ArticlePDF Available

Abstract and Figures

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 differences in strength and balance. Pearson correlations and multiple linear regression analyses evaluated the relationship between strength and balance. No significant bilateral strength differences were observed in knee flexors (KF) and knee extensors (KE) peak torque (PT) scores. No bilateral differences 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). Significant 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 strategies 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.
Content may be subject to copyright.
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).
Conict 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
dierences in strength and balance. Pearson correlations and multiple linear regression analyses evaluated the relation-
ship between strength and balance. No signicant bilateral strength dierences were observed in knee exors (KF) and
knee extensors (KE) peak torque (PT) scores. No bilateral dierences 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). Signicant
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
http://mjssm.me/?sekcija=article&artid=259
4 DOI 10.26773/mjssm.230905
BILATERAL ASYMMETRY IN YOUTH FOOTBALL | C. FRANÇA ET AL.
Introduction
In football, players oen 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-specic 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 dierences between the preferred leg (PL)
and the non-preferred leg (NPL) remains controversial, with
acceptable decits ranging between 10 and 15% (Teixeira et
al., 2014). Pending the players characteristics, the asymme-
tries between limbs can be functional at dierent 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
dierences 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 signicant 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
eects 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 dened 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 dierence 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 identied, 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-
nicant relationship between balance and injury prevention was
previously found (Al Attar et al., 2022). In contrast, dierent 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 dierences 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 benecial 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 signicantly 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 signicant 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, aer 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 signicance. 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 eect size of 0.3 at 0.05 level of signicance.
en, a priori multiple regression analysis indicated a sample
size of 55 participants to achieve 80% power considering an
eect size of 0.15 at 0.05 level of signicance.
Twenty players had the le leg as the preferred limb. Limb
preference was dened 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 dened 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. Aer, ve repetitions of concentric contrac-
tion eorts 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
reect 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 dierences in isokinetic strength
and balance assessments. Eect 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 soware 28.0 (SPSS Inc., Chicago,
IL, USA). e signicance 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% condence 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 dierences 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 dierence 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 signicantly
higher in the PL than in NPL (p≤0.01, trivial eect size). No
overall dierences were seen in balance performance, except
in the LMSI testing, which was substantially better in the PL
(p≤0.01, moderate eect size). Although the results are not
statistically signicant, 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% condence 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 signicant correlation coecients
between isokinetic strength and balance parameters for the PL
and NPL, respectively. Signicant 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 signicant correlations with balance indicators in the
PL analysis. Among balance parameters, the OSI correlated sig-
nicantly 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 coecients 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 coecients 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 signicant 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 dierences 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 oen favored for football-spe-
cic 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 dierences
have been suggested as relevant for injury occurrence (Crois-
ier et al., 2003). In this study, signicant statistical dierences
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 dierences 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
signicant 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 signicant correlation between muscle
strength levels and balance performance was found. PT scores
were signicantly 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 dierent 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 signicant
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 signicant 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
signicantly, 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 inuence the postural balance
dierences between the lower limbs. Besides, the inuence 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 signicant 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).
References
Aagaard, P., Simonsen, E. B., Magnusson, S. P., Larsson, B., &
Dyhre-Poulsen, P. (1998). A new concept for isokinetic
hamstring: quadriceps muscle strength ratio. e American
journal of sports medicine, 26(2), 231-237.
Al Attar, W. S. A., Khaledi, E. H., Bakhsh, J. M., Faude, O.,
Ghulam, H., & Sanders, R. H. (2022). Injury prevention
programs that include balance training exercises reduce
ankle injury rates among soccer players: a systematic
review. Journal of physiotherapy.
Barone, R., Macaluso, F., Traina, M., Leonardi, V., Farina, F., &
Di Felice, V. (2011). Soccer players have a better standing
balance in nondominant one-legged stance. Open Access
Journal of Sports Medicine, 2, 1.
Booysen, M. J., Gradidge, P. J.-L., & Watson, E. (2015). e
relationships of eccentric strength and power with
dynamic balance in male footballers. Journal of Sports
Sciences, 33(20), 2157-2165.
Chtara, M., Rouissi, M., Bragazzi, N. L., Owen, A. L., Haddad,
M., & Chamari, K. (2018). Dynamic balance ability
in young elite soccer players: implication of isometric
strength. J Sports Med Phys Fitness, 58(4), 414-420.
Cohen, J. (1988). 1988: Statistical power analysis for the
behavioural sciences. Hillsdale, NJ: Erlbaum.
Croisier, J.-L., Forthomme, B., Namurois, M.-H.,
Vanderthommen, M., & Crielaard, J.-M. (2002). Hamstring
muscle strain recurrence and strength performance
disorders. e American journal of sports medicine, 30(2),
199-203.
Croisier, J.-L., Ganteaume, S., Binet, J., Genty, M., & Ferret, J.-M.
(2008). Strength imbalances and prevention of hamstring
injury in professional soccer players: a prospective study.
e American journal of sports medicine, 36(8), 1469-1475.
Croisier, J.-L., Réveillon, V., Ferret, J., Cotte, T., Genty, M.,
Popovic, N., Mohty, F., Faryniuk, J., Ganteaume, S., &
Crielaard, J.-M. (2003). Isokinetic assessment of knee
exors and extensors in professional soccer players.
BILATERAL ASYMMETRY IN YOUTH FOOTBALL | C. FRANÇA ET AL.
DOI 10.26773/mjssm.230905 9
Isokinetics and Exercise Science, 11(1).
Fox, K. T., Pearson, L. T., & Hicks, K. M. (2023). e eect
of lower inter-limb asymmetries on athletic performance:
A systematic review and meta-analysis. Plos One, 18(6),
e0286942.
França, C., Ihle, A., Marques, A., Sarmento, H., Martins,
F., Henriques, R., & Gouveia, É. R. (2022). Physical
Development Dierences between Professional Soccer
Players from Dierent Competitive Levels. Applied
Sciences, 12(14), 7343.
Gkrilias, P., Zavvos, A., Fousekis, K., Billis, E., Matzaroglou,
C., & Tsepis, E. (2018). Dynamic balance asymmetries in
pre-season injury-prevention screening in healthy young
soccer players using the Modied Star Excursion Balance
Test—a pilot study. Journal of physical therapy science,
30(9), 1141-1144.
González-Fernández, F. T., Martínez-Aranda, L. M., Falces-
Prieto, M., Nobari, H., & Clemente, F. M. (2022). Exploring
the Y-Balance-Test scores and inter-limb asymmetry
in soccer players: dierences between competitive level
and eld positions. BMC Sports Science, Medicine and
Rehabilitation, 14(1), 1-13.
Hart, N. H., Nimphius, S., Weber, J., Spiteri, T., Rantalainen,
T., Dobbin, M., & Newton, R. (2016). Musculoskeletal
asymmetry in football athletes: a product of limb function
over time.
Helme, M., Tee, J., Emmonds, S., & Low, C. (2021). Does
lower-limb asymmetry increase injury risk in sport? A
systematic review. Physical therapy in sport, 49, 204-213.
Iga, J., George, K., Lees, A., & Reilly, T. (2009). Cross‐sectional
investigation of indices of isokinetic leg strength in youth
soccer players and untrained individuals. Scandinavian
Journal of Medicine & Science in Sports, 19(5), 714-719.
McCurdy, K., & Langford, G. (2006). e relationship between
maximum unilateral squat strength and balance in young
adult men and women. Journal of sports science & medicine,
5(2), 282.
McLester, C. N., Nickerson, B. S., Kliszczewicz, B. M., &
McLester, J. R. (2020). Reliability and agreement of various
InBody body composition analyzers as compared to dual-
energy X-ray absorptiometry in healthy men and women.
Journal of Clinical Densitometry, 23(3), 443-450.
Menzel, H.-J., Chagas, M. H., Szmuchrowski, L. A., Araujo, S.
R., de Andrade, A. G., & de Jesus-Moraleida, F. R. (2013).
Analysis of lower limb asymmetries by isokinetic and
vertical jump tests in soccer players. e Journal of Strength
& Conditioning Research, 27(5), 1370-1377.
Muehlbauer, T., Schwiertz, G., Brueckner, D., Kiss, R., &
Panzer, S. (2019). Limb dierences in unipedal balance
performance in young male soccer players with dierent
ages. Sports, 7(1), 20.
Ruiz-Pérez, I., Ayala, F., Puerta, J. M., Elvira, J. L., De Ste Croix,
M., Hernández-Sánchez, S., & Vera-Garcia, F. J. (2019).
A Bayesian Network approach to study the relationships
between several neuromuscular performance measures
and dynamic postural control in futsal players. Plos One,
14(7), e0220065.
Rutkowska-Kucharska, A. (2020). Asymmetry of lower limb
strength and jumping ability of young soccer players. Acta
of bioengineering and biomechanics, 22(1).
Sannicandro, I., Rosa, R., De Pascalis, S., & Piccinno, A. (2012).
e determination of functional asymmetries in the lower
limbs of young soccer players using the countermovement
jump. e lower limbs asymmetry of young soccer players.
Science & Sports, 27(6), 375-377.
Scinicarelli, G., Oerhaus, C., Feodoro, B., Froboese, I., &
Wilke, C. (2022). Functional performance and interlimb
asymmetries of young football players during single-leg
jump tests. German Journal of Exercise and Sport Research,
52(1), 76-86.
Śliwowski, R., Marynowicz, J., Jadczak, Ł., Grygorowicz, M.,
Kalinowski, P., & Paillard, T. (2021). e relationships
between knee extensors/exors strength and balance
control in elite male soccer players. Peerj, 9, e12461.
Soylu, Ç., Altundağ, E., Akarçeşme, C., & Ün Yildirim, N.
(2020). e relationship between isokinetic knee exion
and extension muscle strength, jump performance,
dynamic balance and injury risk in female volleyball
players. Journal of Human Sport and Exercise 5(13), 502-
514. https://doi.org/10.14198/jhse.2020.153.03
Suchomel, T. J., Nimphius, S., & Stone, M. H. (2016). e
importance of muscular strength in athletic performance.
Sports Medicine, 46(10), 1419-1449.
Teixeira, J., Carvalho, P., Moreira, C., & Santos, R. (2014).
Isokinetic assessment of muscle imbalances and bilateral
dierences between knee extensores and exores’ strength
in basketball, footbal, handball and volleyball athletes.
International Journal of Sports Science 4(1), 1-6.
orpe, J. L., & Ebersole, K. T. (2008). Unilateral balance
performance in female collegiate soccer athletes. e
Journal of Strength & Conditioning Research, 22(5), 1429-
1433.
Van Dyk, N., Bahr, R., Whiteley, R., Tol, J. L., Kumar, B.
D., Hamilton, B., Farooq, A., & Witvrouw, E. (2016).
Hamstring and quadriceps isokinetic strength decits are
weak risk factors for hamstring strain injuries: a 4-year
cohort study. e American journal of sports medicine,
44(7), 1789-1795.
Van Tittelboom, V., Alemdaroglu-Gürbüz, I., Hanssen, B.,
Heyrman, L., Feys, H., Desloovere, K., Calders, P., & Van
den Broeck, C. (2022). Reliability of isokinetic strength
assessments of knee and hip using the Biodex System 4
dynamometer and associations with functional strength
in healthy children. Frontiers in Sports and Active Living,
4, 817216.
Wislo, U., Castagna, C., Helgerud, J., Jones, R., & Ho, J.
(2004, Jun). Strong correlation of maximal squat strength
with sprint performance and vertical jump height in elite
soccer players. Br J Sports Med, 38(3), 285-288. https://doi.
org/10.1136/bjsm.2002.002071
Yamada, R. K. F., Arliani, G. G., Almeida, G. P. L., Venturine,
A. M., Santos, C. V. d., Astur, D. C., & Cohen, M. (2012).
e eects of one-half of a soccer match on the postural
stability and functional capacity of the lower limbs in
young soccer players. Clinics, 67(12), 1361-1364.
Zakas, A. (2006). Bilateral isokinetic peak torque of quadriceps
and hamstring muscles in professional soccer players with
dominance on one or both two sides. Journal of Sports
Medicine and Physical Fitness, 46(1), 28.
... The same trend has been reported in youth soccer. In a sample of 88 male youth soccer players aged 15.9 ± 1.6 years, knee muscle strength (expressed by the PT of KE and KF) was able to explain between 18% and 22% of the variance observed in balance measures [18]. In another study, muscle strength and power measures, assessed using jumping tasks, showed medium to large correlations with balance measures [15]. ...
... Concerning knee muscle strength, body mass, and BF% were significantly and positively correlated to knee flexors and knee extensors performance (peak torque values) in male adult hockey players [20] and male youth weightlifters [21]. In youth soccer, only one study was found on the relationship between knee muscle strength, assessed through the isokinetic dynamometer and balance measures [18]. The results indicate that knee muscle strength has a positive contribution to balance outputs, with knee flexors and knee extensors explaining between 18 and 22% of the variance observed in the balance scores for the preferred leg and the non-preferred leg, respectively [18]. ...
... In youth soccer, only one study was found on the relationship between knee muscle strength, assessed through the isokinetic dynamometer and balance measures [18]. The results indicate that knee muscle strength has a positive contribution to balance outputs, with knee flexors and knee extensors explaining between 18 and 22% of the variance observed in the balance scores for the preferred leg and the non-preferred leg, respectively [18]. However, data on the influence of body composition variables is still scarce among youth soccer population. ...
Article
Full-text available
Background Muscle strength and balance abilities have been related to game performance injury prevention. This study aimed to (1) assess knee muscle strength performance based on intra- and inter-limb asymmetries, (2) investigate the relationships between knee muscle strength, body composition, and balance, and (3) analyze the variance in the overall stability index (OSI) explained by knee muscle strength indicators after controlling for age and body composition. Methods The sample included 104 young male soccer players (16.0 ± 1.6 years). Body composition, knee muscle strength, and balance ability were assessed. A paired samples t-test was used to examine differences between the preferred leg (PL) and the non-preferred leg (NPL). Pearson correlations and hierarchical multiple regression were conducted to explore the relationships between strength, body composition, and balance. Results No significant inter-limb asymmetries were detected for the knee flexors (KF) and knee extensors (KE) performance. The hamstrings-to-quadriceps ratio does not suggest the existence of intra-limb asymmetries (p = 0.06). Significant and negative correlations were found between KF (r = -0.38, p ≤ 0.01) and KE (r = -0.58, p ≤ 0.01) for the PL. Concerning the NPL, KE also revealed a significant and negative relationship with balance (r = -0.30, p ≤ 0.01). Significant and positive relationships were found between bodyweight, fat-free mass, and balance. After controlling for age and body composition, knee muscle strength could explain between 13% and 30% of the variance observed in the OSI (NPL and PL, respectively). Conclusions The results underline knee muscle strength as an important predictor of balance. Thus, training programs targeting knee muscle strength development should be considered during the season periodization by coaches and their staff throughout the players’ developmental stages.
Article
Full-text available
Inter-limb asymmetry refers to an imbalance in performance between the left and right limbs. Discrepancies throughout asymmetry research does not allow practitioners to confidently understand the effect of inter-limb asymmetries on athletic performance. Therefore, this review summarized the current literature using a meta-analytic approach, conforming to the Preferred Reporting Items for Systematic Review and Meta-Analyses guidelines to identify the association between inter-limb asymmetry and athletic performance. A literature search using PubMed, Web of Science and SPORTDiscus databases yielded 11-studies assessing the effect of inter-limb asymmetries, measured via unilateral jump performance, on bilateral jump, change of direction (COD) and sprint performance in adult sports players. The quality of evidence was assessed via a modified Downs and Black checklist and in compliance with the Grading of Recommendations Assessment Development and Evaluation. Correlation coefficients were transformed via Fishers z (Zr), meta-analysed and then re-converted to correlation coefficients. Egger’s regression presented no significant risk of bias. Vertical jump performance was not significantly affected by asymmetry (Zr = 0.053, r = 0.05; P = 0.874), whereas COD and sprint both presented significant weak associations (COD, Zr = 0.243, r = 0.24; Sprint, Zr = 0.203, r = 0.2; P < 0.01). The results demonstrate that inter-limb asymmetries seem to present a negative impact to COD and sprint performance but not vertical jump performance. Practitioners should consider implementing monitoring strategies to identify, monitor and possibly address inter-limb asymmetries, specifically for performance tests underpinned by unilateral movements such as COD and sprint performance.
Article
Full-text available
In soccer, physical development is crucial for developing optimal performance. This study aimed to assess and compare the physical development of elite and non-elite professional soccer players. Seventy-eight male professional football players divided into four competitive levels participated in this study: the elite group (EG), the non-elite group A (NEG-A), the non-elite group B (NEG-B), and the under 23 group (U23). Body composition, static strength, lower-body explosive strength, flexibility, and balance were assessed. No significant statistical differences between elite and non-elite players were seen in body composition parameters. However, the EG performed better in static strength, lower-body explosive strength, flexibility, and balance, even after adjusting for the effects of chronological age. The analysis showed that the competitive level (group) explained 25% to 29% of the variance observed in the lower-body explosive strength tasks. Sports staff and coaches in different age categories or competitive levels should include specific lower-body explosive strength content during soccer training to promote players' long-term development towards the elite level.
Article
Full-text available
Question: What is the effect of injury prevention programs that include balance training exercises on the incidence of ankle injuries among soccer players? Design: Systematic review of randomised trials with meta-analysis. Participants: Soccer players of any age, sex or competition level. Interventions: The experimental intervention was an injury prevention program that included balance training exercises. The control intervention was the soccer team's usual warm-up program. Outcome measures: Exposure-based ankle injury rates. Results: Nine articles met the inclusion criteria. The pooled results of injury prevention programs that included balance training exercises among 4,959 soccer players showed a 36% reduction in ankle injury per 1,000 hours of exposure compared to the control group with an injury risk ratio (IRR) of 0.64 (95% CI 0.54 to 0.77). The pooled results of the Fédération Internationale de Football Association (FIFA) injury prevention programs caused a 37% reduction in ankle injury (IRR 0.63, 95% CI 0.48 to 0.84) and balance-training exercises alone cause a 42% reduction in ankle injury (IRR 0.58, 95% CI 0.41 to 0.84). Conclusions: This meta-analysis demonstrates that balance exercises alone or as part of an injury prevention program decrease the risk of ankle injuries. PROSPERO CRD42017054450.
Article
Full-text available
Background The postural stability seems to be important in the physical development of the soccer player and the specific tasks related to the game. In addition, it is related to the injury risk and therefore, with the injury prevention and retraining processes. In this context, the Y Balance Test (YBT) is presented as a tool to assess dynamic postural control. Objective This study aimed to explore the differences and possible correlations in the YBT scores and inter-limb asymmetry for anterior (ANT), posteromedial (PM) and posterolateral (PL) directions by category and field position in soccer players. Methods 173 males soccer players aged between 14 and 33 years old agreed to participate. Five categories and six field position were considered in this study. A standardised protocol was used at multiple clubs during the pre-season assessment of musculoskeletal function in soccer players. All the players performed the Y Balance Test (YBT) (official YBT Kit), assessing the dominant and non-dominant leg for three YBT directions (anterior-AN, posteromedial-PM and posterolateral-PL), inter-limb asymmetry and composite score. Results For AN, amateur and semiprofessional obtained the highest values for Dominant and Non-Dominant legs (Rangemean = 101.8–109.4%) and the lowest level in PRO players (mean: 62.0%). Concerning PM-PL, semiprofessional (Rangemean = 126.4–132.7%, dominant and non-Dominant respectively), followed by professional and amateur reported higher scores compared to youth categories. Inter-limb asymmetry showed higher values in lower age categories. The best composite scores were detected in semiprofessionals (Rangemean = 113.3–126.7% for dominant and Rangemean = 113.8–129.7% for non-Dominant leg), compared with the rest of the categories and for each field position evaluated. Conclusion Comparisons between field-positions revealed that centre-backs were worse than wingers and forwards. In order to explain variations in dynamic balance between competitive levels within the same age-group, special considerations about training programmes and related co-variables should be considered.
Article
Full-text available
Background This study aimed to analyze the reliability of concentric isokinetic strength assessments (knee and hip) using the Biodex System 4 in healthy children and assess the association with functional strength tests (sit-to-stand [STS], lateral-step-up [LSU]). Methods 19 children (6–12 years) were included. Knee and hip flexion and extension, and hip abduction and adduction were tested at 60 and 90°/s. Results Relative and absolute reliability at 60°/s tended to show better results compared to those at 90°/s. Intra class correlations (ICCs) of knee flexion and extension at 60°/s were good (0.79–0.89). For hip flexion, extension, abduction and adduction at 60°/s ICCs were moderate to good (0.53–0.83). The smallest detectable change (SDC) values (expressed in %) were highly variable. The SDC% for knee flexion and extension and hip abduction at 60°/s were around 50%. Positive associations were found between hip extension and abduction isokinetic strength and the STS test. Conclusion Concentric isokinetic strength assessments in healthy children using the Biodex System 4 were found reliable for knee flexion and extension and hip abduction. Limited associations were found between concentric isokinetic strength tests and functional strength tests.
Article
Full-text available
Background Strength and balance are important factors for soccer players to be successful. This study’s aim was to determine the relationship between lower-limb muscle strength and balance control in elite male soccer players ( n = 77). Methods Concentric isokinetic strength (peak torque of quadriceps (PT-Q) and hamstrings (PT-H), hamstrings/quadriceps (H/Q) ratio) was measured for the dominant and non-dominant leg at angular velocities of 60°s ⁻¹ and 240°s ⁻¹ , as well as the total work for extensors (TW-Q) and flexors (TW-H) for both legs (at an angular velocity of 240°s ⁻¹ only). Balance score (BAL score) was used for unilateral assessment of balance control using a Delos Postural System Test measurement tool. Hierarchical multiple regression analyses were performed to predict balance control using isokinetic knee strength performance for dominant and non-dominant legs. Results Final modelling included peak torque of hamstrings at 240°s ⁻¹ and peak torque of the quadriceps at 240°s ⁻¹ for the non-dominant leg ( R ² = 19.6%; p ≤ 0.001) and only peak hamstring torque at 240°s ⁻¹ for the dominant leg ( R ² = 11.3%; p = 0.003) as significant predictors of balance score. Conclusion Findings indicate that balance control is widely influenced by peak hamstring torque and peak quadriceps torque at high angular velocity particularly in the non-dominant leg i.e., the supporting leg in soccer players.
Article
Full-text available
Background Jumps are predominant components in football (soccer). Interlimb functional difference in single-leg jump performance is a risk factor for lower extremities injuries. Screening uninjured athletes is essential to design prevention strategies and implement individual training interventions. The aims of this cross-sectional study were (1) to provide age-specific mean values and limb symmetry index (LSI) in young football players, (2) to detect age effect on LSI and interlimb functional differences and (3) to investigate the association of age with single-leg functional performance and LSI. Methods A total of 146 male football players (age 14.2 ± 2.3) performed the countermovement jump, jump for distance, side hop and speedy jump tests. Descriptive statistics, mean values (dominant/non-dominant) and LSI were provided according to age groups (U11–U19). Two-way mixed analysis of variance (ANOVA), one-way ANOVA and Pearson’s correlation were used for the statistical analysis. Results Participants showed on average perfect LSI (103.8 ± 14.2%) amongst all tests and age groups. Interlimb functional differences occurred in three out of four tests (p < 0.05), without age interaction (p > 0.05). Age effect was positively associated with single-leg functional performance (p < 0.05), but not with LSI (p > 0.05), in all tests and age groups. Conclusion An LSI ≥100% in single-leg jump tests is proposed as a benchmark in young football players, but interlimb performance differences may occur without age interaction. Nevertheless, the growth process plays a crucial role in the development of functional capacities: older players may show a higher single-leg jump performance, but not a higher LSI, than younger players. In football practice, preventive intervention is advisable to counteract interlimb performance differences, for which unilateral strength, power and plyometric training is recommended.
Article
Full-text available
ABSTRACT Objective The aim of this review was to synthesize the current understanding relating to the risks of lower body functional asymmetry with injury in athletic populations. Methods: An iterative data mining and sampling approach was used to construct a search phrase from key words, which were used to identify studies within SPORTdiscus, Medline and Academic Search Complete databases. Additional references were sourced from the reference lists of these articles Results: After screening, 31 papers were reviewed, from an initial search identifying 302 possible studies. A total of 6228 participants were involved in the studies, of which soccer players were the most observed sporting demographic (n =2171). Of the 31 studies reviewed eight found no statistical association between lower limb asymmetry and injury risk, 10 studies provided partial statistically significant evidence and a further 10 provided statistically significant statistical links. Conclusion: Overall moderate to lower quality evidence for functional asymmetry as a risk factor for injury in sport was observed. While the possibility of a relationship between lower limb functional asymmetry and injury risk cannot be ruled out, further higher quality investigations, adopting standardised methodologies, is required.
Article
Full-text available
Purpose: The aim of this study was to evaluate the potential correlation between asymmetry of lower limb muscle torque, asymmetry of vertical ground reaction force during take-off in young soccer players and their jumping abilities. Methods: Twenty-three young soccer player (16.9 ± 0.64 years old) participated in measurements. An isokinetic dynamometer, the Biodex System, was applied to test muscle torque (PT) of the knee flexors and extensors. The vertical ground reaction force (vGRF) was recorded from two Kistler plates. Jumping abilities were assessed with the horizontal (HJ) and vertical jump (VJ) tests. The asymmetry index (AI) was used to assess the asymmetry of the limbs. Results: The asymmetry index showed the highest asymmetry (over 10%) for the PT under static conditions for knee flexors and extensors. The correlation (-0.432, p = 0.038) was found between the asymmetry of vGRF and the height of the VJ. There was no correlation between the muscle torque and the height of the vertical jump. However, a correlation between the HJ length and muscle torque for flexors and extensors of the right and left lower limb was found. Conclusions: The asymmetry of the muscle torque of the flexors and extensors of the knee joint does not correlate with the results of both jumping ability tests. There was a statistically significant correlation between the vGRF asymmetry index during take-off and the height of the VJ. In the HJ, such a relationship was not found.
Article
Full-text available
In soccer, the dominant leg is frequently used for passing and kicking while standing on the non-dominant leg. Consequently, postural control in the standing leg might be superior compared to the kicking leg and is further enhanced with increasing age (i.e., level of playing experience). Unfortunately, leg differences in postural control are associated with an increased risk of injuries. Thus, we examined differences between limbs in unipedal balance performance in young soccer players at different ages. Performance in the Lower Quarter Y Balance Test (YBT-LQ) of the dominant and non-dominant leg and anthropometry was assessed in 76 young male soccer players (under-13 years [U13]: n = 19, U15: n = 14, U17: n = 21, U19: n = 22). Maximal reach distances (% leg length) and the composite scores were used for further analyses. Statistical analyses yielded no statistically significant main effects of leg or significant Leg × Age interactions, irrespective of the measure investigated. However, limb differences in the anterior reach direction were above the proposed cut-off value of >4 cm, which is indicative of increased injury risk. Further, statistically significant main effects of age were found for all investigated parameters, indicating larger reach distances in older (U19) compared to younger (U13) players (except for U15 players). Although reach differences between legs were non-significant, the value in the anterior reach direction was higher than the cut-off value of >4 cm in all age groups. This is indicative of an increased injury risk, and thus injury prevention programs should be part of the training of young soccer players.