sensors
Article
Eﬀects of Novel Inverted Rocker Orthoses for First
Metatarsophalangeal Joint on Gastrocnemius Muscle
Electromyographic Activity during Running:
A Cross-Sectional Pilot Study
Rubén Sánchez-Gómez 1
, Carlos Romero-Morales 2,*
, Álvaro Gómez-Carrión 1,
Blanca De-la-Cruz-Torres 3
, Ignacio Zaragoza-García 1
, Pekka Anttila 4, Matti Kantola 4 and
Ismael Ortuño-Soriano 1
1
Nursing Department, Faculty of Nursing, Physiotherapy and Podiatry, Universidad Complutense de
Madrid, 28040 Madrid, Spain; rusanc02@ucm.es (R.S.-G.); alvaroalcore@hotmail.com (Á.G.-C.);
izaragoz@ucm.es (I.Z.-G.); iortunos@ucm.es (I.O.-S.)
2
Faculty of Sport Sciences, Universidad Europea de Madrid, Villaviciosa de Odón, 28670 Madrid, Spain
3
Department of Physiotherapy, University of Seville, c/Avicena, s/n, 41009 Seville, Spain; bcruz@us.es
4
Applied Science of Metropolia Univesity, Podiatry Department, 01600 Helsinki, Finland;
pekka.anttila@metropolia.ﬁ (P.A.); Matti.Kantola@metropolia.ﬁ (M.K.)
*
Correspondence: carlos.romero@universidadeuropea.es
Received: 15 April 2020; Accepted: 3 June 2020; Published: 5 June 2020


Abstract: Background: The mobility of the ﬁrst metatarsophalangeal joint (I MPTJ) has been related
to the proper windlass mechanism and the triceps surae during the heel-oﬀ phase of running gait;
the orthopedic treatment of the I MPTJ restriction has been made with typical Morton extension
orthoses (TMEO). Nowadays it is unclear what eﬀects TMEO or the novel inverted rocker orthoses
(NIRO) have on the EMG activity of triceps surae during running. Objective: To compare the TMEO
eﬀects versus NIRO on EMG triceps surae on medialis and lateralis gastrocnemius activity during
running. Study design: A cross-sectional pilot study. Methods: 21 healthy, recreational runners were
enrolled in the present research (mean age 31.41 ± 4.33) to run on a treadmill at 9 km/h using aleatory
NIRO of 6 mm, NIRO of 8 mm, TMEO of 6 mm, TMEO of 8 mm, and sports shoes only (SO), while
the muscular EMG of medial and lateral gastrocnemius activity during 30 s was recorded. Statistical
intraclass correlation coeﬃcient (ICC) to test reliability was calculated and the Wilcoxon test of all ﬁve
diﬀerent situations were tested. Results: The reliability of values was almost perfect. Data showed
that the gastrocnemius lateralis increased its EMG activity between SO vs. NIRO-8 mm (22.27 ± 2.51
vs. 25.96 ± 4.68 mV, p < 0.05) and SO vs. TMEO-6mm (22.27 ± 2.51 vs. 24.72 ± 5.08 mV, p < 0.05).
Regarding gastrocnemius medialis, values showed an EMG notable increase in activity between SO
vs. NIRO-6mm (22.93 ± 2.1 vs. 26.44 ± 3.63, p < 0.001), vs. NIRO-8mm (28.89 ± 3.6, p < 0.001), and vs.
TMEO-6mm (25.12 ± 3.51, p < 0.05). Conclusions: Both TMEO and NIRO have shown an increased
EMG of the lateralis and medialis gastrocnemius muscles activity during a full running cycle gait.
Clinicians should take into account the present evidence when they want to treat I MTPJ restriction
with orthoses, and consider the inherent triceps surae muscular cost relative to running economy.
Keywords: triceps surae; ﬁrst metatarsophalangeal joint; surface electromyography
1. Introduction
Coterill [1] was the ﬁrst author who described painful osteoarthritis (OA) of the ﬁrst
metatarsophalangeal joint (IMTPJ), which is known as hallux rigidus (HR). HR is the last stage
Sensors 2020, 20, 3205; doi:10.3390/s20113205
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of the IMTPJ degeneration, with functional hallux limitus [2] (FHL) at the beginning of the pathological
progress [3]. Joint disease is thought to be caused by repetitive impacts on the dorsal aspect of the base
of the proximal phalanx of the hallux by the ﬁrst metatarsal head during the propulsion phase of gait
and running in feet with multifactorial biomechanical and/or structural deﬁcits [4]. The limitation
of IMTPJ has been linked to gait problems [5] and its consequences on ankle, knee, hip, or low back
during running [6].
The treatment of this injury has been addressed in several conservative non-surgical and surgical
ways.
Non-surgical management is valid to treat HR in the earliest stages [7,8] and includes
ultrasound therapy, inﬁltrative drugs, shoe modiﬁcations, hallux bandages, manual mobilization,
ﬂexor strengthening, and orthoses to improve the joint problems. There are a few references on
treatment of OA using plantar insoles in HR and FHL. Traditional Morton’s extensions are orthoses
with a ﬂat light modiﬁcation under the ﬁrst ray that has been used to treat HR [9–11] to avoid the
impact between the proximal phalanx and ﬁrst metatarsal bones. This opens the IMTPJ dorsally but
restricts its dorsiﬂexion movement, while rocker-sole footwear modiﬁcations have shown a reduction
in the peak pressure under the IMTPJ. This decreases the average gait cycle that is spent in the stance
phase [12] and increases muscle activity of the lower limb [13]. However, there is no reference to either
the inverted rocker-sole orthoses eﬀects or the eﬀect of footwear modiﬁcations on muscle activity
during running.
On the other hand, running economy (RE) has been described as the oxygen cost of running at a
given speed in every case [14] and factors such as biomechanics and muscular fatigue can inﬂuence
the RE [15]. Additionally, barefoot running has shown diﬀerences in biomechanical behaviour [16]
and muscular responses [17,18] when it is compared with classical running shoes. Compared to
fatigue, strength training added to a normal training program for distance running can improve RE
between 2% and 8%. An increase in muscle mass training programs around the proximal region
of the lower limb, such as quadriceps or hamstring [19], or around the distal regions, such as the
triceps surae [20] with plantarﬂexion and dorsiﬂexion ankle exercises, has shown some beneﬁts on RE.
Accordingly, triceps surae and its relationship with the windlass mechanism [21] in the propulsion
phase of gait and running has been reported to provide between 8% and 17% of the elastic energy
that is needed for the heel-oﬀ phase [22,23] toward a suitable IMTPJ dorsiﬂexion [24,25]. However,
the electromyography (EMG) eﬀects in the triceps surae with limited dorsiﬂexion of the IMTPJ that
is induced by any orthotic dorsiﬂexion restriction has never been studied. Understanding the EMG
activity of this muscle will allow us to understand if the subjects could be increasing their energy cost
during running, which is very important for an eﬃcient RE [19]. However, no previous research has
studied the eﬀect of a novel inverted rocker orthoses (NIRO) on the EMG activity of the triceps surae
compared to traditional Morton’s extension orthoses (TMEO) during running in the healthy population.
Because of the restricting IMTPJ eﬀect of TMEO and its inﬂuence on the windlass mechanism that is
linked with the triceps surae [24,25], we hypothesized that TMEO (6 mm and 8 mm) may increase
the EMG activity of the gastrocnemius medialis and lateralis muscles compared to the shoe only (SO)
condition during running activity; in addition, regarding previous muscular activity changes that are
reported with classical rocker soles [13], we hypothesized that NIRO (6 mm and 8 mm) may reduce the
EMG of gastrocnemius medialis and lateralis compared to TMEO (6 mm and 8 mm), and this may
increase EMG compared to SO in healthy people during running activity.
2. Materials and Methods
The public institutional review board at Virgen Macarena-Virgen del Rocío hospitals, reviewed
and approved the present study (certiﬁcate number f7f4a6567676d7ba7163bce0d15e7f98c9f33354).
Ethical and human criteria were followed according to the Declaration of Helsinki, and signed informed
consent was obtained from all subjects.
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2.1. Design and Sample Size
The statistics unit at the Spanish public university used software to assess the suitable sample
size to perform this cross-sectional observational study and to study the diﬀerence in the EMG
changes in the gastrocnemius medialis and lateralis muscles between SO, NIRO 6 mm, NIRO
8 mm, TMEO 6 mm, and TMEO 8 mm groups during running. Previous data on the triceps surae
showed 7.0 ± 0.6 millivolts (mV) wearing 9-mm heel lifts compared to 4.9 ± 0.6 mV wearing typical
shoes [26]. Taking into account a statistical power of 80%, β = 20%, a 95% conﬁdence interval (CI), and
α = 0.05, 30 subjects were needed to complete the study. Considering the typical loss of 20% subjects,
24 participants were recruited. However, three individuals were excluded from the study because
they felt pain and discomfort during the EMG assessment. Reporting of Observational Studies in
Epidemiology (STROBE) [27] criteria and a randomly consecutive sampling technique were followed
to develop the present research.
2.2. Subjects
The following inclusion criteria were used to select the participants: (1) healthy participants,
between 18 and 30 years old; (2) recreational runners with 3–4 h of training per week with more than
1 year of experience; (3) neutral foot posture index (FPI) with values between 0 and +5 points according
to a validity tool [28]; and (4) no injuries or pain at the time of the test. The exclusion criteria were as
follows: (1) any lower limb injury during the last 6 months; (2) less movement in either foot joint than
what is required to perform the optimal biomechanics according to normal values [29,30]; or (3) under
the inﬂuence of any drugs eﬀects at the time of the measurements. Body mass index (BMI) was taken
into account to select a homogeneous sample, using Quetelet’s equation as follows: BMI = weight
(kg)/height (m2) [31].
2.3. Instrumentation and Assessments
Neurotrac® Simplex Plus (Verity Medical Ltd., Braishﬁeld, UK) EMG electronic device with a
USB-Bluetooth [32] was used to study the triceps surae activity during the running test. The recording
range on the device was 0.2 mV to 2000 mV, with a sensitivity of 0.1 mV RMS, 10 m of free wireless
(Bluetooth) connection range and an accuracy of 4% of the reading from mV +/− 0.3 mV to 200 Hz,
with a bandpass ﬁlter of 18 Hz +/− 4 Hz to 370 Hz +/− 10% for readings below 235 mV. The signal
was assessed using self-adhesive circular surface electrodes that were 30 mm in diameter and made of
high-quality hydrogel and conductive carbon ﬁlm to detect the electrical action of the muscle ﬁbers.
The signal from each electrode was captured by the receiver module and ﬁltered automatically by the
Neurotrac® software (Verity Medical Ltd., Braishﬁeld, UK). It was sent by a unidirectional radioelectric
secure connection to the computer and it was digitally transformed by the software to generate activity
patterns data for each electrode.
2.4. Materials
NIRO was made using a ﬂat sheet of ethylene-vinyl acetate (EVA) with a semi-rigid density
that was 3 mm thick, without any orthotic element that could interface with normal biomechanical
behaviour of the foot. NIRO had an inverted rocker composed of EVA medium that was 5 cm long,
2 cm wide, and 6 mm thick. Its proximal and distal edges were smoothly polished, and it was placed
on the IMTPJ. The whole orthotic was covered with an EVA soft layer that was 1 mm thick (Figure 1).
The TMEO was made with the same ﬂat sheet of semi-rigid EVA that was 3 mm thick without any
orthotic element and with a rectangular ﬂat piece of EVA medium (6 mm thick) that was placed under
the IMTPJ area and it was covered with an EVA soft layer that was 1 mm thick (Figure 2). The neutral
SOs were “New Feel PW 100M medium grey” (ref. number: 2018022). NIRO and TMEO were made in
an external orthopedic laboratory that was blinded to the study protocol.
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Figure 1. Novel inverted rocker orthotic (NIRO). 
A flat sheet of ethylene-vinyl acetate (EVA) with an inverted rocker piece of EVA medium 6 
mm thick under IMTPJ (bulked raised shape) covered with a yellow EVA soft layer that was 1 mm 
thick. 
 
Figure 2. Typical Morton’s extension orthotic (TMEO). 
A flat sheet of ethylene-vinyl acetate (EVA) with a rectangular flat piece of EVA medium 6 mm 
thick under IMTPJ covered with a black EVA soft layer that was 1 mm thick. 
2.5. Procedure 
The podiatric clinician researcher (RSG) performed a physical assessment of the subjects and 
applied the eligibility criteria. To visualize the muscle belly, each subject was asked to perform 
plantarflexion of the ankle joint for a few seconds. The surface electrodes were then placed 
longitudinally onto the most prominent bulge of the gastrocnemius medialis and lateralis, based on 
the “European recommendations for surface EMG” [33]. The subjects were then asked to stand on 
one leg in the tip-toe position using their dominant foot for 5 s to set the maximal voluntary 
contractions that were needed in the strongest limb to calibrate the software and to normalize EMG 
data amplitudes for each test [34]. This was followed by acclimatization of subjects to a motorized 
treadmill at 5.17 km/h for 3 min [17]. The participants were divided randomly in gastrocnemius 
lateralis or medialis group by choosing a sealed envelope that assigned them to one group or 
another to begin the test; after that, they selected one of the five sealed envelopes with each of the 
five different conditions of the study (SO, NIRO 6 mm, NIRO 8 mm, TMEO 6 mm, TMEO 8 mm) to 
set randomly the order of the test. The 11 subjects who began with medialis gastrocnemius 
assessments, did the lateralis test following the same randomized protocol for each of the five 
different conditions and vice versa for the 12 participants who began with the lateralis test (Figure 
3). Three running trials at 9 km/h [35] under five different conditions (SO, NIRO 6 mm, NIRO 8 
mm, TMEO 6 mm, and TMEO 8 mm) on the same day were randomly performed. The duration of 
each trial was 1 min. For each subject, the mean EMG muscle activity pattern [36] of the 
gastrocnemius medialis of the dominant leg was recorded during the last 30 s of each 1-min trial, 
which was performed three times, leaving 5 min of rest between each test [37]. To avoid a potential 
imbalance, the same condition was added to contralateral foot. The same protocol was performed to 
Figure 1. Novel inverted rocker orthotic (NIRO).
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Figure 1. Novel inverted rocker orthotic (NIRO). 
A flat sheet of ethylene-vinyl acetate (EVA) with an inverted rocker piece of EVA medium 6 
mm thick under IMTPJ (bulked raised shape) covered with a yellow EVA soft layer that was 1 mm 
thick. 
 
Figure 2. Typical Morton’s extension orthotic (TMEO). 
A flat sheet of ethylene-vinyl acetate (EVA) with a rectangular flat piece of EVA medium 6 mm 
thick under IMTPJ covered with a black EVA soft layer that was 1 mm thick. 
2.5. Procedure 
The podiatric clinician researcher (RSG) performed a physical assessment of the subjects and 
applied the eligibility criteria. To visualize the muscle belly, each subject was asked to perform 
plantarflexion of the ankle joint for a few seconds. The surface electrodes were then placed 
longitudinally onto the most prominent bulge of the gastrocnemius medialis and lateralis, based on 
the “European recommendations for surface EMG” [33]. The subjects were then asked to stand on 
one leg in the tip-toe position using their dominant foot for 5 s to set the maximal voluntary 
contractions that were needed in the strongest limb to calibrate the software and to normalize EMG 
data amplitudes for each test [34]. This was followed by acclimatization of subjects to a motorized 
treadmill at 5.17 km/h for 3 min [17]. The participants were divided randomly in gastrocnemius 
lateralis or medialis group by choosing a sealed envelope that assigned them to one group or 
another to begin the test; after that, they selected one of the five sealed envelopes with each of the 
five different conditions of the study (SO, NIRO 6 mm, NIRO 8 mm, TMEO 6 mm, TMEO 8 mm) to 
set randomly the order of the test. The 11 subjects who began with medialis gastrocnemius 
assessments, did the lateralis test following the same randomized protocol for each of the five 
different conditions and vice versa for the 12 participants who began with the lateralis test (Figure 
3). Three running trials at 9 km/h [35] under five different conditions (SO, NIRO 6 mm, NIRO 8 
mm, TMEO 6 mm, and TMEO 8 mm) on the same day were randomly performed. The duration of 
each trial was 1 min. For each subject, the mean EMG muscle activity pattern [36] of the 
gastrocnemius medialis of the dominant leg was recorded during the last 30 s of each 1-min trial, 
which was performed three times, leaving 5 min of rest between each test [37]. To avoid a potential 
imbalance, the same condition was added to contralateral foot. The same protocol was performed to 
Figure 2. Typical Morton’s extension orthotic (TMEO).
A ﬂat sheet of ethylene-vinyl acetate (EVA) with an inverted rocker piece of EVA medium 6 mm
thick under IMTPJ (bulked raised shape) covered with a yellow EVA soft layer that was 1 mm thick.
A ﬂat sheet of ethylene-vinyl acetate (EVA) with a rectangular ﬂat piece of EVA medium 6 mm
thick under IMTPJ covered with a black EVA soft layer that was 1 mm thick.
2.5. Procedure
The podiatric clinician researcher (RSG) performed a physical assessment of the subjects and
applied the eligibility criteria. To visualize the muscle belly, each subject was asked to perform
plantarﬂexion of the ankle joint for a few seconds. The surface electrodes were then placed longitudinally
onto the most prominent bulge of the gastrocnemius medialis and lateralis, based on the “European
recommendations for surface EMG” [33]. The subjects were then asked to stand on one leg in the tip-toe
position using their dominant foot for 5 s to set the maximal voluntary contractions that were needed
in the strongest limb to calibrate the software and to normalize EMG data amplitudes for each test [34].
This was followed by acclimatization of subjects to a motorized treadmill at 5.17 km/h for 3 min [17].
The participants were divided randomly in gastrocnemius lateralis or medialis group by choosing a
sealed envelope that assigned them to one group or another to begin the test; after that, they selected
one of the ﬁve sealed envelopes with each of the ﬁve diﬀerent conditions of the study (SO, NIRO 6 mm,
NIRO 8 mm, TMEO 6 mm, TMEO 8 mm) to set randomly the order of the test. The 11 subjects who
began with medialis gastrocnemius assessments, did the lateralis test following the same randomized
protocol for each of the ﬁve diﬀerent conditions and vice versa for the 12 participants who began with
the lateralis test (Figure 3). Three running trials at 9 km/h [35] under ﬁve diﬀerent conditions (SO,
NIRO 6 mm, NIRO 8 mm, TMEO 6 mm, and TMEO 8 mm) on the same day were randomly performed.
The duration of each trial was 1 min. For each subject, the mean EMG muscle activity pattern [36] of
the gastrocnemius medialis of the dominant leg was recorded during the last 30 s of each 1-min trial,
which was performed three times, leaving 5 min of rest between each test [37]. To avoid a potential
imbalance, the same condition was added to contralateral foot. The same protocol was performed to
assess another gastrocnemius EMG activity pattern. Subjects were blinded to which of the ﬁve random
conditions that they were wearing, and the results were used to test the hypothesis.
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assess another gastrocnemius EMG activity pattern. Subjects were blinded to which of the five 
random conditions that they were wearing, and the results were used to test the hypothesis. 
 
Figure 3. Randomized flow chart. Abbreviations: SO = shoe only; NIRO = novel inverted rocker 
orthoses; and TMEO = traditional Morton extension´s orthoses. 
2.6. Statistical Analysis 
To test for reliability in the present research, within-day trial-to-trial intraclass correlation 
coefficient (ICC) and the standard error of measurement (SEM) were calculated for the subjects 
under the five conditions for each muscle during the running test [14]. According to Landis and 
Koch [38], coefficients of ICC that were lower than 0.20 indicated a slight agreement, 0.20–0.40 
indicated fair reliability, 0.41–0.60 indicated moderate reliability, 0.61–0.80 indicated substantial 
reliability, and 0.81–1.00 indicated almost perfect reliability. The authors considered coefficients of 
≥0.81 to be appropriate to consider the results of the study as valid. SEM assessed the minimal 
detectable change (MDC) for all measurements. This is known as reliable change index (RCI), and it 
was used to determine the clinical significance of the data [39]. The Shapiro–Wilks test was used to 
assess the normality of the sample, and normal a distribution was present if p >0.05. Demographic 
values were presented as the mean and standard deviation (±SD). The p-values for multiple 
comparisons were corrected with a non-parametric paired Friedman test to prove that all SOs, 
NIROs, and TMEOs conditions were different between them. The Wilcoxon test with Bonferroni’s 
correction was performed to analyze differences between the five different conditions, indicating 
statistically significant differences when p < 0.05 with a 95% CI. All the values that were generated 
using NeuroTrac® software were loaded into Excel® template (Windows® 97–2003), and they were 
analyzed using SPSS version 19.0 (SPSS Science, Chicago, IL, USA). 
3. Results 
The Shapiro–Wilks test showed a non-normal distribution of the sample (p < 0.05), while the 
Friedman test showed that values were different between the five conditions (p < 0.05). All subjects 
were recruited from a biomechanical clinic in Madrid (Spain) over a two-month period (October to 
Figure 3. Randomized ﬂow chart. Abbreviations: SO = shoe only; NIRO = novel inverted rocker
orthoses; and TMEO = traditional Morton extension’s orthoses.
2.6. Statistical Analysis
To test for reliability in the present research, within-day trial-to-trial intraclass correlation coeﬃcient
(ICC) and the standard error of measurement (SEM) were calculated for the subjects under the ﬁve
conditions for each muscle during the running test [14]. According to Landis and Koch [38], coeﬃcients
of ICC that were lower than 0.20 indicated a slight agreement, 0.20–0.40 indicated fair reliability,
0.41–0.60 indicated moderate reliability, 0.61–0.80 indicated substantial reliability, and 0.81–1.00
indicated almost perfect reliability. The authors considered coeﬃcients of ≥0.81 to be appropriate to
consider the results of the study as valid. SEM assessed the minimal detectable change (MDC) for
all measurements. This is known as reliable change index (RCI), and it was used to determine the
clinical signiﬁcance of the data [39]. The Shapiro–Wilks test was used to assess the normality of the
sample, and normal a distribution was present if p >0.05. Demographic values were presented as
the mean and standard deviation (±SD). The p-values for multiple comparisons were corrected with
a non-parametric paired Friedman test to prove that all SOs, NIROs, and TMEOs conditions were
diﬀerent between them. The Wilcoxon test with Bonferroni’s correction was performed to analyze
diﬀerences between the ﬁve diﬀerent conditions, indicating statistically signiﬁcant diﬀerences when
p < 0.05 with a 95% CI. All the values that were generated using NeuroTrac® software were loaded
into Excel® template (Windows® 97–2003), and they were analyzed using SPSS version 19.0 (SPSS
Science, Chicago, IL, USA).
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3. Results
The Shapiro–Wilks test showed a non-normal distribution of the sample (p < 0.05), while the
Friedman test showed that values were diﬀerent between the ﬁve conditions (p < 0.05). All subjects
were recruited from a biomechanical clinic in Madrid (Spain) over a two-month period (October to
November 2019). Forty-ﬁve subjects were asked to participate in the experiment and assessed for
eligibility; 24 did not meet the study entry requirements and three withdrew from the study because of
pain and discomfort. Ultimately, 21 participants (10 males and 11 females) were enrolled into the study.
The participants’ ﬂow chart following the STROBE guidelines, is shown in Figure 4. Sociodemographic
data are shown in Table 1.
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November 2019). Forty-five subjects were asked to participate in the experiment and assessed for 
eligibility; 24 did not meet the study entry requirements and three withdrew from the study 
because of pain and discomfort. Ultimately, 21 participants (10 males and 11 females) were enrolled 
into the study. The participants’ flow chart following the STROBE guidelines, is shown in Figure 4. 
Sociodemographic data are shown in Table 1. 
 
Figure 4. Participant flow chart. 
Table 1. Participant demographics. 
Variable 
n = 21 
Mean ± SD (95% CI) 
Age 
31.41 ± 4.33 
(32.26–35.09) 
FPI (scores) 
3.12 ± 0.17 
(2.07–3.41) 
Weight (kg) 
67.50 ± 8.06 
(62.36–70.06) 
Height (cm) 
170.08 ± 6.91 
(166.9–172.43) 
BMI (kg/m2) 
23.15 ± 3.05 
(21.7–24.7) 
Abbreviations: SD = standard deviation; CI = confidence interval; FPI = foot posture index; and BMI 
= body mass index. 
The reliability of the data obtained from the EMG activity of muscles during running under 
five different conditions is presented as the ICC and SEM, which are shown in Table 2. Most of the 
values reached cut-off values over of 0.81 in the ICC data, which suggests “almost perfect 
reliability” [38], with 0.971 for NIRO-8 mm as the highest value and 0.458 for TMEO-8 mm as the 
lowest for the gastrocnemius lateralis, and 0.894 for TMEO-8 mm as the highest and 0.767 for 
NIRO-8 mm as the lowest for the gastrocnemius medialis. Considering the reference that was 
chosen by the authors, we dismissed TMEO-8 mm values for gastrocnemius lateralis. For SEM, 
0.817 mV was the lowest value set for NIRO-8 mm, and 3.766 mV was the lowest value for TMEO-6 
mm for the gastrocnemius lateralis, and 2.083 mV was the highest value for NIRO-8 mm and 0.326 
Figure 4. Participant ﬂow chart.
Table 1. Participant demographics.
Variable
n = 21
Mean ± SD (95% CI)
Age
31.41 ± 4.33
(32.26–35.09)
FPI (scores)
3.12 ± 0.17
(2.07–3.41)
Weight (kg)
67.50 ± 8.06
(62.36–70.06)
Height (cm)
170.08 ± 6.91
(166.9–172.43)
BMI (kg/m2)
23.15 ± 3.05
(21.7–24.7)
Abbreviations: SD = standard deviation; CI = conﬁdence interval; FPI = foot posture index; and BMI = body
mass index.
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The reliability of the data obtained from the EMG activity of muscles during running under ﬁve
diﬀerent conditions is presented as the ICC and SEM, which are shown in Table 2. Most of the values
reached cut-oﬀ values over of 0.81 in the ICC data, which suggests “almost perfect reliability” [38],
with 0.971 for NIRO-8 mm as the highest value and 0.458 for TMEO-8 mm as the lowest for the
gastrocnemius lateralis, and 0.894 for TMEO-8 mm as the highest and 0.767 for NIRO-8 mm as the
lowest for the gastrocnemius medialis. Considering the reference that was chosen by the authors,
we dismissed TMEO-8 mm values for gastrocnemius lateralis. For SEM, 0.817 mV was the lowest
value set for NIRO-8 mm, and 3.766 mV was the lowest value for TMEO-6 mm for the gastrocnemius
lateralis, and 2.083 mV was the highest value for NIRO-8 mm and 0.326 mV was the lowest value for
TMEO-8 mm for the gastrocnemius medialis. The highest MDC value for TMEO-8 mm was 5.798 mV
and 2.264 mV were the lowest value for the gastrocnemius lateralis. Additionally, 5.775 mV was the
highest value in the NIRO-8 mm group and 0.904 mV was the lowest value in the TMEO-8 mm group
for gastrocnemius medialis.
EMG mean muscle activity in the gastrocnemius medialis and lateralis in SO compared to
NIRO-6 mm and 8 mm and TMEO-6 mm and 8 mm are shown in Table 3. In the gastrocnemius
lateralis, the EMG activity signiﬁcantly increased between the SO and NIRO-8 mm (22.27 ± 2.51
vs. 25.96 ± 4.68 mV; p < 0.05). There was another statistically signiﬁcant increase between SO and
TMEO-6 mm (22.27 ± 2.51 vs. 24.72 ± 5.08 mV, p < 0.05) and vs. TMEO-8 mm (25.49 ± 1.97, p < 0.001),
but the low ICC of the last value invalidated the reliability of this value. For the gastrocnemius medialis,
a statistically signiﬁcant increase in the EMG activity was noted for SO vs. NIRO-6 mm (22.93 ± 2.1
vs. 26.44 ± 3.63, p < 0.001), vs. NIRO-8 mm (28.89 ± 3.6, p < 0.001), vs. TMEO-6 mm (25.12 ± 3.51,
p < 0.05), and vs. TMEO-8 mm (26.38 ± 3.02, p < 0.05). The latter was not considered because of its
low ICC value. In addition, the relationship between NIROs and TMEOs showed that there was a
statistically signiﬁcant increase in NIRO-6 mm and NIRO-8 mm (26.44 ± 3.63 vs. 28.89 ± 3.6, p < 0.05),
and a statistically signiﬁcant decrease in NIRO-8 mm vs. TMEO-6 mm (28.89 ± 3.6 vs. 25.12 ± 3.51,
p < 0.001) and in NIRO-8 mm vs. TMEO-8 mm (28.89 ± 3.6 vs. 26.38 ± 3.02, p < 0.05), although the
latter could not be considered because of its low ICC values.
Sensors 2020, 20, 3205
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Table 2. Reliability ICC of variables with “shoe only” versus 6- and 8-mm of novel inverted rocker orthoses (NIRO) and traditional Morton extension orthoses (TMEO).
Variables
SO
NIRO-6 mm
NIRO-8 mm
TMEO-6 mm
TMEO-8 mm
ICC
(95% CI)
MDC
ICC
(95% CI)
MDC
ICC
(95% CI)
MDC
ICC
(95% CI)
MDC
ICC
(95% CI)
MDC
SEM
0.950
SEM
0.950
SEM
0.950
SEM
0.950
SEM
0.950
Gastrocnemius
lateralis (mV)
0.839
0.932
0.971
0.937
0.458
(0.651–0.935)
1.010
3.560
(0.852–0.973)
1.254
3.477
(0.938–0.988)
0.817
2.264
(0.861–0.975)
1.359
3.766
(0.148–0.777)
2.092
5.798
Gastrocnemius
medialis (mV)
0.848
0.832
0.767
0.872
0.894
(0.649–0.94)
0.913
2.530
(0.637–0.931)
1.707
4.731
(0.501–0.905)
2.083
5.775
(0.723–0.948)
1.408
3.904
(0.77–0.957)
0.326
0.904
Abbreviations: ICC = intraclass correlation coeﬃcient; CI = conﬁdence interval; SEM = standard error of measurement; MDC = minimal detectable change; (mV) = millivolts; SO = shoe
only; and mm = millimeters.
Table 3. Signal amplitudes and comparison values of the mean gastrocnemius lateralis and medialis muscle activities.
SO
NIRO
6 mm
NIRO
8 mm
TMEO
6 mm
TMEO
8 mm
p-Value
SO
p-Value
SO
p-Value
SO
p-Value
SO
p-Value
NIRO
6 mm
p-Value
NIRO
6 mm
p-Value
NIRO
6 mm
p-Value
NIRO 8
mm
p-Value
NIRO
8 mm
p-Value
TMEO
6 mm
Variable
mean (mV)
mean(mV)
mean (mV)
mean (mV)
mean (mV)
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
vs.
gastrocnemius
lateralis
±SD
(95% CI)
±SD
(95% CI)
±SD
(95% CI)
±SD
(95% CI)
±SD
(95% CI)
NIRO 6
mm
NIRO 8
mm
TMEO 6
mm
TMEO 8
mm
NIRO
8 mm
TMEO
6 mm
TMEO
8 mm
TMEO 6
mm
TMEO
8 mm
TMEO
8 mm
22.27 ± 2.51
24.65 ± 4.51
25.96 ± 4.68
24.72 ± 5.08
25.49 ± 1.97
(20.77–23.279) (22.41–26.897) (23.634–28.29) (23.675–27.35) (22.19–27.253)
0.085
<0.05 *
<0.05 *
<0.001 **
0.39
0.88
0.356
0.372
0.67
0.913
22.93 ± 2.1
26.44 ± 3.63
28.89 ± 3.6
25.12 ± 3.51
26.38 ± 3.02
gastrocnemius
medialis
(21.88–23.97)
(24.63–28.24)
(27–30.68)
(23.37–26.87)
(24.88–27.89)
<0.001 **
<0.001 **
<0.05 *
<0.05 *
<0.05 *
0.06
0.67
<0.001 **
<0.05 *
0.22
Abbreviations: mV = millivolts; SO = shoe only; NIRO = novel inverted rocker orthoses; TMEO = traditional Morton extension orthoses; mm = millimeters; ±SD = standard deviation;
p < 0.05 * (95% CI) was considered statistically signiﬁcant; and p < 0.001 ** (95% CI) was considered statistically signiﬁcant.
Sensors 2020, 20, 3205
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4. Discussion
4.1. TMEO and NIRO Eﬀects
This is the ﬁrst study on EMG muscle activity in the gastrocnemius medialis and lateralis under
IMTPJ dorsiﬂexion mobility restrictions by two diﬀerent kinds of orthoses, the TMEO and the NIRO,
in healthy subjects during running. TMEO has been used to treat symptoms of the ﬁrst stages of
OA [9–11] moving away dorsally from the contact between the proximal phalanx of the hallux and ﬁrst
metatarsal head surfaces. However, it is unclear if the eﬀects on the triceps surae activity that were
caused by the windlass mechanism [24] alteration through the IMTPJ caused the restriction. Some
authors have shown the need for proper dorsiﬂexion of the IMTPJ during the push-oﬀ phase to ensure
normal activity of the calcaneus–plantar system [24]. We hypothesized that TMEO would increase the
EMG triceps surae activity that is induced by restriction of IMTPJ dorsiﬂexion. Our results showed
that EMG activity of the gastrocnemius lateralis and medialis increased with TMEO-6 mm and that
there is a further increase with TMEO-8 mm compared to SO (Table 3), although the last one could
not be considered because of the low ICC values. Even knowing that there are no studies related to
EMG activity during running with the orthopedic restriction of IMPTJ dorsiﬂexion, these results are
consistent with other simulated running research [24,25], which showed that engaging the windlass
mechanism by promoting 30◦ of IMTPJ dorsiﬂexion caused the arch to absorb and dissipate more
elastic energy than under normal circumstances, and likely the energy of the triceps surae would be
saved. In the present research, we decreased the windlass capacity through the TMEO, and followed
the lack of storage and release energy in the medial longitudinal arch primary in the heel-oﬀ phase;
this could have been supported by increasing gastrocnemius musculature EMG activity, as shown
by our results, and by sustaining the connection between the IMTPJ and triceps surae through the
windlass mechanism, according with other authors [24,25].
We hypothesized that NIRO would produce less EMG activity on triceps surae than the TMEO
compared to SO. The rationale behind this approach was that its smooth edges and inverted rocker
would produce a slight movement restriction of the IMTPJ; therefore, less eﬀort would be required of
the triceps surae to move the heel up. However, the present research showed surprising results, with a
higher increase in EMG activity in both the gastrocnemius medialis and lateralis muscles (Table 3) with
NIRO compared to TMEO, especially with NIRO-8 mm. This could be partly explained because of
the soft edges of the NIRO, which yielded instability on the IMTPJ and transferred it to triceps surae
in the heel-oﬀ phase. This is consistent with other studies with inverted rocker-sole shoes [40] that
showed increased plantarﬂexion at the ankle joint and an increase in lower limb muscular activity [13].
This conclusion is not consistent with other research that showed increasing toe joint stiﬀness and
increased ankle foot push-oﬀ work by up to 181% [41].
4.2. Osteoarthritis
OA has been deﬁned as one of the most important and incapacitating musculoskeletal disorders in
the world and OA of the IMTPJ, is the most commonly aﬀected region on the foot [42]. This pathology
can involves partial (FHL) or total (HR) rolling fail of the proximal phalanx of the hallux around ﬁrst
metatarsal bone in the last phase of gait [3], and there are a few treatments to relieve them, looking to
avoid contact of the dorsal aspect of theses bones, such as TMEO [9–11] or classical rocker soles [12].
No studies about triceps surae EMG activity and IMTPJ OA using orthoses and/or rocker soles during
running have been reported; nevertheless, our observations with simulated IMTPJ restriction through
TMEO and NIRO, showed an increase of EMG activity pattern of the gastrocnemius medialis and
lateralis, in contrast with a recently study [12] with IMTPJ OA and traditional rocker bottom soles,
which argued that the reduction of the concentric activity of the triceps surae inferred from the forward
displacements of the body center of mass was probably due to passively roll-over of the whole base
of support.
Sensors 2020, 20, 3205
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4.3. Running Economy
Elastic energy is stored and returned by the plantar muscles, plantar aponeurosis, and triceps
surae with the Achilles tendon during the mid-stance and heel-oﬀ phases of running because of its
isometric, concentric, and eccentric stretching–shortening pattern [43,44], which shows that the foot
has an important role in RE. RE is related to diﬀerent biomechanical parameters such as shorter ground
contact times, higher stride frequency, joint stiﬀness, and neuromuscular response [20], speciﬁcally
the pre-activation of gastrocnemius muscular group [14,17,20]. TMEO and NIRO somehow produced
decreased stiﬀness in the IMTPJ by dorsal migration of the I metatarsal bone, and this was shown by the
compensatory increase eﬀect on the gastrocnemius musculature activity that attempts to stabilize IMTPJ
instability when joined with the windlass mechanism. This would cause worse RE [20]. Our obtained
values conﬁrm the results of some studies [45,46], which showed the importance of neuromuscular
pre-activation of the gastrocnemius to increase the leg stiﬀness, anticipating the loading forces and
attenuating the eﬀort of the foot to stabilize the joint as required, improving the energy cost and,
therefore, the RE.
5. Limitations
The sample size that was calculated in a previous study could not be attained because three
individuals were excluded. This must be taken into account when interpreting the results. In addition,
we were not able to assess the “order eﬀect” on our sample because didn’t write the diﬀerent orders
of each participant’s choice, despite the fact that both groups had a similar participant number,
the hypothetical order eﬀect can take over, and we recommended future study designed to improve
this aspect of the assessments.
Because of the short running test duration when NIRO and TMEO were worn, the hypothetical
muscular adaptations of the triceps surae could not be assessed. Longer studies in the future are
needed to determine how the exertion levels can inﬂuence these muscular adaptations during running.
Considering that most ±SD values obtained in the present research are higher than SEM, authors
recommended to have caution in interpreting the results.
6. Conclusions
NIRO and TMEO have shown a high interaction with triceps surae, increasing the gastrocnemius
medialis and lateralis EMG activity during running.
This may be additional evidence of the
biomechanics relationship between IMTPJ and the windlass mechanism connection. Higher values
of the triceps surae EMG activity wearing NIRO and TMEO during running could have a negative
impact on RE; therefore, clinicians should be prescribing them with caution when they want to treat
IMTPJ OA in runners.
Author Contributions: Conceptualization, R.S.-G.; methodology, C.R.-M., M.K. and I.O.-S.; software, I.Z.-G. and
I.O.-S.; validation, Á.G.-C. and P.A.; formal analysis, C.R.-M., B.D.-l.-C.-T. and I.Z.-G.; investigation, R.S.-G. and
P.A.; resources, C.R.-M. and B.D.-l.-C.-T.; data curation, B.D.-l.-C.-T.; writing—original draft preparation; R.S.-G.,
C.R.-M., B.D.-l.-C.-T., I.O.-S., I.Z.-G., P.A. and M.K.; visualization, P.A.; supervision, Á.G.-C. and M.K.; project
administration, B.D.-l.-C.-T. All authors have read and agreed to the published version of the manuscript
Funding: This research received no external fundings.
Conﬂicts of Interest: The authors declare no conﬂict of interest.
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Effects of Novel Inverted Rocker Orthoses for First Metatarsophalangeal Joint on Gastrocnemius Muscle Electromyographic Activity during Running: A Cross-Sectional Pilot Study.
Sánchez-Gómez, Rubén,Romero-Morales, Carlos,Gómez-Carrión, Álvaro,De-la-Cruz-Torres, Blanca,Zaragoza-García, Ignacio,Anttila, Pekka,Kantola, Matti,Ortuño-Soriano, Ismael