59.9 ± 8.3 kg). The requirement to participate was to have classified for the national championship, to train regularly a minimum of five days per week (> 12 h/week) for the last 3 years and not to have any musculoskeletal or neurological disorders, heart or respiratory failures, or any circulatory disturbance that may influence the results of the investigation. Rowers were requested to abstain from caffeine and alcohol consumption for 24 h and to avoid high-intensity training for 48 h before testing. All participants gave their written consent after project information, which was previously approved by the research ethics committee of the University of Alicante (IRB No. UA-2019-07-23).

### *2.2. Procedures*

To determine the contribution of the different variables in rowing ergometer performance, the strength of lower/upper body and 2000 m rowing ergometer performance were tested. Data collection was conducted on three sessions carried out at the same time of the day in a controlled laboratory environment. Dynamic muscle strength and power tests appear to discriminate better between levels of rowing performance than isometric strength tests [28,29]; therefore, during the second session, rowers performed vertical jump tests and the bench pull test (BP). Finally, rowers completed a 2000 m test on a rowing ergometer in the third session. All athletes were familiarized with the testing protocols used in the present study. Pearson correlation statistical test and stepwise multiple linear regression calculations were used to establish strong common variances shared between predictors.

Anthropometric measurements were collected with an astra stadiometer with a mechanical scale to measure height (0.1 cm) and with a body composition analysis device for body mass (0.1 kg), body muscle (0.1 kg) and percentage of fat mass (0.1%) (TanitaBC-545N) [1], through the use of bioelectrical impedance analysis, which is based on the rate at which a weak electrical current travels through the body [30,31].

### 2.2.1. Vertical Jump Test

Three vertical jump types, squat jump (SJ), countermovement jump (CMJ) and repeat jump (RJ) [13], were used to evaluate the lower body power with a jump-mat system (Chronojump, Bosco-System, Barcelona, Spain), capturing 1000 samples per second. Each participant completed three trials of each type of jump with a rest period between actions of 2 min. The best performance was used for data analysis. In SJ, participants began from a flexed position with knees to 90 degrees and hold this position for 3 s before executing jump without any countermovement. The CMJ test started with the standing position, hands on hips and using the countermovement to jump as high as possible after descending to the half squat position. The RJ followed the same procedure that CMJ but with a continuous execution during 30 s. Elastic index (EI) was calculated from the difference between two jump types (SJ and CMJ), mechanical power (MchP) was calculated with Test time (*T* = 30 s), flight time (*ft*) and the number of jumps (*n*) as MchP <sup>=</sup> (*g*2·*T*·*ft*)/[4*n*·(*T*−*ft*)]. Finally, to calculate the resistance index (RI) to fast strength, the average height reached in RJ was related to CMJ height as RI = *h*RJ/*h*CMJ [13]. Power output prediction equations based on body mass, jump height in SJ (*h*SJ) and CMJ (*h*CMJ) were: predicted power SJ = 60.7·*h*SJ + 45.3·body mass—2055 and predicted power CMJ = 51.9·*h*CMJ + 48.9 · body mass—2007 [32].

### 2.2.2. Bench Pull Test

The bench pull test is a specific tool to assess the pulling strength of the upper torso in rowers owing to similar shoulder adduction that take place during rowing stroke [3,24]. BP data were recorded by an optoelectronic encoder (Velowin, Deportec, Murcia, Spain) capturing 500 samples per second from which a dedicated software calculated velocity, power and force output for each repetition. Mean and maximum values of velocity, power and force, both for the entire concentric action and for the propulsive phase during BP were recorded. In such a test, the rower is laying, face down on the bench, whose height from the floor is adjusted according to the length of the rower's arms so that both elbows are in full extension and the arms completely suspended. The barbell is held with hands apart at shoulder level or slightly wider [3,17]. A light load (30% from the 1RM) as the minimum load that can discriminate the different levels of traditional rowers [4], was used for this study. Although some studies did not find a relationship between BP test and rowing performance, it may be a consequence of having used too high intensity to perform BP. The test was cancelled when the rower was unable to flex the arms sufficiently to touch the underside of the bench phase [33].

### 2.2.3. 2000 m Rowing Ergometer Test

The performance test was carried out on a rowing ergometer (Model D; Concept 2, Inc., Morrisville, VT, USA) [11,24,34,35] with the aim of reducing external influences, such as wind, temperature and waves, that could influence the final result [36]. The control of these cofactors is advisable in traditional rowing, practiced in open waters or rough sea, and in Olympic rowing, which is practiced in flat waters, mainly rivers and lakes. Therefore, a rowing ergometer allows for individual testing in a controlled way, providing a valid proxy for rowing performance [4]. The rowers performed an all-out 2000 m test on a rowing ergometer, with the drag factor set to 130 for males and 110 for females [24]. The warm-up consisted of 10 min of moderate intensity (heart rate below 140 beats per minute and 18–20 strokes per minute). The rowers' coach was continuously motivating and giving feedback to the rowers so that they could carry out the test in the shortest time possible. The rowers could see all the power, stroke rate, distance and time information on the screen of the ergometer. Power output, stroke rate and time to complete 2000 m rowing ergometer performance test were recorded.

### *2.3. Statistical Analysis*

The Statistical Package for Social Sciences (SPSS) v.24 program was used to compare the means of variables (IBM, Armonk, NY: IBM Corp). Descriptive statistics (mean ± SD) were used to report the characteristics of conditional factors. Shapiro–Wilk statistical test was used to determine whether the quantitative variables fulfil the criterion of normality. Pearson correlation coefficient (*r*) with 95% confidence intervals (CI) via bootstrapping was used to establish relationships between anthropometric characteristics, jump test, and bench pull test results with rowing performance. The magnitude of the correlation coefficient was interpreted with the following thresholds: 0.0–0.09 (trivial); 0.1–0.29 (small); 0.3–0.49 (moderate); 0.5–0.69 (strong); 0.7–0.89 (very strong); 0.9–0.99 (nearly perfect); and 1.0 (perfect) [37]. A stepwise multiple regression analysis was used to predict the 2000 m test performance. Additionally, a proportional curvilinear allometric scaling of 2000 m test performance was considered to identify key determinants of rowing performance [38,39]. For both regression techniques, predictors were iteratively adding and removing to the predictive model to find the subset of variables resulting in models which explained 50% or more of the variance of the data (R2 > 0.5). The resulting best-fit equations for the best predictive models of rowing performance within the current population for male and female rowers were also shown, together with adjusted R2 to account for non-significant predictors in the regression models. Statistical significance was set at *p* < 0.05.

### **3. Results**

As shown in Table 1, height resulted in strong correlation with performance (*r* = 0.68, *p* = 0.014) in male rowers. Body mass (*r* = 0.83, *p* < 0.001) and body muscle (*r* = 0.81, *p* < 0.001) had a very strong correlation but percentage of body fat and Body Mass Index (BMI) were not correlated with performance test. Jump tests heights (*h*SJ, *h*CMJ and *h*RJ), RI, EI and MP showed low correlation with performance test. Besides, the power output prediction equation based on body mass for SJ showed strong correlation with performance (*r* = 0.58, *p* = 0.048). Slightly higher correlations were found for CMJ (*r* = 0.70, *p* = 0.012). Mean velocity (*r* = 0.84, *p* < 0.001) and mean propulsive velocity (*r* = 0.87, *p* < 0.001) in BP test showed very strong correlation with performance. Likewise, mean power (*r* = 0.85, *p* < 0.001) resulted in very strong correlation and maximum power (*r* = 0.73, *p* = 0.007) showed very strong correlation with performance.

**Table 1.** Relationship of anthropometric and strength determinants with 2000 m rowing ergometer performance.

Female rowers showed strong to nearly perfect correlation between the same anthropometric variables than males with performance: height (*r* = 0.67, *p* = 0.101), body mass (*r* = 0.66, *p* = 0.107) and body muscle (*r* = 0.94, *p* = 0.002). Similarly to male rowers, the power output prediction equation based on body mass for SJ (WSJ) showed strong correlation (*r* = 0.57, *p* = 0.179), and stronger correlation values were found for CMJ (WCMJ) with performance (*r* = 0.59, *p* = 0.159). Finally, female rowers showed strong correlation in mean velocity (*r* = 0.63, *p* = 0.126), mean propulsive velocity (*r* = 0.67, *p* = 0.102) and mean power (*r* = 0.64, *p* = 0.124) in BP test with performance. Furthermore, very strong correlation in maximum power (*r* = 0.79, *p* = 0.036) were found.

The results of the stepwise multiple linear regression analysis in male rowers indicated that body mass is the only predictor variable for anthropometric characteristics explaining 69% (R2 = 0.69, *p* < 0.001) of W2000m. Similarly, the only predictor variable for BP test were the mean propulsive velocity that explained 76% of rowing performance (R<sup>2</sup> = 0.76, *p* < 0.001) (Table 2). The rest of the anthropometric and power variables in jump and BP tests did not contribute significatively and were excluded from the prediction equation. The best predictor of rowing performance among anthropometric characteristics for female rowers was body muscle, accounting for 89% of variance (R<sup>2</sup> = 0.89, *p* = 0.002) and maximum power for BP measures, explaining 62% of rowing performance (R<sup>2</sup> = 0.62, *p* = 0.036). As with the male linear regression models, the inclusion of the remaining variables resulted in models predicting less than 50% of the variance so they were excluded from the equations.

**Table 2.** Stepwise multiple regression analysis and proportional allometric scaling to predict rowing performance in male and female rowers according to anthropometric and power determinants.

SEE: standard error of estimate; W: power; MPV: mean propulsive velocity in bench pull; Pmax: maximum power in bench pull; M: Male; F: Female.

The proportional allometric model relationships between rowing performance (W2000m) and determinants (anthropometric and BP) showed approximately linear associations, indicated by the exponent near unity. The only prediction equation showing power-function characteristics is the mean propulsive velocity for male rowers, which explained 76% of the variance. The proportional slopes of the rest of the predicting equations gave similar prediction power (R2) but with lower estimate errors. The same variables that explain most of the variance in the stepwise linear regression were the remaining predictor variables making a significant contribution to the proportional allometric model. The remaining predicting variables lead to models explaining less than half the variance of rowing performance, so they were excluded from the equations.

### **4. Discussion**

The main aim of the study was to analyze the relationship of anthropometric and strength determinants with 2000 m rowing ergometer performance in traditional rowing. According to several studies, high-performance rowers of both sexes are usually heavier and taller than low-performance counterparts [9,40,41]. Our results are in accordance with other studies in which height, body mass and body muscle correlated with better performance [34,42,43]. Furthermore, among all variables, body mass for male rowers and body muscle for female rowers were the best predictors of rowing performance. Akça [1] found taller and heavier Olympic male college rowers (185.8 cm and 80.2 kg) compared to traditional rowers of our study (178.4 cm and 77.3 kg). This difference can be due to traditional rowing requiring shorter and lighter rowers in some boat positions for hydrodynamic reasons, so the crew must be not homogeneous [11]. For that reason, some physical advantages of heavier and taller rowers in Olympic rowing could become a disadvantage in boat hydrodynamics and rowing technique in traditional rowing [4,11,27].

In line with our findings, Yoshiga and Higuchi [44] reported that rowing performance is highly influenced by body size, in such a way that large body size increased the rowing performance. However, these authors found that female rowers were slower than males when both groups were matched based on the body size, possibly due to the larger body fat of females which deteriorates the rowing performance. Nevertheless, the differences between sexes in rowing performance were reduced when the fat-free mass was taken into consideration. Therefore, the results support the idea that muscle mass and fat-free mass are key factors related to rowing performance [45], especially in female rowers [19].

 Nevertheless, it is difficult to combine high level of musculature with low percentage of body fat [9]. In this study, body fat showed a small correlation with rowing performance [1,34]. This finding was consistent with previous studies which showed significant differences between age categories, although the differences between elite and sub-elite categories were minimal [4,41]. Despite this fact, it seems accepted that rowers with low body fat percentage perform a shorter time in the 2000 m test [4,48].

In this study, traditional male rowers had a low percentage of body fat of 11.9%, which is in accordance with Majumdar et al. [34], who reported similar values (11.1%) in the combined group (light body mass and open category), although elite rowers showed lower percentage of body fat (7.8%). The same trend is observed when the female group (20.9%) were compared with female elite rowers (16.3%). The difference between sex showed higher body fat percentage in the female group compared with the male group (Δ56.8%), usually accumulated around hips and thighs due to physiological and hormone characteristics [49]. Hence, rowers with high height and lean body mass values as well as low percentage of body fat seem to contribute to a greater power output stroke [9].

In the present study, there was a strong correlation in both sexes between power output in SJ (WSJ) and CMJ (WCMJ) with performance when body mass was considered. Greater muscle volume of the vastus lateralis could explain variance in rowing ergometer performance, sprint, and endurance capacity [50]. Battista et al. [8] did not find correlations between jump test and endurance test, probably since body mass, an important variable for rowing performance, has not been considered as an additional factor to assess vertical jump height [50]. To solve this problem, different authors proposed methods to estimate the lower limb power output during squat jump adding body mass variable to the equations [32,51]. Considering the contributions of these authors, the correlations between rowing performance test and jumps increased considerably. Comparing sex categories, our results showed a strong to very strong correlation between lower limb strength and rowing performance, although results were only significant in the male group. Similarly, Ingham et al. [20] reported a higher correlation between lower limb strength and rowing performance in male than female rowers, although these differences in correlation were considerably larger than our results.

The BP test, in contrast to Olympic rowing, showed higher correlations with rowing performance for all variables compared to jump tests. This fact can be due to the major contribution of the upper body in traditional rowing stroke. High correlation values between BP power average and rowing performance suggest that upper body power is one of the most important factors influencing the performance of traditional rowing, possibly due to the use of lower limbs in a flexed position and the greater degree of body extension compared to Olympic rowing [13]. In the same way, our results showed that mean propulsive velocity in BP was the best predictor of rowing performance for male rowers and maximum power in BP for female rowers, both for linear and proportional curvilinear (allometric) models, suggesting approximately linear associations between these determinants and rowing performance. The fact that seats are fixed in traditional rowing reduces legs freedom of movement and consequently their intervention in the stroke, although legs still have an important role in the first phase of paddling (isometric contraction). Comparing sex categories, male group showed higher correlation between BP power average and 2000 m test. These differences could be related to higher values of body muscle and less percentage of body fat of the male group since BP, 2000 m test is strongly correlated with the ratio between power and body mass [4]. Moreover, Attenborough, Smith, and Sinclair [52] studied the upper contribution to rowing performance of female rowers compared to male rowers and suggest to spend time to specific upper body conditioning as a key factor to improve rowing performance in the female category.

### *4.1. Limitations*

In this study, the vertical jump test was performed to evaluate the power of the lower body by the similarity of the jumping movement gesture and the first phase of the leg drive. Further research could perform other measures, such as the Wingate test, to assess lower-body power, more specifically, peak power, mean power and fatigue index, and the possible relationship with traditional rowing performance.

Another limitation of this study is the small sample size and, therefore, the findings of the study should be interpreted with caution. Future studies are required to confirm our results in a larger

population, including other traditional rowing modalities and nationalities to avoid a possible bias derived from an exclusively Spanish sample.

### *4.2. Practical Applications*

The results of this study provide further insight regarding the influence of different determinants of anthropometry and strength on 2000 m rowing ergometer performance in traditional rowing. This study demonstrates that some anthropometric characteristics may influence rowing success and a higher correlation between the upper body and rowing performance than lower body. Therefore, coaches should consider it to perform effective talent identification programs for rowing and training planning.

### **5. Conclusions**

In summary, the data presented within this investigation suggest that large values of height, body mass and body muscle were highly correlated with 2000 m rowing ergometer performance in traditional rowing. Furthermore, body mass for male rowers and body muscle for female rowers were found to be good predictors. The main results showed a strong correlation of WSJ and WCMJ with 2000 m rowing ergometer performance in traditional rowing. However, BP variables were those that most strongly correlated with performance, highlighting the relevant role of the upper trunk in traditional rowing. Mean propulsive velocity in BP in male rowers and maximum power in bench pull in female rowers were found to be good predictors of rowing performance.

**Author Contributions:** Conceptualization, S.S.-A., A.P.-T. and B.P.; Data curation, A.P.-T. and J.M.J.-O.; Formal analysis, S.S.-A., A.P.-T. and B.P.; Investigation, S.S.-A. and J.M.J.-O.; Methodology, A.P.-T., J.M.J.-O. and B.P.; Project administration, S.S.-A.; Resources, A.P.-T. and J.M.J.-O.; Software, S.S.-A. and B.P.; Supervision, B.P.; Validation, S.S.-A. and B.P.; Visualization, J.M.J.-O.; Writing—original draft, S.S.-A.; Writing—review and editing, S.S.-A., A.P.-T., J.M.J.-O. and B.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was supported by a pre-doctoral grant (ACIF/2018/209) from the Generalitat Valenciana, Spain, and vice-rectorate program of research and knowledge transfer for the promotion of R+D+I at the University of Alicante (Ref. GRE18-19).

**Acknowledgments:** The authors would like to express their gratitude to Jennifer Guerra, from the Department of Chemistry and Biochemistry, and to Elena Gandía García, from the Department of World Languages and Cultures, at the University of Nevada, Las Vegas, NV, USA, for providing assistance in preparing this manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## **E**ff**ects of Achilles Tendon Moment Arm Length on Insertional Achilles Tendinopathy**

### **Takuma Miyamoto 1, Yasushi Shinohara 2,\*, Tomohiro Matsui 3, Hiroaki Kurokawa 1, Akira Taniguchi 1, Tsukasa Kumai <sup>4</sup> and Yasuhito Tanaka <sup>1</sup>**

Received: 21 August 2020; Accepted: 22 September 2020; Published: 23 September 2020

**Abstract:** Insertional Achilles tendinopathy (IAT) is caused by traction force of the tendon. The effectiveness of the suture bridge technique in correcting it is unknown. We examined the moment arm in patients with IAT before and after surgery using the suture bridge technique, in comparison to that of healthy individuals. We hypothesized that the suture bridge method influences the moment arm length. An IAT group comprising 10 feet belonging to 8 patients requiring surgical treatment for IAT were followed up postoperatively and compared with a control group comprising 15 feet of 15 healthy individuals with no ankle complaints or history of trauma or surgery. The ratio of the moment arm (MA) length/foot length was found to be statistically significant between the control group, the IAT group preoperatively and the IAT group postoperatively (*p* < 0.01). Despite no significant difference in the force between the control and preoperative IAT groups, a significantly higher force to the Achilles tendon was observed in the IAT group postoperatively compared to the other groups (*p* < 0.05). This study demonstrates that a long moment arm may be one of the causes of IAT, and the suture bridge technique may reduce the Achilles tendon moment arm.

**Keywords:** insertional Achilles tendinopathy; Achilles tendon moment arm; suture bridge method

### **1. Introduction**

The Achilles tendon, attached to the posterior process of the calcaneus, is the largest and strongest tendon in the body. It is composed of the gastrocnemius muscle, originating from the condyle of the femur, and the soleus muscle, originating from the upper tibia. The gastrocnemius and soleus muscles, collectively known as the triceps surae, flex the ankle joint during gait. In the gait cycle, balance is maintained against gravity during the stance phase, and flexing the ankle joint during the toe-off phase provides a forward propulsive force [1].

The contraction of the triceps surae is exerted as a rotational force (torque) around the ankle axis, which can be calculated theoretically as the product of the muscle cross-section and the moment arm (MA) (Figure 1) [2–4].

In general, the length of the MA is known to change depending on the angles of plantarflexion and dorsiflexion of the ankle joint, which is different in each individual [2–4]. However, the length of the MA of the triceps surae has been reported to change depending on the angle of plantar- and dorsiflexion of the ankle joint [5–7]. Yet, it has also been reported that motion does not cause any change in the length of the MA [8,9], and that the plantarflexion moment length, from the plantarflexion region to the dorsiflexion region, is different in each individual, with little change caused by motion [2–4]. Thus, the changes in the length of the MA in motion has attracted great attention. Moreover, the shape

of the foot has been shown to influence the Achilles tendon MA [10]. However, these studies examined the length of the MA only in healthy individuals and it has not been investigated in patients with Achilles tendon disorders.

**Figure 1.** The contraction of the triceps surae can be calculated theoretically as the product of the triceps surae muscle cross-section area and the Achilles tendon moment arm using the lever rule.

The center of rotation (COR) and tendon excursion (TE) methods are mainly used for the measurement of the lever arm of Achilles tendon [5], with MRI and ultrasound generally used for these methods, respectively. Manja et al. [10] proposed a new method using a weight-bearing X-ray. The method allowed the evaluation of the morphology and anatomical position of the bone and soft tissue. Therefore, we are now able to investigate in various patients with Achilles tendon disorders.

Insertional Achilles tendinopathy (IAT), an Achilles tendon disorder, is caused by the traction force of the Achilles tendon [11,12]. This traction force is influenced by the rotational torque of the ankle joint, and the moment arm length is proportional to the plantar flexion torque of the ankle joint [13]. Therefore, when force is applied to the ankle joint, the Achilles tendon moment arm length is inversely proportional to the traction force applied to the insertion of the Achilles tendon. On the other hand, with a short moment arm, the circular arc associated with ankle rotation is thought to shorten, reducing the length displacement of the muscle-tendon complex, and further reducing the consumption of elastic energy of the Achilles tendon [13,14]. Thus, the moment arm of the ankle joint is expected to influence the traction force applied to the insertion of the Achilles tendon, the length displacement of the muscle-tendon complex, and the elastic energy.

Surgical treatments for IAT include para-tendon excision, adhesion detachment, intra-tendon degeneration excision, and calcaneal wedge osteotomy [15,16]. Of these, we preferred surgical treatment of insertional Achilles tendinosis with reattachment of the Achilles tendon using the suture bridge technique (Figure 2). This method firmly fixes the enthesis by pressing the tendon with the surface, and good outcomes have been reported with this method [17].

However, due to the lack of basic research on this surgical technique, its effectiveness remains unknown. This study aimed to examine the moment arm length as an anatomical feature in patients with IAT. We hypothesized that the suture bridge method for IAT influences the moment arm length.

ķ ĸ Ĺ ĺ **Figure 2.** We performed a reconstruction of the Achilles tendon with a suture bridge technique. -**1** : Bone spurs, calcifications in the tendon, and Haglund's prominence is removed. -**2** : Two Swive Locks with Fiber Tape inserted in the proximal hole. -**3** : Fiber Tape is passed through the Achilles tendon and inserted into the prepared distal hole using Swive Lock. -**4** : The tails on the distal row flushed to the anchor. (**A**): Preoperative X-ray. (**B**): Postoperative X-ray for the same case.

### **2. Materials and Methods**

This is a retrospective study. Fourteen individuals (16 feet) who required surgical treatment for IAT at our hospital between 2014 and 2019 were included in the IAT group. Those with a history of collagen diseases or trauma other than Achilles tendon injuries, and those who could not be followed up for more than one year after surgery were excluded from the study. As a result, 8 individuals (10 feet; IAT group) were examined in this study. Fifteen healthy individuals (with no complaints of ankle joint injuries nor history of surgical trauma) were included for examination in the control group.

The reconstruction in all IAT cases was performed using the Achilles SpeedBridge system (Arthrex, FL, USA). After extending the Achilles tendon in the midline and partly separating it from the insertion, the osteophytes, intra-tendinous degeneration site, and posterior superior calcaneus at the enthesis were excised. Subsequently, fiber tape was used to fix the Achilles tendon to the calcaneus using four knotless anchors. We analyzed a clinical evaluation by the American Orthopedic Foot and Ankle Society ankle/hindfoot scale (AOFAS scale) and visual analog pain scale (VAS) 1 year after the surgery.

At 6 months to 1 year after the surgery, 1.5T MRI (Vantage Titan Ver3.0, Canon, Tokyo, Japan) images were taken to evaluate the morphology of the Achilles tendon insertion area and abnormal signal in the tendon before and after surgery.

Plain X-ray images were used to measure the lever arm. The images of the affected side were taken for patients in the IAT group, while those of the healthy side were taken for patients in the control group. As described by Manja et al. [10], who used plain X-ray images of the feet during weight-bearing, the weight-bearing plane was taken as the plantar axis from the lateral image; a straight line perpendicular to this plantar axis, passing through the center of the Achilles tendon, was set as the central axis of the Achilles tendon. The shortest distance from the central axis of the Achilles tendon to the center of the first metatarsal head parallel to the plantar axis was defined as the length of the sole. A perfect circle was drawn to fit the pulley surface of the talus, and the center point was used as the center of motion. The shortest distance from the center of motion to the central axis of the Achilles tendon was defined as the MA length. In addition, the angle between the Achilles tendon moment arm and the line from the center of the ankle joint movement to the insertion of the Achilles tendon was defined as the α angle (Figure 3). The central axis of the Achilles tendon at the time of postoperative measurement was the midpoint from the most dorsal surface to the bottom of the insertion (Figure 4). Also, the ratio of the MA length/foot length was used to standardize the difference in foot size. Furthermore, in order to take the influence of body weight into account, the force applied to the Achilles tendon at the time of heel raise can be calculated as the rotational force around the ankle joint axis. Then, the force applied to the Achilles tendon, F, can be estimated by:

$$\mathbf{F} = \mathbf{w} \times (\mathbf{L} - \mathbf{M}\mathbf{A}) / \mathbf{M}\mathbf{A}$$

where w is the body weight, MA is the Achilles tendon moment arm length, and L is the length of the sole [14,18,19] (Figure 5). In the present study, this formula was used to calculate the force applied to the Achilles tendon, and the average value of three measurements performed by one examiner was used.

Both the control group and the IAT group were examined, and all measurements were taken preand post-operatively. Statistical tests were performed using Microsoft Excel. Data were tested using unpaired student *t*-tests, and effects were considered significant at 95% CI and *p* < 0.05. All subjects gave their informed consent for inclusion before they participated in the study. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of Nara Medical University (Approval number: 2740).

**Figure 3.** *Cont*.

**Figure 3.** The measurement of the lateral weightbearing radiograph were (A) the centerline of the Achilles tendon insertion, (B) plantar axis, (C) the center of rotation of the ankle, (L) the length of the sole, (MA) the Achilles tendon moment arm (α). The angle between the Achilles tendon moment arm and the line connecting the insertion point and the center of rotation.

**Figure 4.** B: Plantar axis. Af: Front edge of the Achilles tendon insertion. Ar: Rear edge of the Achilles tendon insertion. The central axis of Ar and Af was defined as Ap (the central axis of post-operated Achilles tendon insertion).

**Figure 5.** (A) The centerline of the Achilles tendon insertion, (C) the center of rotation of the ankle, (L) the length of the sole, and (MA) the Achilles tendon moment arm. (F) Traction force of the Achilles tendon. (W) Reaction force by body weight. We calculated F using principle of lever.

### **3. Results**

The average height of subjects in the healthy group was 163 ± 7.4 cm, with average body weight and BMI of 64.5 <sup>±</sup> 12.1 kg and 24.3 <sup>±</sup> 4.8 kg/m2, respectively. The average height of subjects in the IAT groups was 163.6 <sup>±</sup>13.3 cm, with average body weight and BMI of 73.5 <sup>±</sup> 11.0 kg and 27.7 <sup>±</sup> 4.5 kg/m2, respectively. Thus, there was no significant difference between the groups for these items (Table 1).

**Table 1.** Patient Characteristics.

Abbreviations: IAT, insertional Achilles tendinopathy. BMI, body mass index. Values are given as mean ± standard deviation (SD).

The AOFAS scale and VAS improved in all cases (AOFAS scale: Preoperative averaged 71.7, postoperative averaged 92.3; VAS: Preoperative averaged 5.0, postoperative averaged 0.83).

MRI assessment after surgery for IAT revealed Achilles tendon insertion running from the proximal knotless anchor insert to the distal knotless anchor insert via the suture bridge technique in all cases (Figure 6). Also, MRI findings showed the disappearance of abnormal signals in the Achilles tendon in all cases.

**Figure 6.** The MRI images of T2-weighted images of the same patient. ↔: Length of Achilles tendon insertion. (**A**): Pre-operation of the Achilles tendon insertion. (**B**): Post-operation of the Achilles tendon insertion. We can see the postoperative Achilles tendon insertion presence from the proximal knotless anchor insert to the distal knotless anchor insert.

There was no significant difference in the α angle between the control and IAT groups. The ratio of MA length/foot length was 0.30 <sup>±</sup> 0.9 <sup>×</sup> 10−3, in the control group, 0.31 <sup>±</sup> 1.0 <sup>×</sup> 10−2, in the IAT group preoperatively, and 0.27 <sup>±</sup> 0.2 <sup>×</sup> 10−<sup>1</sup> in the IAT group postoperatively, showing significant differences between these groups (*p* < 0.05). The force applied to the Achilles tendon was 148.5 ± 24.9 kg in the control group, 166.2 ± 23.0 kg in the IAT group preoperatively, and 200.8 ± 25.4 kg in the IAT group postoperatively. Despite no significant difference in the force between the control and preoperative IAT groups, a significantly higher force to the Achilles tendon was observed in the IAT group postoperatively, compared to the other groups (*p* < 0.05) (Figure 7).

**Figure 7.** Radiographic Measurement. (**A**): Comparison of the α angle between the control and pre-operative IAT groups. (**B**): Comparison of the ratio of MA length/foot length between the control group, the preoperative IAT group, and the postoperative IAT group, showing significant differences between these groups (*p* < 0.05). (**C**): Comparison of the force applied to the Achilles tendon between the control group, the preoperative IAT group, and the postoperative IAT group, showing that a significantly higher force to the Achilles tendon was observed in the IAT group postoperatively, compared to the other groups (*p* < 0.05). Values are given as mean ± SD.

### **4. Discussion**

COR method and TE methods are mainly used for the measurement of the lever arm of Achilles tendon [5]. It has been reported that the TE method shows a shorter moment arm than the COR method [5]. However, it has also been reported that there is no difference in measurements between the two methods [20]. Thus, there is no unified view on this issue. Furthermore, the Achilles tendon moment arm has been shown to be influenced by weight-bearing [21], and an evaluation method using weight-bearing X-ray has been developed for taking images during weight-bearing [10]. It is also feasible to use radiographs to measure the Achilles tendon moment arm [10]. In the present study, we examined the group of patients with IAT before and after surgery, in comparison to a control group of healthy individuals. In order to measure the moment arm and evaluation of the morphology of the bone during weight-bearing in each group, we decided to use a weight-bearing X-ray for evaluation. Also, the usage of an X-ray is a simple method and allows us to perform anatomical calculations easily. Therefore, we determined that it was a good method in our study.

A study using this measurement method reported a moment arm length of 64.3 mm [10]. In the present study, the average moment arm lengths of the healthy and IAT groups were 51.7 mm and 53.7 mm, respectively, both of which were shorter than the previously reported value. However, the length of the sole of subjects in the previous report was 185.7 mm, in contrast to 171 mm and 170.1 mm in healthy and IAT groups, respectively, in the present study. This suggests that the moment arm length may be influenced by the difference in the physical constitution.

In addition, Manja et al. [10] showed that the moment arm length and α angle are influenced by the difference in the shape of the foot, such as flat and concave feet. In the present study, no significant difference in α angle between the control and experimental groups was observed, indicating that the results are not influenced by the difference in the shape of the subject's foot, even in flat and concave feet.

To date, there have been no reports comparing the Achilles tendon moment lengths between healthy individuals and patients with IAT, but many factors are known to influence the pathophysiology of IAT. Lyman et al. [22] stated that it is caused by the traction force of the Achilles tendon during ankle movement. In the present study, those with IAT had a significantly longer Achilles tendon moment arm than healthy individuals. However, there was no significant difference in the force applied to the Achilles tendon between two groups. This indicates that the traction force of the Achilles tendon, which is the cause of IAT, is not influenced simply by the force applied to the Achilles tendon.

It has been shown that the Achilles tendon exerts approximately 4 to 6 times its weight during running [23,24], stretching approximately 8% of the total length of the Achilles tendon [25]. It has also been shown that, despite the smaller force applied during walking than in running, approximately 7% of the total length of the Achilles tendon is stretched during walking [26]. This indicates that the insertion of the Achilles tendon is influenced not only by simple tension but also by the elastic energy due to the stretching of the Achilles tendon [27]. In fact, triceps surae eccentric exercise has been recommended as a conservative therapy for IAT [28,29], and the extension of the Achilles tendon is a major cause of IAT. Our data revealed that individuals with IAT had a longer moment arm length than healthy individuals. This suggests that, with a long moment arm, the arc length associated with the ankle rotation lengthens, increasing the length displacement of the muscle-tendon complex, and further increasing the elastic energy consumption of the Achilles tendon [13,14]. Therefore, the elasticity of the Achilles tendon is shown to be greatly responsible for the development of IAT.

In addition, any measurements of the moment arm length before and after surgery using the suture bridge technique have not been reported to date. As a surgical treatment for IAT, the calcaneal wedge osteotomy takes the influence of the moment arm into account and aims to reduce stress on the insertion of the Achilles tendon by shortening the calcaneus length and slightly lifting the insertion of the Achilles tendon [30]. The suture bridge technique used in the present study has also been reported to have good long-term outcomes [17,31]. This method can treat various pathological conditions that influence IAT with minimal invasion [31]. Furthermore, by comparing the preoperative and postoperative data in this study, the postoperative Achilles tendon moment arm was found to be shortened. On the other hand, the force applied to the Achilles tendon was significantly increased in individuals with IAT compared to healthy individuals. This indicates that this method may not reduce the force applied to the Achilles tendon. Rather, it may reduce the length displacement of the Achilles tendon by reducing the Achilles tendon moment arm, thereby reducing the energy applied to the Achilles tendon.

There are three limitations to this study. First, the number of cases was small. The study needs to proceed with an increased number of cases, and proper statistical analysis needs to be performed. Secondly, the Achilles tendon insertion is an area, but for the sake of simplicity, it was considered to be a point. Performing 3D analyses allows more precise analyses, and we will need to investigate the area of the Achilles tendon insertion. Third, the moment arm measurement method used in this study was the static evaluation. An X-ray or MRI scan is required to measure the Achilles tendon moment arm after the reconstruction of Achilles tendon insertion using the suture bridge technique. However, it is difficult to measure the dynamic change of the moment arm before and after surgery with the measurement methods reported to date. Therefore, a new measurement method needs to be developed for measuring a dynamic moment arm using X-ray or ultrasonic waves in the future.

### **5. Conclusions**

In this study, we examined the moment arm in patients with IAT before and after undergoing the suture bridge technique compared to that of healthy individuals. The Achilles tendon moment arm was significantly longer in patients with IAT. We hypothesized that the suture bridge technique for IAT influences the moment arm length. To test this, we examined the changes in the moment arm length of patients with IAT before and after the surgery by analyzing the X-ray images of the lateral side of their feet during under weight-bearing. This study demonstrates that a long moment arm may be one of the causes of IAT, and the suture bridge technique may reduce the Achilles tendon moment arm. These findings suggest that it is important to consider the influence of the Achilles tendon moment arm in the treatment of IAT.

**Author Contributions:** Conceptualization, Y.S.; methodology, Y.S. and T.M. (Takuma Miyamoto); software, T.M. (Takuma Miyamoto); validation, T.M. (Takuma Miyamoto), Y.S. and Y.T.; formal analysis, T.M. (Takuma Miyamoto); investigation, T.M. (Takuma Miyamoto); resources, Y.T., T.K., A.T., H.K. and T.M. (Tomohiro Matsui); data curation, Y.T., T.K., A.T., H.K. and T.M. (Tomohiro Matsui); writing—original draft preparation, T.M. (Takuma Miyamoto); writing—review and editing, Y.S. and Y.T.; visualization, T.M. (Takuma Miyamoto); supervision, Y.S. and Y.T.; project administration, Y.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **Immediate E**ff**ects of an Inverted Body Position on Energy Expenditure and Blood Lactate Removal after Intense Running**

### **Moo Sung Kim <sup>1</sup> and Jihong Park 2,\***

Received: 5 September 2020; Accepted: 21 September 2020; Published: 23 September 2020

**Abstract:** We compared the immediate effects of a cool-down strategy including an inverted body position (IBP: continuous 30-s alternations of supine and IBP) after a short period of an intense treadmill run with active (walking) and passive (seated) methods. Fifteen healthy subjects (22 years, 172 cm, 67 kg) completed three cool-down conditions (in a counterbalanced order) followed by a 5-min static stretch on three separate days. Heart rate, energy expenditure, blood lactate concentration, fatigue perception, and circumference of thighs and calves were recorded at pre- and post-run at 0, 5, 10, 20, and 30 min. At 5 min post-run, subjects performing the IBP condition showed (1) a 22% slower heart rate (*p* < 0.0001, ES = 2.52) and 14% lower energy expenditure (*p* = 0.01, ES = 0.48) than in the active condition, and (2) a 23% lower blood lactate than in the passive condition (*p* = 0.001, ES = 0.82). Fatigue perception and circumferences of thighs and calves did not differ between the conditions at any time point (F10,238 < 0.96, *p* < 0.99 for all tests). IBP appears to produce an effect similar to that of an active cool-down in blood lactate removal with less energy expenditure. This cool-down strategy is recommended for tournament sporting events with short breaks between matches, such as Taekwondo, Judo, and wrestling.

**Keywords:** cool-down strategy; heart rate; fatigue perception

### **1. Introduction**

Progressive overload is an essential principle in developing physical fitness, and it often requires fatigue-inducing training [1]. Fatigue is defined as a physiological (and psychological) constraint that results in failure to maintain a certain amount or intensity of voluntary muscle contractions [2]. By overloading exercise-induced fatigue, athletes may experience a temporary (e.g., a few days) performance reduction, referred to as functional overreaching [3]. Nonfunctional overreaching is also considered as the normal process of training adaptation, although it may take a longer time (e.g., a few weeks) to restore function [4]. Overtraining results in performance decrease with persistent physical and/or psychological fatigue for an extended period of time (e.g., a few months) [5]. While athletes and coaches seek optimal training adaptations, inadequate physiological (and psychological) recovery between training sessions can lead to overtraining [6]. Therefore, cool-down strategies to balance training stimulation and physical recovery are just as important as the training program.

Since "active recovery", defined as submaximal exercise or movement immediately following a training session, was first introduced in 1975 [7], its relative superiority over passive recovery has been reported in many studies [8–11]. Specifically, faster decreases of blood lactate [9], intramuscular pH level (due to reduced H+) [12], and oxyhemoglobin [13] as well as faster phosphocreatine resynthesis [14] are known advantages although the most effective working intensity remains controversial [9,15]. Enhancement of oxidation due to a higher rate or volume of blood flow to the working muscles [15] enhances the lactic acid removal [16] and prevents H+ release [17], which allows faster recovery from exercise-induced fatigue. Because of the increase in blood flow, practicing active recovery is preferred, especially when multiple exercise bouts are performed.

If the rate or volume of blood flow is an important causal factor, an inverted body position (IBP) might expedite the recovery process. This gravity-independent position (elevation of the lower body) decreases vascular hydrostatic pressure, resulting in increased venous return and lymphatic drainage [18]. According to the Frank−Starling mechanism [19], an increased venous return increases myocardial stretch (preload), which results in increased stroke volume and cardiac output [20]. Therefore, IBP activity could increase peripheral circulation in the lower extremities and enhance the process of physiological recovery. Previously, a position of head-down tilt at 15◦ was shown to increase stroke volume, mean arterial pressure, and cutaneous vascular conductance [21,22]. Another study [23] used the same position for 5 min and reported a 36% increase in tibial microvascular flow. Although an acute increase of cardiovascular function from holding a head-down tilt position is evident [21–23], changes of blood lactate removal and fatigue perception have not been evaluated.

Hence, the primary purpose of this study was to examine how a 10-min cool-down protocol containing IBP affects physiological and psychological recovery after intense running compared to the same duration of active (walking) and passive (seated) cool-down strategies. Since the effects of increasing blood flow were achieved on the 15◦ head-down tilt [21–23], our IBP protocol had a maximal inclination (up to 90◦) in expectation of the greatest effects of circulation of blood and body fluids [18]. Considering results from previous head-down tilt studies [21–23], the effects of elevation [18], and the Frank−Starling mechanism theory [19], we hypothesized that the lactate removal rate at the end of IBP would be greater than that of passive recovery at the same time point. Due to more voluntary muscle contractions and movements, we further expected higher values of heart rate and energy expenditure in the active recovery condition than in the IBP and passive recovery conditions.

### **2. Materials and Methods**

### *2.1. Experimental Design*

We used a three (condition) × six (time) single-blinded cross-over design with repeated measures of time. To objectively increase exercise-induced blood lactate, a treadmill run was implemented. In this approach, we assumed that heart rate increase by exercise correlates linearly with the amount of blood lactate. To test the condition effect, dependent measurements among the conditions were compared at each time point. Each condition had a 10-min cool-down activity that consisted of 5 min of IBP, walking on the treadmill or sitting on a chair, and another 5 min of static stretching (same activity for each condition). Therefore, the post-run 5-min time point reflected the acute effect of the IBP whereas the post-run 20- and 30-min time points corresponded to the combined effect of IBP and stretching. The temperature and relative humidity in the laboratory were maintained at 25 ◦C and 50% during the data collection period. One researcher guided the treadmill run and conducted the measurements while being blinded to the cool-down condition by leaving the laboratory while another researcher conducted the cool-down protocol. The study was approved by the Institutional Review Board and was conducted in accordance with the Declaration of Helsinki.

### *2.2. Subjects*

Fifteen healthy active adults (5 females: 20.4 ± 0.6 years, 161.4 ± 3.3 cm, 57.8 ± 4.2 kg; 10 males: 22.9 ± 0.9 years, 176.5 ± 1.9 cm, 72.1 ± 0.7 kg) who were participating in club sports (>240 min/week) were recruited for this study. Subjects had no history of lower-extremity surgery and were free from musculoskeletal injury within the last six months. Subjects with current lower-extremity musculoskeletal pain, anemia, diabetes, or cardiovascular disease were excluded. Prior to participation, all subjects gave informed consent, as approved by the University's Institutional Review Board.

### *2.3. Procedures*

Each subject visited the laboratory three times (with a week wash-out period) on separate days at the same time of day (Figure 1).

**Figure 1.** Testing procedures, IBP: inverted body position. Heart rate, energy expenditure, fatigue perception, circumference of lower-extremity, and blood lactate were recorded at each measurement time.

Upon arrival at the laboratory for the first time, subjects read and signed the informed consent form. At each visit, subjects were fitted with a heart rate monitor, and asked to lie down on a treatment table for a 10-min rest to achieve cardiovascular stability before measuring their resting heart rate.

Pre-run measurements were recorded in the order of heart rate, energy expenditure, fatigue perception, circumference of the thighs and calves, and blood lactate (measurements were recorded in the same order at all other time points). Subjects then performed an exercise protocol on the treadmill (Jog Forma, Technogym S.p.A, Gambettola, Italy) where the run began at 4 km/h without treadmill inclination and the speed and inclination increased by 1 km/h and 0.5% every minute. The exercise protocol was ended when the subjects reached the target heart rate (90% of the maximal heart rate calculated using the Karvonen formula) [24]. The post-run 0-min measurements were recorded immediately. After a set of measurements, the researcher left the laboratory and a different researcher came into the laboratory to guide the cool-down conditions.

Subjects executed the three cool-down conditions (IBP, active, and passive) in a counterbalanced order. For the IBP condition, subjects performed five sets of continuous alternating positions on the inversion table. Each set was defined as a position change from A to B for 30 s (Figure 2A,B) followed by a position change from B to A (Figure 2A,B). The duration of inversion was determined to prevent discomfort due to increased intracranial pressure [25]. To change positions, the researcher asked a subject to press a button on the inversion table. For the active condition, subjects walked on the treadmill at speeds of 5 and 6 km/h for females and males, respectively, for 5 min. The walking treadmill speed was determined in our pilot study where we wanted it to be neither too fast (to avoid additional fatigue) nor too slow (no effect on active recovery). For the passive condition, subjects remained seated on a chair for 5 min. After the next set of measurements (post-run 5 min), a 5-min static stretch of the hip adductors, extensors, abductors, and flexors, and knee extensors with a 30-s hold on each side (the same protocol was applied for each condition) was performed. After completion of the static stretch, the post-run 10-, 20-, and 30-min measurements were recorded.

**Figure 2.** The inverted body position, once subjects moved from (**A**) a supine position to (**B**) the IBP, 30 s were counted and then they were returned to the starting position (**A**). It took 17 s to move from position A to B or vice versa. Subjects then spent about 13 s in the IBP (**B**) before returning to position (**A**).

### *2.4. Measurements*

The total running time, average treadmill speed, and inclination were recorded for each subject during each treadmill run.

For the blood lactate measurements, a finger-pricking lancet needle (26 G, Lancets, Moa, Korea) was administered to collect 0.7 μL blood samples on lactate test strips (Lactate Plus Lactate Test Strips, Nova Biomedical, Waltham, MA, USA). The strips were then inserted into a lactate meter (Lactate Plus, Nova Biomedical, USA) for analysis. The blood lactate analysis using this device was previously validated [26].

To measure heart rate and energy expenditure, the subjects wore a heart rate monitor consisting of a Polar M400 GPS watch on their wrist and a Polar H7 strap on their chests throughout the experiment (Polar Electro Oy, Kempele, Finland). After the device was wirelessly connected to the mobile application (Polar Beat), the subject's age, sex, height, and mass were entered. All data recorded throughout the experiment were downloaded from a website (http://flow.polar.com).

The perception of subjective fatigue level was quantified using a modified 10-cm visual analogue scale [27,28] with the terms "unfatigued" and "fatigued" at each end. Subjects were asked to mark their fatigue level at each measurement time.

Using a tape measure, the circumferences of the thighs and calves were recorded (to the nearest 0.1 cm) at the midpoint between the anterior superior iliac spine and the superior pole of the patella and the thickest circumference on the triceps surae, respectively. Both sides were recorded each time (left side first).

### *2.5. Statistical Analyses*

Our sample size was calculated based on a pilot study conducted in our laboratory. We expected a mean difference in blood lactate concentration of 3 mmol/L with a standard deviation of 3.5 mmol/L (an effect size of 0.86). These calculations estimated that 14 individuals would be necessary in each condition (an alpha of 0.05 and a beta of 0.2). This calculation was supported by a similar previous study (*n* = 14) that reported a difference of 3.17 mmol/L [10].

Descriptive data (means and 95% confidence intervals) were calculated and checked for normality using the Shapiro−Wilk test. To test condition effects over time, a mixed model analysis of variance (random variable: subjects; fixed variables: condition and time) and Tukey's test were performed on each dependent measurement. To determine practical significance, Cohen's d effect size [29] was also calculated when statistical significance was found. To test for a correlation between the blood lactate concentration and fatigue perception, Pearson's correlation coefficient was calculated. SAS software (SAS 9.3, Institute Inc., Cary, NC, USA) was used for all tests (*p* < 0.05).

### **3. Results**

The mean ± 95% confidence interval total running time was 7 min and 39 s ± 46 s at a treadmill speed of 7.3 ± 0.4 km/h and an inclination of 1.7 ± 0.2%. The mean treadmill speed and inclination at the end of each run were 11.1 ± 0.8 km/h and 3.6 ± 0.4%, respectively.

The blood lactate concentration was different among the three conditions at post-run 5 and 10 min (condition × time interaction: F10,238 = 3.04, *p* = 0.001; condition effect: F2,238 = 12.38, *p* < 0.0001; Table 1). Specifically, the IBP condition (6.7 mmol/L, 23%, *p* = 0.001, ES = 0.82) and the active condition (6.9 mmol/L, 20%, *p* = 0.01, ES = 0.75) showed lower blood lactate values than the passive condition (8.6 mmol/L) at post-run 5 min. At post-run 10 min, the active condition showed lower blood lactate values than the passive condition (4.4 vs. 6.0 mmol/L, 27%, *p* = 0.02, ES = 0.90). Regardless of the condition (time effect: F5,238 = 314.95, *p* < 0.0001), the blood lactate values decreased gradually after running and they did not return to the pre-run level (1.3 mmol/L) at the end of the experiment (post-run 30-min: 3.1 mmol/L, *p* < 0.0001, ES = 2.41; Figure 3A).

**Figure 3.** (**A**) Changes in blood lactate and fatigue perception regardless of conditions. Error bars are upper and lower limits of 95% confidence intervals. † Different from other time points in blood lactate (*p* < 0.0001). ‡ Different from post-run 0, 5, and 10 min in fatigue perception (*p* < 0.0001). (**B**) Correlation between blood lactate and fatigue perception. There is a high correlation (r = 0.75) between blood lactate and fatigue perception.

The heart rate values differed between the conditions at each time point (condition × time interaction: F10,238 = 8.39, *p* < 0.0001; condition effect: F2,238 = 17.61, *p* < 0.0001; time effect: F5,238 = 1243.89, *p* < 0.0001; Table 1). The IBP condition (96 bpm, 22%, *p* < 0.0001, ES = 2.52) and the passive condition (103 bpm, 17%, *p* = 0.002, ES = 1.92) resulted in lower heart rate values than the active condition (123 bpm) at post-run 5 min.

Energy expenditure among the three cool-down conditions at each time point did not differ (no condition × time interaction: F10,238 = 1.65, *p* = 0.09; time effect: F5,238 = 224.40, *p* < 0.0001; Table 1). However, the total energy expenditure between the conditions was different (condition effect: F2,238 = 4.81, *p* = 0.01) in that subjects in the active condition had a 14% higher total energy expenditure compared to the IBP condition (219 vs. 189 kcal, *p* = 0.01, ES = 0.48; Figure 4).

**Figure 4.** Changes in total energy expenditure regardless of time, IBP: inverted body position; Error bars are upper limits of 95% confidence intervals, † Condition effect (F2238 = 4.81, *p* = 0.01): Active was recorded at higher total energy expenditure values compared with the IBP (219 vs. 189 kcal, 14%, *p* = 0.01, ES = 0.48).

**Table 1.** Changes in heart rate, energy expenditure, blood lactate, and fatigue perception.

IBP: inverted body position; ICC: intraclass correlation coefficient, † Different from the IBP (23%, *p* = 0.001, ES = 0.82) and active (20%, *p* = 0.01, ES = 0.75) conditions at post-run 5 min; ‡ Different from the active condition (27%, *p* = 0.02, ES = 0.90) at post-run 10 min; # Different from the IBP (22%, *p* < 0.0001, ES 2.52) and passive (17%, *p* = 0.002, ES = 1.92) conditions at post-run 5 min.

Fatigue perception did not differ among the three cool-down conditions at each time point (no condition × time interaction: F10,238 = 0.13, *p* = 0.99; no condition effect: F2,238 = 0.87, *p* = 0.42; time effect: F5,238 = 245.80, *p* < 0.0001; Table 1). Regardless of the condition, fatigue perception was 87% higher after running (6.8 cm, *p* < 0.0001, ES = 4.63) and returned to the pre-run level (0.9 cm) at post-run 20 min (1.2 cm, *p* = 0.55; Figure 3A).

The circumference of the left (no condition × time interaction: F10,238 = 0.28, *p* = 0.99; condition effect: F2,238 = 3.72, *p* = 0.03; time effect: F5,238 = 3.88, *p* = 0.002; Table 2) and right (no condition × time interaction: F10,238 = 0.25, *p* = 0.99; no condition effect: F2,238 = 1.26, *p* = 0.29; time effect: F5,238 = 6.66, *p* < 0.0001; Table 2) thighs, and left (no condition × time interaction: F10,238 = 0.96, *p* = 0.48; condition effect: F2,238 = 22.32, *p* < 0.0001; time effect: F5,238 = 41.33, *p* < 0.0001) and right (no condition × time interaction: F10,238 = 0.79, *p* = 0.64; condition effect: F2,238 = 12.13, *p* < 0.0001; time effect: F5,238 = 44.57, *p* < 0.0001) calves did not differ between the conditions at each time point.

Blood lactate was highly correlated with fatigue perception (r = 0.75, *p* < 0.0001; Figure 3B). Good to excellent measurement consistency (ICC values from 0.75 to 0.99; Tables 1 and 2) was calculated for all measurements [30].

**Table 2.** Changes in circumference of lower-extremities.

IBP: inverted body position; ICC: intraclass correlation coefficient, No condition × time interactions in circumference of left (F10,238 = 0.28, *p* = 0.99) and right (F10,238 = 0.25, *p* = 0.99) thigh, and circumference of left (F10,238 = 0.96, *p* = 0.48) and right (F10,238 = 0.79, *p* = 0.64) calf.

### **4. Discussion**

The purpose of this study was to examine how the IBP condition consisting of continuous alternation between 30-s IBP and a supine position for 5 min would help physiological and psychological recovery after intense running relative to active (walking) and passive (sitting) recovery methods. While walking is a commonly recommended method [10,31] and sitting is considered as a control, we were interested in comparing these existing and frequently practiced methods to a recovery condition containing IBP. During the initial 5 min of the recovery period, the IBP was effective in reducing the

blood lactate and heart rate to levels similar to the active and passive conditions. In other words, the IBP condition produced an effect similar to that of the active condition in blood lactate with less energy expenditure. This observation suggests that the recovery condition including IBP could be an alternative recovery strategy after a short bout of intense exercise. Since blood lactate removal is dependent on the amount of blood flow to working muscles, and the process is facilitated by muscle contraction [17], performing IBP could be beneficial for individuals who need a fast recovery while minimizing energy consumption.

The IBP and active (walking at a speed of 5 to 6 km/h) strategies in this study reduced blood lactate by 32% (9.8 to 6.7 mmol/L) and 27% (9.4 to 6.9 mmol/L), respectively, at 5 min postexercise (post-run 0 min). The effects of recovery strategies with active movements [10,31,32], electrotherapy [33], and massage [31] were previously reported. Comparable corresponding values at similar time points in other studies included a 19% (14.9 to 12.0 mmol/L) reduction with a 5-min treadmill run (at 50% of maximal speed) [32] and a 49% (3.9 to 2.0 mmol/L) reduction with 6 min of motor level electrotherapy (at 9, 8, and 7 Hz for 2 min each via 8 electrodes) [33]. At 10 min postexercise (after subjects had finished a 5-min static stretch in our study), blood lactate was reduced by 45% (9.8 to 5.4 mmol/L) and 53% (9.4 to 4.4 mmol/L) in the IBP and active conditions, respectively. Other studies reported reductions of 40% (14.9 to 8.9 mmol/L) with a 10-min treadmill run (at 50% of the maximal speed) [32], 51% (11.7 to 5.7 mmol/L) with a 10-min free style swim (at 65% of each individual's best 200-m front crawl swim velocity) [31], and 39% (11.6 to 7.1 mmol/L) with a 10-min massage (using the effleurage, petrissage, tapotement, and compression techniques) [31]. As indicated by these indirect comparisons at 5 and 10 min postexercise, our IBP condition (a combination of the 5-min IBP and static stretch) appears to be as effective as other recovery activities. Although not directly measured, we believe that the effect of the IBP could have been attributed to hemodynamic changes similar to those of using compression garments [34,35]. According to men's and women's Taekwondo Kyorugi schedules at the Olympic Games in Rio in 2016, the rests between matches ranged from 360 min to less than 60 min. In the Asian Games in Jakarta−Palembang in 2018, the rest times in the men's fencing épée ranged from 80 min to 175 min. During such short time periods, a recovery activity enhancing a rapid blood lactate removal with minimal energy expenditure would contribute to the performance in the subsequent match. Because all of the comparable recovery strategies reported require an electrical device [33], massage personnel [31], or to expend more energy [31,32], our method with IBP could be the best option. Although the blood lactate values in the three conditions in our study became similar at 20 min postexercise, IBP is still beneficial because it resulted in a longer time to rest with a low blood lactate level.

Fatigue perception did not differ among the three recovery conditions at each time point. Previously, electrotherapy (motor level stimulation via 8 electrodes) and the passive (sitting on a chair) recovery method reduced perceived exertion by 46% and 38% compared with active recovery, respectively [33]. Another study [32] reported 18% less perceived exertion with passive (sitting on a chair) recovery than with active (treadmill run at 50% maximal speed) recovery [32]. Although the results of the Borg scales (0 to 10 points: [33]; 6–20 points: [32]) are comparable to our fatigue perception because both are measures of subjective physical condition, the differences of the measurement characteristics could explain why there was no difference across the conditions in our study. The Borg scale matches a numeric scale with descriptions of exertion, asking subjects to indicate their exertion level, whereas our measurement simply asked subjects to place themselves between "unfatigued" and "fatigued" on a visual analogue scale. Measurements were also taken during exercise using the Borg scale and after exercise in our study. Since subjects in previous studies felt less exertion in the passive condition than those in the active condition [32,33], providing different descriptions (e.g., fresh or recovered) or using a more detailed numeric scale may have produced a different result. Interestingly, blood lactate and fatigue perception values had a strong correlation in our study (r = 0.75, r2 = 0.57). Thus, at least 57% of an individual's perceived fatigue can be explained by blood lactate and vice versa. However, fatigue perception returned to the pre-run level at 20 min postexercise (post-run 20 min)

whereas blood lactate levels did not. This observation has several practical implications. First, people may feel complete recovery although their bodies are still in the process of physiological recovery. Second, a complete physiological recovery may be unnecessary for psychological (perceptual) recovery. Third, in addition to blood lactate, there could be other factors which contribute to the level of fatigue perception. A future study should examine the relationships between psychological, physiological, and athletic performance.

Similarly, there were no changes in the circumferences of lower extremities at any time point. Given that a tape measure was used to obtain circumferences in many previous studies [34–37], the measurement reproducibility in our study was strong (ICC value of 0.99 for between-session reliability in all measurements), measurement technique or errors were unlikely to contribute to the results. In fact, we did not even observe a circumference change at the post-run measurements, which could be explained by the exercise duration and mode (specific energy system). Previously reported postexercise extremity circumference was induced by resistance [34,35] or endurance exercise [36,37]. Therefore, an approximately 7-min all-out treadmill run may not have been sufficient to increase leg swelling. Exercise-induced transient hypertrophy, also known as "the pump", is thought to be caused by the accumulation of intracellular fluid from reactive hyperemia [38] or osmolytic activity [39]. In endurance activities, the expansion of extracellular volume [37] due to increased plasma volume and sodium retention [40] is thought to be responsible for oedematous swelling. While we assumed that our intense treadmill run mostly used the combined energy system of anaerobic glycolysis, it produced neither of the physiological mechanisms, resulting in no circumference change. Because body fluid removal is an important facet of physical recovery, a future investigation should examine how the IBP strategy affects the increased circumference caused by either resistance or prolonged endurance exercise. Appendicular body composition analysis along with skinfold thickness would provide better accuracy in the assessment of exercise-induced anthropometric changes.

There are several limitations and assumptions in our study that may benefit future research. Although good criterion-related validity and high reliability in the estimation of energy expenditure using the same heart rate monitoring device were established [41], the major contributing factors such as the physical activity and sedentary levels [42], and percentage of body fat [43] were not reflected. Additionally, a lower heart rate recorded during the IBP might have underestimated the whole-body metabolism and energy consumption. Therefore, we acknowledge the level of accuracy in estimating energy expenditure as a limitation. While accepted, our hypothesis was based on increased blood flow for a body position with 15◦ head-down tilt [21–23] and the lack of direct measures on blood flow parameters limit us to explain and interpret the observed benefits. As an elevated blood pressure during and after exercise is typical, performing IBP immediately after exercise may be harmful by applying additional pressure to blood vessels. When comparing the inversion duration (90◦ for 2 min) in previous reports [44,45] to ours (90◦ for 13 s: Figure 2), we assume that the possibility of additional blood pressure was minimal. Additionally, our subjects' heart rate at the last minute of the treadmill run and the beginning of IBP were 183 and 121 bpm, respectively. A 2-min interval for the post-run measurements resulted in a 34% reduction in heart rate (183 to 121 bpm). Considering that both heart rate and blood pressure are linearly related to exercise intensity [46], a reduced heart rate value indirectly supports the idea that our subjects performing IBP were not in a deleterious situation. However, future studies should attempt to compare the change of blood flow volume and pressure during the recovery period to resolve our limitations. Lastly, the contraindications of IBP should be addressed since the IBP may be unsafe for pregnant individuals or those with abnormal blood pressure or other cardiovascular diseases, eye or ear injuries, or herniated discs.

According to our data, a 10-min recovery strategy consisting of 5 min of IBP and a 5-min static stretch was the most effective of the tested protocols in removing blood lactate and minimizing energy expenditure during the acute recovery period (the first 30 min, postexercise). Therefore, the recovery condition including our IBP protocol is an appropriate strategy to enhance physical and psychological recovery. We particularly recommend this IBP protocol for athletes who complete several matches

(events) within a few hours on a single day (e.g., tournaments for Taekwondo, Judo, weightlifting, fencing, wrestling, sprint cycling, and swimming). In the final stage of matches (e.g., after quarterfinals), the fatigue level would be high. In this situation, physical fitness and stamina can play bigger roles than technical and tactical strategies. Therefore, we recommend practicing our IBP method between matches. If an IBP device is unavailable, peer-assisted IBP or a supine position with legs elevated against a wall could be used instead.

**Author Contributions:** Conceptualization, M.S.K. and J.P. Methodology, J.P. Formal analysis, M.S.K. and J.P. Investigation, J.P. Data curation, M.S.K. Writing—original draft preparation, M.S.K. and J.P. Writing—review and editing, J.P. Supervision, J.P. Project administration, J.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** This study received no specific grant from any funding agency. We thank Chae Rin Kim, Chaerin Yeom, and Gihum Kown for their data collection and reduction.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article*

## **Ultrasound-Guided Percutaneous Needle Electrolysis and Rehab and Reconditioning Program for Rectus Femoris Muscle Injuries: A Cohort Study with Professional Soccer Players and a 20-Week Follow-Up**

**Fermín Valera-Garrido 1,2,3,\*, Sergio Jiménez-Rubio 4,5, Francisco Minaya-Muñoz 1,2,6, José Luis Estévez-Rodríguez 5,7 and Archit Navandar <sup>8</sup>**

Received: 4 September 2020; Accepted: 3 November 2020; Published: 8 November 2020

**Abstract:** Rectus femoris muscle strains are one of the most common injuries occurring in sports such as soccer. The purpose of this study was to describe the safety and feasibility of a combination of percutaneous needle electrolysis (PNE) and a specific rehab and reconditioning program (RRP) following an injury to the rectus femoris in professional soccer players. Thirteen professional soccer players received PNE treatment 48 h after a grade II rectus femoris muscle injury, followed by a the RRP 24 h later. Assessment of recovery from injury was done by registering the days taken to return to train (RTT), return to play (RTP), and structural and functional progress of the injured muscle was registered through ultrasound imaging and match-GPS parameters. Also, adverse events and reinjuries were recorded in the follow up period of twenty weeks. The RTT registered was 15.62 ± 1.80 days and RTP was 20.15 ± 2.79 days. After fourteen days, the ultrasound image showed optimal repair. Match-GPS parameters were similar before and after injury. There were no relapses nor were any serious adverse effects reported during the 20-week follow-up after the RTP. A combination of PNE and a specific RRP facilitated a faster RTP in previously injured professional soccer players enabling them to sustain performance and avoid reinjuries.

**Keywords:** muscle strain; quadriceps; football; invasive physiotherapy; post-injury performance; reinjury; return to play

### **1. Introduction**

Professional soccer players face significant mechanical, metabolic, and physiological demands as a result of their participation in the sport [1]. Regarding to these demands, scientific evidence shows, for example, that elite soccer players cover total distances of 11,173 ± 524 m [2], with 600–831 m ran at speeds exceeding 7 m·s−<sup>1</sup> [3] and over 160 accelerations between 1.5–2 m·s−<sup>2</sup> [4]. Given these demands, soccer has a high risk of injury [5,6]; in fact, at the elite level, up to two injuries per player per season occur [7]. Muscle strains are among the most frequent non-contact injuries [8], and 81–92% of all muscle injuries in professional soccer affect four muscle groups: the hamstrings, the adductors, the quadriceps, and the calf muscles, with the quadriceps being the most affected after the hamstrings and the adductors [9,10]. The biggest muscle among the quadriceps, the rectus femoris, is the anatomical area that is most affected during a tear in the quadriceps [10,11], representing 19% of all non-contact muscle injuries [10,12]. Among all rectus femoris injuries, 46.1% are grade I injuries (representing an edema during MRI evaluation), 37.3% are grade II (representing a partial tear during MRI evaluation) and 4.9% are grade III (representing a complete tear during MRI evaluation) [10].

The rectus femoris is composed of fast-twitch type II fibers [13] and is vulnerable to an injury because of its biarticular nature spanning over the hip and knee joint [12]. It has an important relationship with the iliopsoas helping in the flexion of the hip [14,15], and previous research has shown that a weak or an under-active psoas can result in the rectus femoris generating a greater hip flexion force, resulting in a rectus femoris strain [14–16]. It is mainly injured during kicking or sprinting [15]. In kicking, the rectus femoris plays an important role where it is subjected to changes in muscular length during the reduction of moment of inertia at the knee in the leg cocking phase (when the hip and knee both flex) to the subsequent increase in knee extension velocity in the leg acceleration phase [15,17]. Similarly, in sprinting, specifically during acceleration and deceleration, the quadriceps absorb a large amount of energy [15,17] when they are activated eccentrically as knee extensors while the hip flexes simultaneously. In such a scenario, the adoption of a correct posture is key in order to maintain stability and facilitate force transmission across the lower-limb joints, thereby avoiding compensation generated by an absence of coordination of the lumbopelvic complex [14,15,17].

Moreover, its high recurrence rate in the weeks following return to play (RTP) is a cause of concern in professional soccer [18–20]. Its recurrence rate is between 5% and 6.4% in the short-term (the first week following RTP) [21,22], between 50% and 77% in the medium-term (between the first and second month after RTP), and 15% in the long term (between 5–6 months after RTP) [22]. For these reasons, a successful management of these injuries constitutes a challenge for clinicians in order to define specific protocols of treatment that integrate biological stimulus [23–25] and reconditioning programs adapted for elite competition [17].

Ultrasound (US)-guided percutaneous needle electrolysis (PNE) is a minimally invasive technique that consists of the application of a galvanic current through an acupuncture needle under direct ultrasound visualization [26]. PNE stimulates a local inflammatory response leading to increased cellular activity and repair of the affected area [27]. It has proven to be effective in injuries such as tendinosis (for example, in patellar tendonitis [28], lateral epicondylitis [29], subacromial pain syndrome [30], etc.), and plantar heel pain [31]. The histological and functional evidence observed in an animal model of muscle lesion [32,33] demonstrates that the application of PNE during muscle regeneration induces a decrease in pro-inflammatory mediators (TNF-α and IL-1β), and an increase in the expression of anti-inflammatory proteins (PPAR-γ) and vascular endothelial growth factor (VEGF). The recovery time of the damaged muscle tissue is also reduced. However, such studies in humans are limited to case studies analyzing acute muscle injury [34,35]. Previous studies [36] have used different strength programs in participants with previous injuries to the rectus femoris in order to assess the effectiveness of the program. A review on the rectus femoris injuries has stressed the need to propose interventions through criteria-based reconditioning programs for this injury in order to ensure an optimal RTP scenario to sustain performance and prevent reinjuries [37]. Such criteria-based reconditioning interventions have been shown to be successful in the case of hamstring injuries [38,39].

The identification of elimination of potential risk factors for an injury can be aided by following a cohort of players in a single center over a large span of time. If the same rehabilitation program is applied to all players of the cohort, this also allows the evaluation of a particular program and helps look at the medium to large term development of the players. This has been seen in the case of hamstring injuries, where following a cohort of professional and youth players helped discarding isokinetic strength (of the hamstrings and quadriceps) [40] and flexibility [41] as weak indicators for a potential hamstring injury. In terms of medium to long-term evaluation of the program, the application of a criteria based reconditioning program showed the absence of reinjuries in a span of twenty-four weeks following injury [38] and the players actually managed to improve their performance as a result of individualized training they underwent [39]. However, to the best of the authors' knowledge, such programs following an injury to rectus femoris have not been described in the literature.

Hence, the aim of this study was to assess the safety and feasibility of a combination of percutaneous needle electrolysis and a specific rehab and reconditioning program (RRP) in professional soccer players with an acute muscle injury to the rectus femoris. A secondary aim of the study was to analyze possible reinjuries following RTP in the short, medium and long term. Based on previous research, the hypothesis was that the PNE and RRP program would reduce the layoff time, but the players' physical performance at RTP would be at a lower level as compared to the pre-injury levels.

### **2. Methods**

### *2.1. Study Design*

A single exposure cohort study design was used where a single group of players was followed over the course of three seasons. In case of sustaining an injury to the rectus femoris, their rehabilitation and performance was monitored in the short, medium, and long term.

### *2.2. Participants*

Players from a single club playing in the top professional soccer division of the Spanish soccer league system, commonly known as *LaLiga* were followed over three seasons, i.e., the 2017–2018, 2018–2019, and 2019–2020 seasons. To be eligible for the study, participants were required to meet the inclusion criteria of being professional male soccer players, over 18 years of age and having suffered the acute onset of anterior thigh pain, confirmed via ultrasound imaging [42–45] as a grade II [46] injury to the rectus femoris muscle located at the myotendinous junction. The sports medicine physician confirmed the diagnosis 48 h after the injury incidence. Additional eligibility criteria are described in Table 1.

Thirteen soccer players (age = 27.92 ± 3.17 years; height = 180.92 ± 3.42 cm; mass = 74.07 ± 3.42 Kg; 61.5% of whom were injured while kicking) (Table 2) were finally included in the cohort and volunteered to participate in the study. All participants provided written informed consent, and procedures were conducted under the guidance of the technical and medical staff of the team, always adhering to the ethical principles for medical research involving human subjects under the Declaration of Helsinki.

**Table 2.** Profile of professional soccer players.

\* CoD: change of direction; D: dominant; ND: non-dominant; Kg: kilogram; cm: centimeter.

### **3. Intervention Protocol**

### *3.1. US-Guided Percutaneous Needle Electrolysis (PNE)*

The PNE technique was performed under ultrasound guidance on the muscle injury using an intensity of 1.5–2 mA during 3 s, five times (1.5–2:3:5), according to the protocol by Valera-Garrido and Minaya-Muñoz [47], 48 h after the injury because electromyographic recovery of endplate noise in muscles treated percutaneously with galvanic current is relatively quick (72 h), coinciding with the inflammatory reaction [47]. A specifically developed medically certified device (Physio Invasiva®, PRIM Physio, Spain) was used (Figure 1). The device produced a continuous galvanic current through the cathode (modified electrosurgical scalpel with the needle) while the patient held the anode (handheld electrode). A GE® LOGIQ™ E9 ultrasound machine with a ML6–15 linear transducer (GE Healthcare®, Wisconsin, USA) was used. During the PNE technique, the player was placed in the supine position on a reclining bed at an elevation of 45 degrees with both legs flat resting on the bed. Prior to inserting a needle, the underlying skin was cleaned with isopropyl alcohol and chlorhexidine (Lainco® 2%). The transducer, enclosed in a non-sterile rolled latex covered over non-sterile ultrasound gel, was placed on the target area. Subsequently, an acupuncture needle 0.30 mm <sup>×</sup> 30 mm (Physio Invasiva® needles, PRIM Physio, Spain; uncoated steel needle with rigid metal handle with guide, Korean type) was inserted using a short-axis approach at a 80-degree angle with skin and advanced toward the muscle injury according to the technique described by Valera-Garrido and Minaya-Muñoz [47] (Figure 2). A physiotherapist with more than 10 years of experience in ultrasound evaluation and over fifteen years of experience in invasive therapy applied the PNE technique. Oral paracetamol was used when necessary for the purpose of pain relief in the first 24 h after PNE intervention.

**Figure 1.** Medically certified device (Physio Invasiva®, PRIM Physio, Spain). (**A**) Device; (**B**) cathode (modified electrosurgical scalpel with the needle); (**C**) needle holder; (**D**) needle; (**E**) anode (handheld electrode).

**Figure 2.** Percutaneous needle electrolysis (PNE) technique for rectus femoris muscle injury. (**A**) Graphic scheme in transverse plane. Short-axis approach at an 80-degree angle into the area of muscle injury (dark red part). (**B**) B-mode ultrasound image of PNE technique in which arrowhead represents the tip of the needle in a short-axis approach in area of muscle injury. (**C**) B-mode ultrasound image of PNE technique in which the white area represents the hydrogen gas released during the electrolysis. RF: rectus femoris; VM: vastus medialis; VL: vastus lateralis; VI: vastus intermedius; IT: intramuscular tendon of RF muscle.

### *3.2. Rehab and Reconditioning Program (RRP)*

The rehab and reconditioning program was divided into two parts: an indoor rehab (IR) program and an on-field reconditioning (OFR) program. The IR program, focused on restoring the athlete's neuromuscular function [48], had a duration of approximately 8–9 days post-PNE. This program was further divided into two phases. The initial phase (Phase 1, Table 3) was based on the optimization of range of motion (ROM) and strength. Progressive muscular tensions at different ROMs and movement velocities [49] were prescribed to protect it against future demands and promote connective tissue repair (48 h, 72 h & 96 h post-PNE-Table 3). Hip, pelvic and thoracic spine mobility, single leg strength training, and dynamic tasks that combined isometric activation with stretch-shortening cycle of the lower limb muscles were included in this initial phase. The second phase (Phase 2, Table 4) began 5–7 days post-PNE and focused on tasks based on the absorption and production of force in different planes. This acted as a link toward the progression of movement skills in order to optimize lineal and multidirectional movements [38,39,50]. Players were instructed to perform the included exercises within a pain score of 2 on a numerical rating scale from 0 to 10, where 0 represented no pain and 10 was the most extreme pain possible. Players were encouraged to increase the load with minor pain corresponding to 2 of 10; that is, if pain was ≤1 of 10, they were encouraged to increase the load, and if pain was ≥3 of 10, the load was reduced [51].

The OFR program (Phase 3, Table 5) focused on an optimal reconditioning to prepare the player for the competition demands through technical and coordination drills (OF-1, OF-2, OF-3, and OF-4), which were increased in terms of complexity and demands of decision making [38,52]. In addition, global positioning system (GPS) and accelerometer data were used to training load monitoring during the OFR program. When the player successfully completed all item of the RRP and progression criteria, they were declared fit to train with the team.

ROM: range of motion; EQI: eccentric-quasi-isometric; ISO: isometric tension; SSC: stretch-shortening cycle; DOMS: delayed onset muscle soreness; PNE: percutaneous needle electrolysis.

CMJ: countermovement jump; BW: body weight.

 3.

OF-1: on-field 1 (days 9–10 post PNE); OF-2: day 11 post PNE; OF-3: days 13–14 post PNE; OF-4: day 15 post PNE; RSA: repeated sprint ability; CoD: change of direction; PNE: percutaneous needle electrolysis.

### **4. Variables and Data Analysis**

### *4.1. Return to Training and Return to Play*

The number of days between the injury and the return to training (RTT), return to play (RTP 1), and following match (RTP 2) were counted.

### *4.2. Ultrasound*

Ultrasound scanning of the injured area was performed every three days with the help of Compare Assistant LOGIQ™ software (GE Healthcare®, Milwaukee, WI, USA) to compare prior examinations with the current one for confidence.

### *4.3. GPS Match Variables*

GPS data (WIMU PRO™, Real Track Systems®, Almeria, Spain) was collected for all players from all matches in which they participated before (PRE: PRE 1 and PRE 2) and after (RTP: RTP 1 and RTP 2) injury. This pre-injury data would serve as a control group for this cohort, as it would permit the comparison with the aim of evaluating the effects of the intervention, on the performance of the injured player. This could only be registered on 10 players, since three players were injured in the preseason, and there were no data from official competitions prior to the injury.

The variables used to compare the performance represented actions where the quadriceps are known to be the most active: in accelerations, high intensity runs, and sub-maximal and maximal sprints [53]. The variables measured were (a) total distance covered in m (as a reference value); (b) distance covered at high intensities (between 18.1–21.0 Km·h<sup>−</sup>1) in m; (c) distance covered at very high intensities (21.1–24.0 Km·h<sup>−</sup>1) in m; (d) distance covered at sprint velocities (above 24.0 Km·h<sup>−</sup>1) in m; (e) the peak speed registered in Km·h<sup>−</sup>1; (f) the peak acceleration registered in m·s<sup>−</sup>2; and (g) the explosive distance (i.e., the distance covered when the acceleration exceeded 1.2 m·s<sup>−</sup>2) in m·min<sup>−</sup>1.

### *4.4. Statistical Analysis*

The GPS data recorded were compared, pre-injury vs. post-injury values, using a repeated measures ANOVA. Post-hoc corrections were made with Bonferroni corrections considering the following analyses only: PRE 2 vs. RTP 1, PRE 2 vs. RTP 2, PRE 1 vs. RTP 1, and PRE 1 vs. RTP 2. All calculations were carried out with Jamovi 1.2.17 (version 1.2) with α = 0.05.

### *4.5. Adverse Events*

The soccer players were asked to report any adverse events that they experienced during or after the PNE technique and RRP. Adverse events can be defined as sequelae of medium to long term in duration, with moderate to severe symptoms, perceived as distressing and unacceptable and requiring further treatment [54].

### *4.6. Follow-Up*

After RTP, the players were followed up in the short term (1 week), medium term (8 weeks) and long term (20 weeks) to assess a possible re-injury and adverse effects.

### **5. Results**

Participants returned to full team training (RTT) in 15.62 ± 1.80 days and returned to play (RTP 1) in 20.15 ± 2.79 days and played a total of 94.6 ± 1.20 min. The following match (RTP 2) was in 29.62 ± 3.95 days following the index injury with a match time 95.1 ± 1.14 min.

### *5.1. Ultrasound*

Initially, all players presented images showing a fibrillar defect in the myotendinous junction of rectus femoris muscle with surrounding edema, without fluid collection/hematoma drainable, consistent with a grade II strain injury (Figure 3A). In 92.3% of the cases, the ultrasound examination in the day 7 post-PNE showed a significant change in the extent of edema and fiber disruption (Figure 3B). One player needed one additional session of PNE (7 days post-PNE-1). Ultrasound follow-up at 14 days showed an almost complete disappearance of the injury pattern of the muscle in all players in the ultrasound image (Figure 3C).

**Figure 3.** Transverse B-Mode ultrasound images of the evolution of the injured muscle. (**A**) Initial. 48 h post-injury. Ultrasound imaging shows muscle fiber discontinuity at the level of tear (arrowhead), with surrounding echogenic muscle edema (asterisk). (**B**) Ultrasound imaging at seven days post-PNE. (**C**) Ultrasound imaging at 14 days post-PNE.

### *5.2. GPS Parameters*

On comparing the data from the GPS data (Figure 4), there were significant differences between the GPS recorded distances at high intensities (F (3,27) = 4.67, *p* = 0.009, η<sup>2</sup> = 0.087) and distances covered at sprint speeds (F (3,27) = 2.98, *p* = 0.049, η<sup>2</sup> = 0.108) but no significant differences for distances covered at very high intensities (F (3,27) = 0.699, *p* = 0.561, η<sup>2</sup> = 0.010). Post-hoc comparisons showed a significant decrease between PRE-1 and RTP 1 in distances covered at high intensities (*p* = 0.007). All other post-hoc comparisons between PRE and RTP data showed no significant differences between data registered at PRE and POST. A comparison of the total distance run showed significant differences between the matches, with the distances run during RTP 2 being significantly higher than PRE-2 (*p* < 0.001) and PRE-1 (*p* < 0.001); and the distance run at RTP 1 was higher than that at PRE-2 (*p* = 0.007).

The variation in peak speed (Figure 5) was not significant between the different instances (F (3,27) = 2.49, *p* =0.136, η<sup>2</sup> = 0.126). However, a significant difference was observed for the peak acceleration registered (F (3,27) = 4.74, *p* = 0.009, η<sup>2</sup> = 0.257), although post-hoc comparisons showed no significant differences between the different instances. The explosive distance showed significant differences between the instances (F (3,27) = 13.4, *p* < 0.001, η<sup>2</sup> = 0.481), and post-hoc comparisons showed significantly higher values for RTP 2 compared to PRE-2 (*p* < 0.001) and PRE-1 compared to RTP 1 (*p* < 0.001).

**Figure 4.** Absolute total distance and absolute distances run in different velocity ranges by the participants in matches before (PRE) and after (POST) a rectus femoris injury. \* indicates a significant difference between instances at *p* < 0.05. \*\* indicates a significant difference between instances at *p* < 0.001.

**Figure 5.** Peak speed, peak acceleration, and explosive distance covered by the participants in matches before (PRE) and after (POST) a rectus femoris injury. \*\* indicates a significant difference between instances at *p* < 0.001.

### *5.3. Adverse Events*

There were no serious adverse events reported, whereas minor adverse events were common. It included pain during PNE (100%) and local soreness after treatment (23.1%) that resolved spontaneously within 24–48 h without any intervention.

### *5.4. Follow-Up*

No player suffered a reinjury 20 weeks following RTP. Also, there were no dropouts during the development of the study.

### **6. Discussions**

This study describes the intervention of percutaneous needle electrolysis treatment along with a rehab and reconditioning program following an injury to the rectus femoris in professional soccer players. Ultrasound images during the reconditioning and performance parameters after return to play were registered to assess the effectiveness of the intervention. A short-term, medium-term, and long-term follow-up was also carried out, and the results showed that the injured muscle recovered completely and were no reinjuries or adverse effects reported in the 20 weeks following the index injury.