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DOI 10.26773/mjssm.230905 3
Received: 01 Jun 2023 | Accepted after revision: 09 August 2023 | Early access publication date: 01 September 2023 | Final publication date: 15 September 2023
© 2023 by the author(s). License MSA, Podgorica, Montenegro. This article is an open access article distributed under the terms and conditions of
the Creative Commons Attribution (CC BY).
Conict of interest: None declared.
Bilateral Asymmetry and the Relationship Between
Unilateral Isokinetic Strength and Balance
Performance in Male Adolescent Football Players
Cíntia França1,2,3, Francisco Martins1,2, Diogo V. Martinho4, Andreas Ihle5,6,7, Adilson Marques8,9, Hugo Sarmento4,
Filipe Manuel Clemente10,11, Krzysztof Przednowek12, Pedro Campos2,13,14, Élvio Rúbio Gouveia1,2,6*
Affiliations:
1
Department of Physical Education and Sport, University of Madeira, Funchal, Portugal,
2
LARSYS, Interactive Technologies Institute, Funchal,
Portugal,
3
Research Center in Sports Sciences, Health Sciences, and Human Development (CIDESD), Vila Real, Portugal,
4
University of Coimbra, Research
Unit for Sport and Physical Education (CIDAF), Faculty of Sport Sciences and Physical Education, Coimbra, Portugal,
5
Department of Psychology, University
of Geneva, Geneva, Switzerland,
6
Center for the Interdisciplinary Study of Gerontology and Vulnerability, University of Geneva, Geneva, Switzerland,
7
Swiss
National Centre of Competence in Research LIVES—Overcoming Vulnerability: Life Course Perspectives, Lausanne, Switzerland,
8
CIPER, Faculty of Human
Kinetics, University of Lisbon, Lisbon, Portugal,
9
ISAMB, Faculty of Medicine, University of Lisbon, Lisbon, Portugal,
10
Escola Superior Desporto e Lazer,
Instituto Politécnico de Viana do Castelo, Rua Escola Industrial e Comercial de Nun’Álvares, Viana do Castelo, Portugal,
11
Instituto de Telecomunicações,
Delegação da Covilhã, Lisboa, Portugal,
12
Institute of Physical Culture Sciences, Medical College, University of Rzeszów, Rzeszów, Poland,
13
Department
of Informatics Engineering and Interactive Media Design, University of Madeira, Funchal, Portugal,
14
WoWSystems Informática Lda, Funchal, Portugal
Correspondence: Élvio Rúbio Gouveia. ORCID 0000-0003-0927-692X, address: Universidade da Madeira, Campus da Penteada, 9020-105 Funchal.
Email: erubiog@staff.uma.pt
Abstract
Muscle strength and balance ability have been related to injury prevention and game actions performance in football.
The aims of this study are twofold: (1) to examine bilateral asymmetries in isokinetic strength and balance assessments,
and (2) to evaluate the relationship between muscle strength and balance measures. Eighty-eight male adolescent
football players were assessed for body composition (InBody 770), isokinetic strength (Biodex System 4 Pro Dynamom-
eter), and balance performance (Biodex Balance System). Paired samples t-tests were conducted to determine bilateral
dierences in strength and balance. Pearson correlations and multiple linear regression analyses evaluated the relation-
ship between strength and balance. No signicant bilateral strength dierences were observed in knee exors (KF) and
knee extensors (KE) peak torque (PT) scores. No bilateral dierences were found for balance measures, except in the
lateromedial stability index, which was better while performing with the preferred leg (p ≤ 0.01; d = 0.29). Signicant
correlations were found between KE PT and balance tasks (p ≤ 0.01). KF and KE PT shared between 18 and 22% of the
common variance in the overall stability index in the non-preferred and preferred leg, respectively. Monitoring strate-
gies of bilateral asymmetries may be crucial to enhance performance in tasks underpinned by unilateral movements,
such as changes of direction and sprints, and to identify players at risk of injury. Adopting these strategies during the
early stages of football training might be crucial for players’ long-term development.
Keywords: soccer, peak torque, knee exors, knee extensors, youth
Cite this article: França, C., Martins, F., Martinho, D.V., Ihle, I., Marques, A., Sarmento, H., Clemente, F.M., Przednowek,
K., Campos, P., & Gouveia, É.R. (2023) Bilateral Asymmetry and the Relationship Between Unilateral Isokinetic
Strength and Balance Performance in Male Adolescent Football Players. Montenegrin Journal of Sports Science and
Medicine, 19 (2), Ahead of print. https://doi.org/10.26773/mjssm.230905
ORIGINAL SCIENTIFIC PAPER
@MJSSMontenegro
BILATERAL ASYMMETRY IN YOUTH FOOTBALL
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4 DOI 10.26773/mjssm.230905
BILATERAL ASYMMETRY IN YOUTH FOOTBALL | C. FRANÇA ET AL.
Introduction
In football, players oen perform jumping, sprinting, and
rapid change of direction actions (Wislo et al., 2004). Since
these movements are associated with force-time characteristics
(Suchomel et al., 2016; Wislo et al., 2004), muscle strength is
recognized as one critical physical attribute of football players.
Greater muscle strength has been associated with enhanced
sport-specic skills performance and decreased injury rates
(Suchomel et al., 2016).
Meanwhile, lower-body strength assessments (i.e., squat
or the half-squat) tend to be bilateral and fail to examine
strength asymmetries in the lower limbs. e existence of
bilateral strength dierences between the preferred leg (PL)
and the non-preferred leg (NPL) remains controversial, with
acceptable decits ranging between 10 and 15% (Teixeira et
al., 2014). Pending the player’s characteristics, the asymme-
tries between limbs can be functional at dierent magnitudes
(Sannicandro et al., 2012). Previous research has reported that
morphological adaptations between the kicking and support
limbs of football players might occur in response to repetitious
asymmetrical loading patterns resulting from their functional
dierences within the context of their sport (Hart et al., 2016).
As an example, relationships between levels of training ex-
posure and asymmetrical loading exposure (kicking limb vs
support limb) were observed, with distinct morphological ad-
aptation noted between limbs (Hart et al., 2016).
Although some literature suggests that signicant imbal-
ances can be associated with increased musculoskeletal inju-
ry occurrences (Croisier et al., 2008; Van Dyk et al., 2016), a
recent systematic review reported that relationship as low to
moderate, and there is inconsistency in the ndings (Helme
et al., 2021). Despite these inconsistencies, knowing the asym-
metry level between limbs is helpful to address the detrimental
eects of possible inter-limb asymmetry. A recent study de-
scribed that inter-limb asymmetries seem to present a negative
impact on tasks underpinned by unilateral movements, such
as changes of direction and sprints (Fox et al., 2023). ere-
fore, monitoring strategies should be adopted to identify and
address inter-limb asymmetries according to individual needs.
Frequently, muscle strength has been dened as the ability
to produce a force on an external object or resistance (Suchomel
et al., 2016). us, isokinetic strength assessments have become
popular, safe, and reliable in the sports literature. Several inves-
tigations have examined bilateral strength imbalances through
isokinetic strength tests in professional football (Croisier et
al., 2008; Menzel et al., 2013; Van Dyk et al., 2016). Among 46
male professional footballers aged 24.8±3.2 years, the authors
reported a mean of bilateral strength asymmetry of 9.14% at
peak torque (PT) for concentric knee exion (KF) while per-
forming at 60º/s (Menzel et al., 2013). In the same testing con-
ditions, another study showed a mean dierence of 7% at KF
and 6.5% at KE between the PL and the NPL in footballers aged
23.8±2.12 years (Teixeira et al., 2014). Although these strength
asymmetries have been identied, the scores reported are with-
in the respective average values. Besides strength comparison
between limbs, research has also analyzed the hamstrings to
quadriceps ratio (H:Q ratio) due to its relationship with injury.
For isokinetic strength testing at 60º/s, lower values than 0.60
for the H:Q ratio has been associated with injuries, such as an-
terior cruciate ligament injuries and hamstring strains (Crois-
ier et al., 2002). However, most data on lower-limb strength
asymmetries are available at the professional football level, and
a detailed analysis still lacks among youth.
Meantime, football players frequently perform actions using
unilateral stances, such as kicking, passing, and dribbling, which
demands balance control. Better balance ability, both in stable
and dynamic conditions, was observed in elite players compared
to their non-elite counterparts (França et al., 2022). Besides, a sig-
nicant relationship between balance and injury prevention was
previously found (Al Attar et al., 2022). In contrast, dierent re-
ports have emerged concerning bilateral asymmetries in balance
measures (Gkrilias et al., 2018; González-Fernández et al., 2022;
McCurdy & Langford, 2006; orpe & Ebersole, 2008). While
no dierences were observed among healthy adults (McCurdy
& Langford, 2006) and female collegiate football players (orpe
& Ebersole, 2008), substantial bilateral asymmetries were found
in youth footballers (Gkrilias et al., 2018; González-Fernández et
al., 2022). Additionally, in youth football, inter-limb asymmetry
in dynamic balance performance was reported (Gkrilias et al.,
2018; González-Fernández et al., 2022), but no data concerning
stability is available.
Of note, muscle strength levels may be benecial to bal-
ance control (Booysen et al., 2015; Śliwowski et al., 2021).
e ability to generate eccentric strength and power pre-
sented moderate correlations with the Y-Balance scores on the
NPL in male footballers (Booysen et al., 2015). Among vol-
leyball players, both the KF and KE PT were signicantly cor-
related with the overall stability index (OSI) in both legs’ per-
formance (Soylu et al., 2020). However, most of the previous
research designed to examine the relationship between muscle
strength and balance performance in sports has favored elite
players or non-athlete populations, and importantly, detailed
knowledge regarding youth is still lacking.
erefore, the aims of this study are twofold: (1) to ex-
amine bilateral asymmetries according to the PL and NPL
in strength and balance, and (2) to evaluate the relationship
between strength and balance measures. First, it was hypothe-
sized that superior levels of strength and balance would be ob-
served in the performance of the PL, and secondly, it was ex-
pected a signicant and positive relationship between strength
and balance.
Methods
Study design
e study followed a descriptive cross-sectional design. e
study protocol was approved by the Faculty of Human Kinet-
ics Ethics Committee (CEIFMH N°34/2021) and followed the
Declaration of Helsinki. All the assessments were conducted
in a physical performance laboratory by trained sta from the
research team. All participants were volunteers, and informed
consent was obtained from their respective legal guardians.
e study was conducted during the sports season
2021/2022, aer two months of training sessions. Each partic-
ipant was assessed on two consecutive days, with a rest interval
of at least 12h regarding the latest training session.
Participants
Eighty-eight male adolescent football players aged
15.9±1.6 years participated in this study. e optimal sample
size calculation was performed using G*Power 3.1. A priori
paired-sample t-test (two-tails) indicated a sample size of 54
participants to achieve 80% power to detect an interaction ef-
fect size of 0.5 at 0.05 level of signicance. In the second anal-
BILATERAL ASYMMETRY IN YOUTH FOOTBALL | C. FRANÇA ET AL.
DOI 10.26773/mjssm.230905 5
ysis, a priori Pearson product-moment correlation showed a
total sample of 84 participants to achieve 80% power to detect
an interaction eect size of 0.3 at 0.05 level of signicance.
en, a priori multiple regression analysis indicated a sample
size of 55 participants to achieve 80% power considering an
eect size of 0.15 at 0.05 level of signicance.
Twenty players had the le leg as the preferred limb. Limb
preference was dened as the leg that is preferred when kick-
ing a ball. Participants had at least three years of football train-
ing experience and competed at the regional level in Portugal.
Training frequency was four times per week plus one match
during the weekend.
Body composition
Body composition was evaluated during the early morn-
ing while participants were fasting. Stature was assessed to the
nearest 0.01 cm using a stadiometer (SECA 213, Hamburg,
Germany). A hand-to-foot bioelectrical impedance analysis
(InBody 770, CA, USA) was used for measurement (McLester
et al., 2020), with participants only wearing their underwear.
Among the body composition variables, body mass, body fat
percentage (BF%), total fat-free mass (FFM), and segmental
FFM of the PL and the NPL were retained for analysis.
Isokinetic testing
Isokinetic measurements were performed on the hamstrings
and quadriceps muscles using the Biodex System 4 Pro Dyna-
mometer (Shirley, NY, USA) (Van Tittelboom et al., 2022). Be-
fore data collection, a 5-minute warm-up in a reclining bicycle
(Technogym Xt Pro 600 Recline, Cesena, Italy) was performed.
en, participants were seated in the dynamometer following
the manufacturer’s guidelines. e lever arm of the dynamome-
ter was aligned with the lateral epicondyle of the knee, while the
trunk, the evaluated thigh, and the leg were stabilized with belts.
e range of motion was dened as participants carrying the
knee extension to its maximum range. en, participants were
asked to ex the kneed until 90º of exion. As recommended, in-
dividual calibration for gravity correction was performed at 30º
of knee exion. During testing, participants were asked to keep
their arms crossed with the hand on the opposite shoulder hold-
ing the belts, and verbal support was given throughout the tests.
ree repetition trials were given before testing to ensure the
correct execution. Aer, ve repetitions of concentric contrac-
tion eorts of knee exion and knee extension were performed
at 60º/s, with a 60 s interval. is testing speed has been recom-
mended to assess strength. For analysis, the peak torque (PT),
the relative peak torque/body weight (PT/BW), and the conven-
tional hamstring-to-quadriceps (H:Q) strength ratio for KF and
KE in the PL and NPL were calculated. e H:Q conventional
ratio was used since it was generally measured during concentric
contraction (Aagaard et al., 1998) and was calculated by dividing
the mean concentric KF PT by the mean KE concentric PT over
the ve repetitions. Finally, the limb symmetry index (LSI) was
calculated using the equation (1):
Balance testing
Balance assessment was performed using the Biodex Bal-
ance System SD (Biodex, Shirley, NY, USA). For testing, par-
ticipants were barefoot in an upright position, arms placed
laterally to the body, and feet set shoulder‐width apart. Before
testing, the equipment was adjusted to the height of the partic-
ipants. A single training session was allowed before data col-
lection to ensure correct execution and minimize learning ef-
fects during the testing phase. e rest interval between testing
conditions was set at 60 s. Bilateral comparison consisted of a
protocol performed in a unilateral stance. Level 4 was the most
stable, and level 1 was the most unstable. e testing scores
reect the level of deviation from the horizontal position.
erefore, lower scores indicate better balance performance
(Yamada et al., 2012). For analysis, the Overall Stability Index
(OSI), Anteroposterior Stability Index (APSI), and Laterome-
dial Stability Index (LMSI) were used.
Statistics
Descriptive statistics are presented as means ± standard
deviation. All data were checked for normality using the
Kolmogorov-Smirnov test. Paired samples t-tests were con-
ducted to identify bilateral dierences in isokinetic strength
and balance assessments. Eect size (d) was interpreted using
d-Cohen as follows (Cohen, 1988): d < 0.2 (small), 0.2 > d <
0.6 (moderate), 0.6 > d < 1.2 (large), and d > 1.2 (very large).
e Pearson product-moment correlation was used to explore
the relationships between isokinetic strength and balance tests
according to each leg. Correlations values were interpreted ac-
cording to their size (Cohen, 1988): 0.10 > r < 0.29 (small),
0.30 > r < 0.49 (medium), 0.50 > r < 1.0 (large). Finally, mul-
tiple linear regression analyses were performed to determine
the association between isokinetic strength and balance per-
formance. To avoid heteroscedasticity, the dependent variable
was transformed using the log. All analyses were performed
using IBM SPSS Statistics soware 28.0 (SPSS Inc., Chicago,
IL, USA). e signicance level was set at 0.05.
Results
Table 1 resumes descriptive statistics concerning age and
body composition.
Table 1. Descriptive statistics for body composition of adolescent male football players (n = 88)
Variable Mean (95% CI) SD
CA (years) 15.9 (15.5 - 16.2) 1.6
Stature (cm) 172.3 (170.5 - 174.1) 8.3
Body mass (kg) 63.5 (61.5 - 65.5) 9.3
FFM (kg) 55.7 (54.0 - 57.5) 8.3
BF (%) 11.7 (10.6 – 12.9) 5.3
95% CI (95% condence interval), SD (standard deviation), CA (chronological age), FFM (fat-free mass), BF (body fat)
Table 2 presents the descriptive statistics and bilateral
comparison in strength and balance assessments. No signif-
icant dierences between the PL and NPL were observed for
KF PT/BW, KE PT/BW, and H:Q strength ratio in isokinet-
ic strength parameters. LSI analysis showed a dierence of
1.7% for KF PT and 1.2% for KE PT values when the PL was
6 DOI 10.26773/mjssm.230905
BILATERAL ASYMMETRY IN YOUTH FOOTBALL | C. FRANÇA ET AL.
compared to the NPL.
Concerning body composition, FFM was signicantly
higher in the PL than in NPL (p≤0.01, trivial eect size). No
overall dierences were seen in balance performance, except
in the LMSI testing, which was substantially better in the PL
(p≤0.01, moderate eect size). Although the results are not
statistically signicant, better overall balance scores were
achieved while performing with the PL compared to the NPL.
Table 2. Descriptive statistics and comparison of preferred and non-preferred leg performance in isokinetic dynamometer
and unilateral balance assessment (n = 88)
Parameter Preferred leg Non-preferred leg Paired comparisons
Mean (95% CI) SD Mean (95% CI) SD t p d LSI (%)
Isokinetic strength
KF PT (Nm) 90.0 (85.7 – 94.4) 20.7 88.5 (84.1 – 93.0) 21.2 1.184 0.24 0.07 98.3
KF PT/BW (Nm/kg) 1.36 (1.28 – 1.45) 0.38 1.35 (1.27 – 1.44) 0.37 0.499 0.62 0.03
KE PT (Nm) 151.0 (141.7 – 160.3) 43.8 152.8 (144.2 – 161.3) 40.2 1.000 0.32 0.04 98.8
KF PT/BW (Nm/kg) 2.32 (2.15 – 2.49) 0.78 2.35 (2.18 – 2.52) 0.76 0.960 0.34 0.04
H:Q strength ratio (%) 0.62 (0.59 – 0.65) 0.15 0.60 (0.57 – 0.63) 0.15 1.762 0.08 0.13
Body composition
FFM (kg) 8.93 (8.60 – 9.25) 1.46 8.86 (8.54 – 9.18) 1.43 4.947 ≤ 0.01** 0.05
Balance
OSI (º) 2.14 (1.70 – 2.58) 2.06 2.37 (1.93 – 2.81) 2.07 1.775 0.08 0.11
APSI (º) 1.64 (1.22 – 2.06) 1.97 1.73 (1.32 – 2.15) 1.95 0.739 0.46 0.05
LMSI (º) 1.02 (0.85 – 1.20) 0.84 1.27 (1.08 – 1.46) 0.92 2.567 ≤ 0.01** 0.29
95% CI (95% condence interval), SD (standard deviation), KF (knee exors), KE (knee extensors), PT (peak torque), BW (bodyweight), H:Q
(hamstrings/quadriceps), FFM (fat-free mass), OSI (overall stability index), APSI (anteroposterior stability index), LMSI (lateromedial stability
index), LSI (limb symmetry index), ** p ≤ 0.01
Tables 3 and 4 show the signicant correlation coecients
between isokinetic strength and balance parameters for the PL
and NPL, respectively. Signicant correlations were found be-
tween KF and KE PT/BW, both for the PL (r = 0.80, p≤0.01) and
the NPL (r = 0.74, p≤0.01). e KE PT/BW presented strong
and negative correlations with balance indicators both for the
PL (p≤0.01) and the NPL (p≤0.01). In contrast, KF/PT only
showed signicant correlations with balance indicators in the
PL analysis. Among balance parameters, the OSI correlated sig-
nicantly and positively with the APSI and LMSI in both eval-
uations. Overall, the PL analysis observed the highest number
of relationships between isokinetic strength and balance tasks.
Table 3. Correlation coecients between isokinetic strength and balance in the preferred leg assessments
Parameter 1. 2. 3. 4. 5.
1. KF PT/BW - 0.80** -0.27* -0.27* -0.20
2. KE PT/BW - -0.48** -0.44** -0.40**
3. OSI - 0.97** 0.63**
4. APSI - 0.44**
5. LMSI -
KF (knee exion); KE (knee extension); PT/BW (Peak Torque/Bodyweight); OSI (overall stability index); APSI (anteroposterior
stability index); LMSI (lateromedial stability index); * p ≤ 0.05; ** p ≤ 0.01
Table 4. Correlation coecients between isokinetic strength and balance in the non-
preferred leg assessments
Parameter 1. 2. 3. 4. 5.
1. KF PT/BW - 0.74** -0.16 -0.16 -0.11
2. KE PT/BW - -0.40** -0.40** -0.34**
3. OSI - 0.97** 0.79**
4. APSI - 0.63**
5. LMSI -
KF (knee exion); KE (knee extension); PT/BW (Peak Torque/Bodyweight); OSI (overall stability index); APSI
(anteroposterior stability index); LMSI (lateromedial stability index); * p ≤ 0.05; ** p ≤ 0.01
Finally, the results of linear regression analyses with isoki-
netic strength (PT/BW) predicting balance performance are
summarized in Figures 1 and 2. e model explained between
18 and 23% of the variance observed in the OSI performance
for the NPL and PL, respectively. In both cases, KF PT/BW and
KE PT/BW were signicant predictors of the model. However,
the strongest unique contribution for OSI scores was seen in KE
PT/BW (PL: β = -0.712, p≤0.01, and NPL: β = -0.595, p≤0.01).
BILATERAL ASYMMETRY IN YOUTH FOOTBALL | C. FRANÇA ET AL.
DOI 10.26773/mjssm.230905 7
Discussion
is study examined bilateral asymmetries according to
the PL and NPL leg in muscle strength and balance tasks in
youth football. Also, the relationship between strength and
balance measures was assessed. First, it was hypothesized that
superior levels of strength and balance parameters would be
observed in the performance of the PL. However, no signi-
cant bilateral strength dierences were observed in the isoki-
netic strength assessment or balance testing, except in the
LMSI. Secondly, it was expected a substantial and positive
relationship between strength and balance, which was under-
lined in the present study results. Overall, muscle strength was
positively related to balance performance.
Bilateral asymmetry in youth football
Bilateral asymmetries have been observed in sports with
predominant unilateral movements, such as football. Indeed,
while playing, using the PL is oen favored for football-spe-
cic skills performance (Zakas, 2006). In professional players,
previous research reported a bilateral strength asymmetry
ranging from 7% to 9% when muscle strength was assessed at
an angular velocity of 60º/s (Croisier et al., 2003; Menzel et al.,
2013). However, note that 10-15% bilateral strength dierences
have been suggested as relevant for injury occurrence (Crois-
ier et al., 2003). In this study, signicant statistical dierences
were seen in FFM between the PL and the NPL. However, LSI
ranged between 98.3% and 98.8%, indicating the existence of
small bilateral asymmetries. Previous research reported an
average LSI of 103.8% among players between the U11 and
U19 age categories when the jump performance was evaluated
(Scinicarelli et al., 2022). Still, only one study examining bilat-
eral asymmetry using isokinetic strength was found in youth
football, with mean dierences of nearly 8% being described
at an angular velocity of 60º/s (Rutkowska-Kucharska, 2020).
Indeed, performing an isokinetic strength assessment requires
specialized measuring equipment that is not frequently avail-
able, particularly in youth samples.
Meanwhile, the H:Q strength ratio analysis did not show a
signicant muscle imbalance between KE and KF. e results
indicate an acceptable H:Q score following the literature rec-
ommendations for the 60º/s testing speed (>0.60). In contrast,
values below 0.60 indicate a substantial strength imbalance be-
tween the KE and KF, which could predispose the individual
to a non-contact injury (Croisier et al., 2002). According to the
literature, football training appears to increase the strength of
the knee joint muscles. However, it seems that greater devel-
opment of the quadriceps muscles is favored compared with
the hamstrings muscles (Iga et al., 2009). Moreover, players
with imbalances appeared ve times more likely to sustain a
hamstring strain (Croisier et al., 2003), underlining the im-
portance of testing and monitoring muscle strength levels as a
preventive measure.
Figure 1. Multiple regression analyses with KF and KE PT/BW as predictors of the
OSI scores in the preferred leg analysis.
Figure 2. Multiple regression analyses with KF and KE PT/BW as predictors of the OSI scores in
the non-preferred leg analysis.
8 DOI 10.26773/mjssm.230905
BILATERAL ASYMMETRY IN YOUTH FOOTBALL | C. FRANÇA ET AL.
Association between strength and balance
In this study, a signicant correlation between muscle
strength levels and balance performance was found. PT scores
were signicantly and negatively correlated to balance variables,
indicating that a higher strength output contributes to a lower
deviation from the horizontal position during balance tasks.
is relationship is supported by the multiple regression anal-
yses, which showed that PT values could explain between 18%
and 22% of the variance observed in the OSI scores for the NPL
and the PL, respectively. e results are in line with previous
literature focused on the relationship between muscle strength
and balance, although using dierent methods and protocols
than the ones applied in this study. For example, among 26 foot-
ball players aged 16.2±1.6 years, the values of maximal isomet-
ric strength were able to explain between 22% and 49% of the
variance observed in the Y-balance performance (Chtara et al.,
2018). In other studies, the KF PT was described as a signicant
predictor of the Star Excursion and Y-Balance Tests in athletes
from several contexts (Ruiz-Pérez et al., 2019).
When analyzing the model, KF PT/BW and KE PT/BW
were signicant predictors of OSI in both legs’ performance.
However, the strongest unique contribution was made by KE
PT/BW, emphasizing the role of quadriceps muscle strength
in balance performance. Indeed, there is evidence in sports
literature that the quadriceps muscle strength is the greater
supporter of the knee joint during balance tasks (Śliwowski et
al., 2021; Soylu et al., 2020). Reports in previous research de-
scribed a strong correlation between KE strength and balance
performance in male footballers (Śliwowski et al., 2021) and
female volleyballers (Soylu et al., 2020), which is in line with
the results of the present study.
Concerning balance, overall superior performance was
observed with the PL compared to the NPL, although not
signicantly, except for the LMSI. Past research on this top-
ic has reported controversial results. In football players, some
authors reported no substantial asymmetry in balance per-
formance between the lower limbs (Muehlbauer et al., 2019),
while others said the opposite (Barone et al., 2011). However,
note, that multiple factors could inuence the postural balance
dierences between the lower limbs. Besides, the inuence of
leg dominance on unilateral balance should probably be con-
text-dependent (Sannicandro et al., 2012).
Limitations and Strengths
is study presents some limitations, such as using
cross-sectional data and the lack of assessment of participants’
maturity status. Since strength gains are larger during and af-
ter the peak height velocity, the maturity status may play an
important role in the present study results. Deploying a lon-
gitudinal analysis and controlling maturity status would be far
more informative. However, few studies are available on isoki-
netic strength and balance assessment in such a representative
sample of youth football players, which should be underlined.
is study provides new insights into strength and balance
performance in youth football and emphasizes the positive
contribution of strength to balance performance. Finally,
monitoring strategies of bilateral asymmetries may be crucial
to enhance performance in tasks underpinned by unilater-
al movements, such as changes of direction and sprints, and
to identify players at risk of injury. Adopting these strategies
during the early stages of football training might be crucial for
players’ long-term development.
Conclusion
e results of the present study show no signicant bilat-
eral asymmetries in strength and balance performance among
youth football players. However, a tendency for superior per-
formance levels was seen for the PL. Moreover, strength (PT
KE and PT KF) was able to explain between 18% and 22% of
the variance observed in the OSI, which is believed to be the
best indicator of the overall ability of the individual to balance
the platform. Sports agents and coaches are advised to adopt
monitoring strategies of strength and balance capabilities to
detect inter-limb asymmetries that may compromise tasks
supported by unilateral movements and/or enhance the risk
of injury. ese strategies might be decisive for youth play-
ers’ long-term development. Future research should include
a longitudinal assessment of strength and balance measures
and control for the maturity status as a possible confounder of
physical performance at this age gap.
Acknowledgments
e authors would like to thank all participants and re-
spective legal guardians for participating in this study. is
project has received support from the Portuguese Recovery
and Resilience Program (PRR), IAPMEI/ANI/FCT under the
Agenda C645022399-00000057 (eGamesLab).
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