Effect of plyometric training and
neuromuscular electrical stimulation
assisted strength training on muscular,
sprint, and functional performances in
collegiate male football players
Shahnaz Hasan1, Gokulakannan Kandasamy2, Danah Alyahya1,
Asma Alonazi1, Azfar Jamal3,4, Amir Iqbal5,
Radhakrishnan Unnikrishnan1 and Hariraja Muthusamy1
1 Physical Therapy Department, College of Applied Medical Sciences, Majmaah University,
Al Majmaah, Saudi Arabia
2 School of Health and Life Sciences, Teesside University, Middlesbrough, United Kingdom
3 Department of Biology, College of Science, Al-Zulﬁ-, Majmaah University, Al Majmaah, Riyadh
Region, Saudi Arabia
4 Health and Basic Science Research Centre, Majmaah University, Al Majmaah, Saudi Arabia
5 Rehabilitation Research Chair, College of Applied Medical Sciences, King Saud University,
Riyadh, Saudi Arabia
ABSTRACT
Background: The study’s objective was to analyze the inﬂuence of an 8-week
neuromuscular electrical stimulation (NMES) with a plyometric (PT) and strength
training (ST) program on muscular, sprint, and functional performances in collegiate
male football players.
Methods: Sixty collegiate male football players participated in this randomized
controlled trial single-blind study. All the participants were randomly divided into
two groups: (1) NMES group (Experimental, n = 30) who received NMES assisted ST
and (2) sham NMES group (Control, n = 30) who received sham NMES assisted ST.
In addition, participants from both groups received a PT program; both groups
received intervention on three days a week for 8-weeks. The study’s outcomes, such
as muscular, sprint, and functional performances, were assessed using a strength test
(STN) for quadriceps muscle, sprint test (ST), and single-leg triple hop test (SLTHT),
respectively, at baseline pre-intervention and 8-week post-intervention. The interaction
between group and time was identiﬁed using a mixed design (2 × 2) ANOVA.
Results: Signiﬁcant difference found across the two time points for the scores of
STN: F (1.58) = 5,479.70, p < 0.05; SLTHT: F (1.58) = 118.17, p < 0.05; and ST:
F (1.58) = 201.63, p < 0.05. Similarly, the signiﬁcant differences were found between
groups averaged across time for the scores of STN: F (1.58) = 759.62, p < 0.05 and
ST: F (1.58) = 10.08, p < 0.05. In addition, after 8-week of training, Cohen’s d
observed between two groups a large to medium treatment’s effect size for the
outcome STN (d = 10.84) and ST (d = 1.31). However, a small effect size was
observed only for the SLTHT (d = 0.613).
Conclusions: Findings suggest that the effect of PT and ST with either NMES or
sham NMES are equally capable of enhancing muscular, sprint, and functional
performances in collegiate male football players. However, PT and ST with NMES
How to cite this article Hasan S, Kandasamy G, Alyahya D, Alonazi A, Jamal A, Iqbal A, Unnikrishnan R, Muthusamy H. 2022. Effect of
plyometric training and neuromuscular electrical stimulation assisted strength training on muscular, sprint, and functional performances in
collegiate male football players. PeerJ 10:e13588 DOI 10.7717/peerj.13588
Submitted 10 January 2022
Accepted 24 May 2022
Published 16 June 2022
Corresponding author
Shahnaz Hasan,
sh.ahmad@mu.edu.sa
Academic editor
Tiago Barbosa
Additional Information and
Declarations can be found on
page 14
DOI 10.7717/peerj.13588
Copyright
2022 Hasan et al.
Distributed under
Creative Commons CC-BY 4.0
have shown an advantage over PT and ST with sham NMES in improving muscular
performance and sprint performance among the same participants.
Subjects Anatomy and Physiology, Rehabilitation, Sports Medicine
Keywords Strength, Functional performance, Sprint, Collegiate male football players, Plyometric
training, NMES
INTRODUCTION
Strength and conditioning play an essential role in injury prevention and improving
muscle performance (Wilson et al., 2013). In most sports teams or individual sports such as
netball, football, and volleyball, muscle strength of the quadriceps is crucial for athletes
sporting abilities such as running, sprinting, and jumping (Magalhães et al., 2004).
Movements included in this type of training are powerful and fast concentric contractions
followed by high-intensity eccentric contractions throughout a high-impact reaction force
is proven to enhance performance (da Cunha et al., 2020). Although an athlete’s
performance can be inﬂuenced by multiple factors (i.e., technical, tactical, and physical)
(Aán et al., 2021), the main focus of this study was aimed at muscular, physical, and
functional performance.
There is a vast literature on different strengthening exercises methods to improve
performance and prevent injury in sports (Myer et al., 2011; LaStayo et al., 2003).
Neuromuscular electrical stimulation (NMES) is one method that involves the utilization
of electrical stimuli to trigger contractions of the muscles. This technique is widely used for
strengthening interventions and restoring or preserving the functioning and mass of
muscles in sports (Gobbo et al., 2014). Research shows that NMES is more appropriate in
improving muscle strength and performance when combined with other training such as
plyometric or strength training (Bansal, Zutshi & Munjal, 2020). The quadriceps are the
primary muscle group in the lower limb that controls knee movement and increases
stability during any dynamic or functional movement (Markovic & Mikulic, 2010).
Strength training the quadriceps muscle plays a vital role in improving functional
performance in most sports (Antrich & Brewster, 1986). It is essential to keep the
quadriceps strong to prevent knee injuries by reducing the shear force in the tibiofemoral
joint (Augustsson & Thomee, 2000).
Plyometric training is also known as dynamic or jump training, involves exercises where
muscles are required to exert maximum force throughout short intervals with an overall
goal of increasing power (Markovic & Mikulic, 2010). Plyometric is seen as a popular
training modality, either alone or when combined with other types of training (Markovic &
Mikulic, 2010; Antrich & Brewster, 1986; Augustsson & Thomee, 2000; Hasan et al., 2021).
The available evidence shows that this type of training has several positive changes,
whether this is in athletic performance and muscle functioning abilities (Markovic &
Mikulic, 2010). Strength training, also known as resistance training, is distinguished by the
deliberate action of muscular contractions against extraneous loads. It is acknowledged as
the most convenient approach for strengthening muscles (Wang & Zhang, 2016).
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However, there is previous literature on the effects of NMES and strength training on
improving muscle strength and physical and functional performance (Gomes da Silva
et al., 2018; Paillard, 2018; Basas et al., 2018). Based on our knowledge, there are limited
studies and data looking at NMES and strength training effects with more extended
intervention (exceeding 4 weeks). In addition, there is a scarcity of data on the effects of
sport speciﬁc intervention and gender speciﬁc sample size. Previous research was more
focused on the effect of NMES in post-operative rehabilitation and not on injury
prevention or strength training in football players (Gatewood, Tran & Dragoo, 2017).
Therefore, the study’s objective was to analyze the inﬂuence of an 8-week neuromuscular
electrical stimulation (NMES) with a plyometric and strength training program on
muscular strength, sprint performance, and functional performance in collegiate male
football players. The study hypothesized that the effect of PT and ST with NMES would be
more beneﬁcial than PT and ST with sham NMES on muscular, sprint, and functional
performances in male collegiate football players.
MATERIALS AND METHODS
Study design
A single-blind two-arm parallel group randomized controlled trial study design was used
to determine the beneﬁcial effects of 8-week NMES training program in collegiate male
football players.
Ethical considerations
The study was conducted according to the guidelines of the Declaration of Helsinki and
approved by the Chair of Majmaah University for Research Ethics Committee, Saudi
Arabia (Ethics number: MUREC-Dec. 15/COM-2020/13-2 dated 15 December 2020).
Study population
Two hundred colligate male football players were assessed via a telephone interview to
participate. The team physiotherapist examined one hundred 10 participants who met the
inclusion criteria. Young male participants aged 18 and 25 years who participated in
football training were included in the study. The participants were excluded with a history
of any lower limb surgery, a current injury that affected the lower limb function,
cardio-respiratory disease.
A total of sixty participants were divided randomly between the NMES group
(experimental group): NMES aided strength training and the sham NMES group (control
group): sham NMES aided strength training.
Procedures
The study was performed between 30 December 2020 and 28 July 2021 at the
Rehabilitation center, Majmaah University, Saudi Arabia. Collegiate male football players
were recruited from Majmaah and Riyadh sporting clubs and universities. The NMES
experimental procedures and potential risks were explained to the participant before
signing their informed consent under the Declaration of Helsinki. The College of Applied
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Medical Science, Ethical Sub-Committee of the Majmaah, Saudi Arabia (Ethics Number:
MUREC-Dec. 15/COM-2020/13-2) approved all procedures of this study.
Conclusively, 60 participants included in this study were randomly divided into two
groups: NMES Group (experimental group, n = 30) and sham NMES Group (control
group, n = 30). In addition to plyometric training, both the NMES and sham NMES groups
undertook NMES and sham NMES guided strength training, respectively, three sessions a
week for 8-weeks. Pre- and post-test readings were taken at baseline and post 8-week of
intervention, respectively. The outcome measures for this study were muscular
performance i.e., maximal voluntary isometric contraction (MVIC) of quadriceps muscle
strength measured by ISOMOVE dynamometer (ISO-MANSW-IT Tecno body, Italy;
https://www.tecnobody.com/en/products/detail/isomove), the sprint performance test,
and the single-leg triple hope test. All the participants who completed the study trial were
included for the statistical analysis. A CONSORT ﬂow diagram of the participants is
illustrated in Fig. 1 (Schulz, Altman & Moher, 2010).
Interventions
After familiarizing two training and testing sessions, the NMES and sham NMES
groups underwent an 8-week training program with three sessions per week. Both groups
received the same plyometric training (Tomlinson et al., 2020). In addition, NMES group
received the NMES guided strength training while the sham NMES group received the
sham NMES guided strength training. Strength training includes terminal knee extension
exercises. Plyometric training, including bounding, hurdling, and drop jumping. Before
starting any strengthening or plyometric pieces of training, each participant underwent
a standardized warm-up session for 10–15 min, which included 7–8 min of jogging and
running and stretching exercises for 5–6 min (Silvers-Granelli et al., 2015). Furthermore,
Fig. 2 is depicting the details of groups, interventions including types of exercises, and
outcomes measures.
Neuromuscular Electrical Nerve Stimulation (Martimbianco et al., 2017; Snyder-
Mackler et al., 1995). A NMES guided strength training program was carried out using an
electrotherapeutic device (Endomed 982, Enraf Nonius, Rotterdam, The Netherlands), a
two-channel medium frequency NME stimulator. In this study, it was applied to stimulate
the targeted nerve and muscles, such as the femoral nerve and quadriceps femoris muscles
of both limbs. Participants from both groups were instructed to shave the part and wash
thoroughly with ethanol to clean the area and reduce skin resistance before applying
electrodes over the skin. For the NMES group, a standard carbon rubber electrode in
moistened sponge pads was placed over the femoral triangle and transversely over the
quadriceps muscle motor points of vastus medialis and vastus lateralis muscles (Fig. 3).
Motor points were pointed out as the area that produced the most signiﬁcant visible
muscle contraction when applied electrical stimulation. The electrodes were securely
fastened using Velcro straps. The participants were seated on an Isomove device during the
stimulation, used for quadriceps strength training with knee ﬁxed at 60 -0-degree angles
(0 correspondence to the full extension of the leg) stimulator with a frequency of 2,500 Hz
@ of 75 burst per second delivered with AMF 50 HZ with 5 s, time interval and holding
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time 8 s, ramp up and down 2 s. The intensity was set maximally according to the
participant’s tolerance. It was given for 25 min (Snyder-Mackler et al., 1995; Taradaj et al.,
2013).
For the sham NMES group, the participants followed the NMES parameters and
quadriceps strength training (with knee ﬁxed at 60-0-degree angles) similar to the NMES
group. However, in contrast, the placement of electrodes was positioned away from the
course of the femoral nerve (e.g., VMO and RF), and the intensity was set to very mild just
for the participants’ feelings. Each training session lasted for 25 min (Snyder-Mackler et al.,
1995; Taradaj et al., 2013).
Terminal knee extension exercises: participants sitting with the knee ﬂexed from 60 to
0 angles on the Isomove device and instructed for maximum voluntary isometric
contraction of their quadriceps muscle for three sets of 10 repetitions, three times a week
for 8-weeks.
Bounding: This is plyometric training where enormous strides are used in the running
action and extra time is spent in the air. The participant performed bounding for 30 m, two
Figure 1 Consolidated Standards of Reporting Trials (CONSORT) diagram showing ﬂow of
participants through each stage of a randomized trial.
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sets for the initial 3 weeks, and after three sets of 30 m bounding with a rest period of
3–4 min.
Hurdling: The participant was instructed to jump with both legs over the eight
consecutive cones height of 40 cm, kept in a straight line, 1 m apart for plyometric training
as hurdling. The participant performed two sets of hurdling over eight cones for the initial
2 weeks. Three sets of hurdling were completed over eight cones for the next 6 weeks.
The rest period was 3–4 min between each set of hurdling.
Figure 2 Depicting the details of groups, warm-up activities, interventions including types of
exercises, and outcomes measures.
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Drop jumping: The participant drops to the ground from a stepper (height 40 cm) and
immediately jumps maximally forward. The participant performed two sets of eight
repetitions of drop jumping for 2 weeks, and for the next 6 weeks, three sets of eight
repetitions of drop jumping with a rest time of 2–3 min between each set were completed
(Tomlinson et al., 2020).
For both the NMES and sham NMES groups, plyometric training (bounding, hurdling,
and drop jumping) was performed three sessions weekly for 8-weeks (Tomlinson et al.,
2020).
Outcome measures
Maximal Voluntary Isometric Contraction Strength (STN) Test: We used an ISOMOVE
dynamometer, software version 0.0.1 (ISO-MANSW-IT; Tecnobody, Dalmine (BG), Italy),
to assess the maximum peak torque of quadriceps muscles strength dominant leg before
strength training and after 8-weeks of training. The reliability of quadriceps strength
measurements of the ISOMOVE dynamometer was previously validated (Hasan et al.,
2021). Participants completed the warm-up session and were familiarized with the
equipment before data collection. Participants were seated with the hip at a 90-degree
angle to minimize hip and thigh motion, and straps were applied across the chest,
Figure 3 Electrode placement during NMES guided strength training.
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midthighs, and pelvis to avoid displacements during contraction. The shin pad was ﬁxed at
5.1 cm (2 inches), superior to the medial malleolus. The knee angles are set at 60 degrees of
ﬂexion, producing the most signiﬁcant torque output (Alonazi et al., 2021). Verbal
instruction was given to keep his/her arm crossed over his/her chest, and verbal
encouragement was given to motivate them to attain maximum effort during the 5 s
contractions. Each test included three MVICs at 60-degree angles with a 3-min rest
between the trials series to eliminate fatigue. The peak torque was directly measured by
ISOMOVE software (Fig. 4).
Sprint Test (ST): The sprint test is a reliable (Interclass coefﬁcient correlation =
0.95–0.98) and valid to measure speed performance (Barr et al., 2014; Zabaloy et al., 2021).
Participants were instructed to stand with their forward legs placed closer to the starting
line, and then on verbal command, they started sprinting with a maximal speed over a
50 m distance. All the performances were recorded by a handheld stopwatch (XINJIE,
SW8-2008) times (in seconds) (Hasan et al., 2021; Alonazi et al., 2021; Zafeiridis et al.,
2005) when the participant’s foot touched the ﬁnishing line. The subsequent two sprint test
trials were performed after 5 min recovery period, and the lowest timing of the two scores
was considered the pre-test (baseline) scores.
Single-Leg Triple Hop Test (SLTHT): The SLTH test scores were measured from the
participant performance, as covered the distance in three hops using a measuring tape.
The participants stood on the dominant limb with the toes just behind the starting line and
then completed the three consecutive hops on the same limb. The single-leg triple hop test
performance measured the distance covered from the starting point to where the back
of the participant’s heel hit the ground (please refer to Fig. 5) (Hasan et al., 2021; Alonazi
et al., 2021; Kale & Gurol, 2019; Hamilton et al., 2008). They performed three trials with
Figure 4 Illustration of maximal voluntary isometric contraction 600 strength (STN) test.
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a 3 min’ recovery period. The best of the three scores (i.e., the maximum distance covered)
was taken as the pre-test (baseline) score.
All the outcomes including MVIC strength (STN), sprint performance (ST), and
functional performance (SLTHT) were assessed by only one assessor who was blind to the
study. The intra-observer reliability was found to be excellent (95% CI [0.91–0.97]).
Statistical analysis
A Statistical Programming for Social Studies SPSS software (IBM SPSS Statistics v.26, IBM
Corp., Armonk, NY, USA) was used to analyze the outcomes measures. A Shapiro-Wilk
test of normality was used for the homogenous distribution of collected data. The main
effect of an intervention on the outcome measures across the baseline and 8-week
post-intervention (2-time points), between-group (NMES vs sham NMES groups), and the
interaction between group and time were identiﬁed using a mixed design (2 × 2) two-way
analysis of variance (ANOVA). Further, the comparison of an intervention effect on the
outcome measures within-group across the time points and between-groups at 8-week
post-intervention using a Bonferroni’s multiple comparison test. Additionally, the size of
an intervention effect on outcomes measures was observed within-group across the time
points and between-groups at 8-week post-intervention using a Cohen’s d test. The
magnitude of effect sizes in strength training research for untrained participants (who
received consistently strength training less than 1-year) is as follows: d value <0.50=trivial
Figure 5 Illustration of single leg triple hop test (SLTHT).
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effect size, 0.50–1.25=small effect size, 1.26–2.00=medium effect size, and >2.00=large
effect size (Rhea, 2004). A relationship among outcomes measures were established using
Pearson’s coefﬁcient test. The magnitude of Pearson’s coefﬁcient test i.e., r value between
0.00–0.10, 0.10–0.39, 0.40–0.69, 0.70–0.89, and 0.90–1.00 corresponds to negligible,
weak, moderate, strong, and very strong correlation between the variables, respectively
(Schober, Boer & Schwarte, 2018). The conﬁdence interval (CI) level was set at 95% for
mean, i.e., signiﬁcant level p < 0.05.
RESULTS
Out of 200 telephonic conversations, 110 participants were ready to be examined; out of
110, 25 participants were excluded due to lower limb injury, 12 did not agree to take
NMES, and 13 did not agree due to their availability for 8-weeks’ protocol.
The mean scores (95% CI) obtained for the age, height, weight, and BMI of all the
participants (n = 60) was 22.13 (95% CI [19–25] years), 1.66 (95% CI [1.62–1.70] m),
64.27 (95% CI [55–70] kg), and 23.43 (95% CI [24.40–25.70] kgm−2), respectively.
A Shapiro-Wilk test reported a homogenous distribution (p > 0.05) of descriptive
characteristics and outcomes measures among both the groups, except for age (NMES
group, p = 0.024; sham NMES group, p = 0.010), BMI (NMES, p = 0.044), and SLTHT
(NMES, p = 0.013). A group-wise (n = 30/group) mean scores for the descriptive
characteristic, including age, height, body mass, and BMI of all the participants and
outcomes measures, are presented in Table 1.
Table 1 Depicting descriptive characteristics of the participants, baseline scores of outcomes measures, and normality test using the
Shapiro-Wilk test (95% CI for mean).
Variables
Groups (n = 30/group)
Baseline scores (Mean ± SD)
Shapiro-Wilk test of normality
Min.
Max.
Statistics
df
p-value
Age (years)
NMES
22.20 ± 1.83
19
25
0.918
30
0.024
Control
22.07 ± 1.80
19
25
0.903
30
0.010
Height (m)
NMES
1.65 ± 0.01
1.63
1.68
0.938
30
0.082
Control
1.66 ± 0.02
1.62
1.70
0.958
30
0.282
Body mass (Kg)
NMES
63.33 ± 2.99
55
69
0.954
30
0.214
Control
65.20 ± 2.30
59
70
0.956
30
0.242
BMI (Kg/m2)
NMES
23.23 ± 1.09
20.4
24.9
0.928
30
0.044
Control
23.63 ± 0.75
21.7
25.7
0.942
30
0.102
STN (Nm-2)
NMES
145.20 ± 3.68
136
151
0.942
30
0.101
Control
144.93 ± 3.98
135
153
0.975
30
0.685
SLTHT
NMES
501.30 ± 54.50
390
575
0.907
30
0.013
Control
499.90 ± 51.14
375
586
0.970
30
0.541
ST
NMES
9.19 ± 0.57
7.78
10.37
0.984
30
0.923
Control
9.23 ± 0.40
8.41
9.96
0.981
30
0.856
Note:
Values are mean scores ± standard deviations (SD); BMI, body mass index; NMES, Neuromuscular electrical stimulation group; Control, representing the sham NMES
group; Statistics, t-value of t-test; df, Degree of freedom; p-value, level of signiﬁcance; p insigniﬁcant at >0.05.
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Table 2 represents the main effect of the interventions on the outcome measures across
the two time points (pre- and post), between the groups, and the interaction between time
and group along with the effect size (η). There was a signiﬁcant difference found across
the two time points for the scores of the outcomes STN: F (1.58) = 5,479.70, p < 0.05;
SLTHT: F (1.58) = 118.17, p < 0.05; and ST: F (1.58) = 201.63, p < 0.05. Similarly, the
signiﬁcant differences were found between groups averaged across time for the scores
of the outcomes STN: F (1.58) = 759.62, p < 0.05 and ST: F (1.58) = 10.08, p < 0.05.
However, a non-signiﬁcant difference was observed between groups for the scores of the
outcomes SLTHT: F (1.58) = 1.53, p > 0.05. There was also a signiﬁcant interaction
was observed between time and group for the scores of the outcomes STN: F
Table 2 The main effect of treatment on the outcomes, within-subject factors across the time (pre
and post), between-subject factors between the groups (NMES vs Control), and the interaction
between groups (2) and time (2) using a mixed design 2 × 2 ANOVA test.
Variables
Outcomes
df1
df2
F-value
p-value
η2
Time (2)
STN
1
58
5,479.70
0.001*
0.990
SLTHT
1
58
118.17
0.001*
0.671
ST
1
58
201.63
0.001*
0.777
Time * Groups
(2 × 2)
STN
1
58
1,576.10
0.001*
0.965
SLTHT
1
58
44.38
0.001*
0.433
ST
1
58
24.33
0.001*
0.296
Groups (2)
STN
1
58
759.62
0.001*
0.929
SLTHT
1
58
1.53
0.221
0.026
ST
1
58
10.08
0.002*
0.148
Notes:
* Signiﬁcant value if p < 0.05.
df, Degree of freedom; η2, Eta Squared where η2 = 0.01 indicates a small effect; η2 = 0.06 indicates a medium effect;
η2 = 0.14 indicates a large effect.
Table 3 Pairwise comparison for the scores of outcomes muscular performance (STN), functional
performance (SLTHT), and sprit performance (ST) across two-time points (pre & post) within
each group using Bonferroni’s multiple comparison test. Cohen’s d test was applied for
measuring effect size between two-time points.
Outcomes
Groups
Pre-
intervention
Post-
intervention
Time
(Pre-Post)
p-value
Cohen’s d
STN (ΔMD ± SD)
NMES
145.20 ± 3.68
214.67 ± 4.18
−69.47 ± 0.50
0.001*
17.64^
Control
144.93 ± 3.98
165.90 ± 4.80
−20.97 ± 0.82
0.001*
4.76^
SLTHT (ΔMD ± SD)
NMES
501.30 ± 54.50
540.73 ± 51.78
−39.43 ± 2.72
0.001*
0.74^
Control
499.90 ± 51.14
509.37 ± 50.41
−9.47 ± 0.73
0.004*
0.19
ST (ΔMD ± SD)
NMES
9.19 ± 0.58
7.91 ± 0.57
1.28 ± 0.01
0.001*
2.23^
Control
9.23 ± 0.40
8.61 ± 0.50
0.62 ± 0.10
0.001*
1.36^
Notes:
* Signiﬁcant value if p < 0.05.
^ Large and medium effect size if Cohen’s d value >2.00 and between 1.26–2.00, respectively (Rhea, 2004).
ΔMD, Mean differences; SD, Standard Deviation; NMES, Neuromuscular electric stimulation; ΔMD, Mean differences;
STN, Strength; SLTHT, Single leg triple hop test; ST, Resisted stride.
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(1.58) = 1,576.10, p < 0.05; SLTHT: F (1.58) = 44.38, p < 0.05; and ST: F (1.58) = 24.33,
p < 0.05.
Tables 3 and 4 depict pairwise comparisons using Bonferroni multiple comparisons test
for the scores of the outcomes within-groups across the two-time points and
between-groups at 8 weeks post-intervention, respectively. The ﬁndings within-group
showed signiﬁcant differences (p < 0.05) for the scores of all outcome measures including
STN, ST, and SLTHT across the two-time points of the study (Table 3). However, the
between-group analysis demonstrated a signiﬁcant difference (p < 0.05) for the outcomes
STN (p < 0.001, d = 10.84) and ST (p < 0.002, d = 1.31) except for a non-signiﬁcant
difference for the outcome measure SLTHT (p > 0.05, d = 0.613) (Table 4). In addition,
after 8-week of training, Cohen’s d observed between two groups a large to medium
treatment’s effect size for the outcome STN (d = 10.84) and ST (d = 1.31). However, a small
effect size was observed only for the SLTHT (d = 0.613).
In addition, Pearson’s coefﬁcient test revealed a signiﬁcant (95% CI, p < 0.05) but weak
to moderate correlation between: STN and SLTHT (r = −0.252, p = 0.052), STN and ST
(r = −0.540, p = 0.001), and ST and SLTHT (r = −0.358, p = 0.005) at 8-week
post-intervention (Table 5).
DISCUSSION
The present study used NMES and a plyometric training program to assess muscular
strength, sprint ability, and functional performance in collegiate male football players.
Findings from the 8-week combined program showed improvements in all the above three
Table 4 Pairwise comparison of post-test scores (at 8-weeks) for the outcomes muscular
performance (STN), functional performance (SLTHT), and sprint test (ST) between groups using
Bonferroni multiple comparison test. Cohen’s d test was applied for measuring effect size between
two groups.
Outcomes
NMES
(Mean ± SD)
Control
(Mean ± SD)
NMES vs Control
(ΔMD ± SD)
p-value
Cohen’s d
STN
214.67 ± 4.18
165.90 ± 4.80
48.77 ± −0.62
0.001*
10.84^
SLTHT
540.73 ± 51.76
509.37 ± 50.41
30.93 ± 0.37
0.221
0.613
ST
7.91 ± 0.57
8.61 ± 0.50
−0.70 ± 0.08
0.002*
1.31^
Notes:
* Signiﬁcant value if p < 0.05.
^ Large and medium effect size if Cohen’s d value >2.00 and between 1.26-2.00, respectively (Rhea, 2004).
DMD, Mean differences; SD, Standard Deviation; NMES, Neuromuscular electric stimulation; STN, Strength test;
SLTHT, Single leg triple hop test; ST, sprint test.
Table 5 Correlation between strength test (STN), single-leg triple hop test (SLTHT), and sprint test
(ST) at post-intervention.
Variables
SLTHT Po
(r & p-value)
ST Po
(r & p-value)
STN Po
−0.252 (0.052)*
−0.540 (0.001)*
ST Po
−0.358 (0.005)*
1
Note:
* Signiﬁcant value (2-tailed), if p < 0.05.
Hasan et al. (2022), PeerJ, DOI 10.7717/peerj.13588
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outcomes. A positive correlation is shown for both experimental and controlled groups in
post-intervention variables. Therefore, the results suggest that NMES assisted strength
training combined with plyometric training enhances strength and athletic performance in
adult male college footballers. The current study adds to existing research, proving that
the combination of plyometric training with NMES assisted strength training is an
adequate method to improve muscular strength (Alonazi et al., 2021). Similar to the
present study, previous studies have shown the effect of NMES on strengthening lower
limb muscles in a post-operative population through rehabilitation of knee injuries
(Gatewood, Tran & Dragoo, 2017).
For example, in total knee arthroplasty patients, Walls et al. (2010) studied over a
similar time frame demonstrated signiﬁcant improvement in quadriceps strength and
functional movements with NMES. In support of our results, a review by Kim et al. (2010)
recommended using NMES and exercises together to improve quadriceps strength rather
than exercises alone in anterior cruciate ligament (ACL) reconstruction post-operative
rehabilitation.
One of the most important reasons for the increased strength is muscle activation
potentiation. NMES seems to increase the actin-myosin cross-bridges to calcium, thereby
increasing the muscle’s force-generating capacity (da Cunha et al., 2020; Gomes da Silva
et al., 2018; Bouguetoch, Martin & Grosprêtre, 2021). Our main ﬁndings mainly support
the hypothesis in which we found the use of NMES in addition to plyometric training
signiﬁcantly improved strength and physical performance not just immediately but also
after 8-weeks of intervention. The muscle ﬁber type also inﬂuences the force-generating
capacity of the muscles. Stimulating type II muscle ﬁbers produce a higher speciﬁc force
than type I ﬁbers. This type of stimulation associated with greater expression of fast-twitch
myosin heavy chain isoform through plyometric has proven to increase a muscle’s overall
strength and performance (Taradaj et al., 2013; Smith, Hotze & Tate, 2021; Rahmati,
Gondin & Malakoutinia, 2021). Although the sham NMES group showed marginal
improvements in strength, the magnitude is not the same as the NMES group. Despite the
ever-growing scientiﬁc research around placebo effects, researchers have continued to
present sham procedures with little beneﬁt within clinical research. Brim & Miller (2013)
states that the importance of recognizing the extent of the placebo effect from any speciﬁc
sham-controlled trial is unclear. The placebo effect is well deﬁned for speciﬁc sham
procedures as it produces a more signiﬁcant placebo response in more pharmacological
research, such as effects on pain rather than enhancing muscular strength, ﬁber size, and
physical performance (Hróbjartsson & Gøtzsche, 2004).
In addition, the perception among researchers and the sports staff is that the NMES to
improve strength and performance is more appropriate for the clinical population (who
struggle to contract muscles actively) rather than the physically active or athletic
population (Veldman et al., 2016). Whereas Thomé et al. (2021) and Gondin, Cozzone &
Bendahan (2011) supports the current results as the research states that improvements of
up to 40% of athletes sporting performance have been concluded through observation with
the use of NMES and the cumulative effect of strong, plyometric training and
strengthening protocol (Teixeira et al., 2021a).
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The magnitude of effect size in strength training research has been considered relatively
higher than in social and psychological research (Rhea, 2004). A very large effect size
suggests the practical signiﬁcance of that particular intervention over the study outcomes.
The current study revealed a very high effect size in NMES group (d = 10.84) over sham
NMES group for the outcome STN. It advocates the practical signiﬁcance of NMES,
application of NMES as add-on modality is very important in improving the muscle
strength, thus, enough reason to consider its application in clinical settings.
Besides many beneﬁts, this study exhibits few limitations for generalizing its ﬁnding to
some extent. The study was relatively limited to a short duration of the training period
(only 8-weeks) in the context for improving strength and physical performance, limited to
a very speciﬁc study population (i.e., collegiate male football players), and did not monitor
any external factors, such as additional exercises or physical activities of the collegiate male
football players other than their actual intervention that could affect the validity of the
study ﬁndings (Teixeira et al., 2021b). Therefore, this study cannot be generalized to other
populations.
CONCLUSIONS
The research ﬁndings suggest that the Plyometric and strength trainings in addition to
either NMES or sham NMES for a short-duration training period are equally capable of
enhancing the muscular performance, sprint performance, and functional performance of
collegiate male football players.
NMES has been proven as a training modality to enhance muscular performance, sprint
performance, and functional performance in collegiate male football player; therefore, it
might be applied to other similar competitive endurance sports, such as football and
netball. Future studies will require more than 8-weeks of training that includes a wider
range of endurance athletes, with strict monitoring of external factors that might affect the
validity of the ﬁndings.
ACKNOWLEDGEMENTS
The authors extend their appreciation to the faculty members of Majmaah University,
especially Mr. Raad Ibrahim Alraidan, for their sincere support and assistance in this
research study.
ADDITIONAL INFORMATION AND DECLARATIONS
Funding
This work was funded by the deputyship for Research and Innovation, Ministry of
Education, Saudi Arabia, Project Number No IFP-2020-25. The funders had no role in
study design, data collection and analysis, decision to publish, or preparation of the
manuscript.
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Grant Disclosures
The following grant information was disclosed by the authors:
Research and Innovation, Ministry of Education, Saudi Arabia: IFP-2020-25.
Competing Interests
The authors declare that they have no competing interests.
Author Contributions
 Shahnaz Hasan conceived and designed the experiments, performed the experiments,
analyzed the data, authored or reviewed drafts of the article, and approved the ﬁnal draft.
 Gokulakannan Kandasamy conceived and designed the experiments, analyzed the data,
authored or reviewed drafts of the article, and approved the ﬁnal draft.
 Danah Alyahya conceived and designed the experiments, authored or reviewed drafts of
the article, and approved the ﬁnal draft.
 Asma Alonazi conceived and designed the experiments, authored or reviewed drafts of
the article, and approved the ﬁnal draft.
 Azfar Jamal performed the experiments, analyzed the data, prepared ﬁgures and/or
tables, and approved the ﬁnal draft.
 Amir Iqbal conceived and designed the experiments, analyzed the data, prepared ﬁgures
and/or tables, and approved the ﬁnal draft.
 Radhakrishnan Unnikrishnan performed the experiments, prepared ﬁgures and/or
tables, and approved the ﬁnal draft.
 Hariraja Muthusamy performed the experiments, prepared ﬁgures and/or tables, and
approved the ﬁnal draft.
Human Ethics
The following information was supplied relating to ethical approvals (i.e., approving body
and any reference numbers):
The Chair of Majmaah University for Research Ethics Committee, Saudi Arabia,
granted Ethical approval to carry out the study (Ethics number: MUREC-Dec./COM-
2020/13-2 dated 15 December 2020).
Data Availability
The following information was supplied regarding data availability:
The raw measurements are available in the Supplemental Files.
Supplemental Information
Supplemental information for this article can be found online at http://dx.doi.org/10.7717/
peerj.13588#supplemental-information.
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Effect of plyometric training and neuromuscular electrical stimulation assisted strength training on muscular, sprint, and functional performances in collegiate male football players.
Hasan, Shahnaz,Kandasamy, Gokulakannan,Alyahya, Danah,Alonazi, Asma,Jamal, Azfar,Iqbal, Amir,Unnikrishnan, Radhakrishnan,Muthusamy, Hariraja