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The Effects of Training
Interventions on Interlimb
Asymmetries: A
Systematic Review With
Meta-analysis
Francesco Bettariga, MSc,
1,2
Anthony Turner, PhD,
2
Sean Maloney, PhD,
2
Luca Maestroni, MSc,
2,3
Paul Jarvis, MSc,
2
and Chris Bishop, PhD
2
1
StudioErre, Brescia (BS), Italy;
2
London Sport Institute, School of Science and Technology, Middlesex University,
Greenlands Lane, London, United Kingdom; and
3
ReAct, Bergamo (BG), Italy
ABSTRACT
Interlimb asymmetries have been
recently investigated in athletic popu-
lations. However, the effects of training
interventions on interlimb asymmetries
have been scarcely examined. There-
fore, the aim of this study was to
determine the effects of training inter-
ventions on changes in interlimb
asymmetries from pre- to post-training.
Furthermore, to examine the effects of
training programs on intervention
groups compared with control groups.
A database search was completed
(MEDLINE, CINAHL, and SPORTDis-
cus). Eight studies were then included
in the meta-analysis. Results showed
small reductions in interlimb asymme-
tries in single-leg broad jump (SLBJ)
and change of direction (COD) speed
from pre- to post-training interventions,
whereas moderate effects were found
in single-leg countermovement jump
(SLCMJ) and SL lateral jump. When
comparing the training interventions
with the control groups, results
showed small effects in favor of the
training groups for reducing interlimb
asymmetries in SLBJ and large effects
in SLCMJ and COD speed. Thus,
training interventions can evoke small
to moderate reductions in interlimb
asymmetries from pre- to post-training
programs. Strength training performed
unilaterally or bilaterally may elicit
these reductions. Furthermore, training
interventions showed larger effects
compared with the control groups in
reducing interlimb asymmetries. How-
ever, further research is needed.
INTRODUCTION
The concept of interlimb asym-
metries has been widely investi-
gated in recent years
(4,6,16,17,63,86). By definition, inter-
limb asymmetry refers to an imbalance
or deficit between limbs (e.g., left vs.
right, dominant vs. nondominant,
healthy vs. injured) and should not be
confused with intralimb asymmetry
that refers to an imbalance within the
same limb (e.g., quadriceps-hamstring
ratio) (4,86). Although interlimb asym-
metries in physical qualities, such as
strength, power, and reactive strength,
have been shown to be associated with
reduced physical and sporting perfor-
mance, these findings are not always
consistent (17,38). Interestingly, the
correlation between interlimb asym-
metries and muscle and noncontact
injuries shows equivocal results
(39,48,49,56,60,64,76). Historically,
between-limb differences of 10–15%
have been commonly assumed to be
a meaningful interlimb asymmetry
after injuries (1,68). However, more
recently, such assumptions have been
challenged (20,53), and numerous
studies have highlighted the task-
specific nature of asymmetry
(18,62,67), meaning that an asymmetry
measured from one test should not be
expected to be the same in another test
(5). Thus, any specific threshold rela-
tive to asymmetry is challenging to
unanimously apply when using multi-
ple tests and metrics.
When focusing specifically on physical
performance, interlimb asymmetries in
power, from the single-leg counter-
movement jump (SLCMJ), have been
Address correspondence to Francesco Bet-
tariga, francescobettariga@gmail.com.
KEY WORDS:
interlimb asymmetry; training pro-
gram; physical performance
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shown to be associated with reduced
jump height and slower linear speed
and change of direction (COD) perfor-
mance in youth and adult male soccer
players (r520.51 to 20.77) (7). Sim-
ilarly, jump height and distance asym-
metries in the SLCMJ and single-leg
triple hop (SLTHOP), respectively,
were negatively associated with reduced
sprint performance (r50.49 to 0.59)
and reduced jump performance (r5
20.47 to 20.58) in youth female soccer
players (14). When focusing on different
tests, interlimb differences during the
single-leg drop jump (SLDJ) were re-
ported in adult soccer players, highlight-
ing that larger imbalances in reactive
strength were indicative of slower linear
sprint and COD performance (r50.52
to 0.66) (19). Similarly, in professional
cricket athletes, jump height and reactive
strength asymmetries during the SLDJ
were indicative of slower 505 times
(r50.56 to 0.74) however, the same
relationships were not found in profes-
sional soccer players (8). In support of
these findings, Raya-Gonzalez et al. (73)
did not find any significant relationships
between asymmetry scores (e.g., SLCMJ,
COD, mean and peak power) and sprint-
ing or jumping performance in youth
soccer players. Finally, between-limb
asymmetries in concentric power from
isoinertial crossover tests did significantly
correlate with negative performance in
COD and sprint (r50.41–0.51; p,
0.05) in youth handball players, even
though asymmetries in SLCMJ, single-
leg broad jumps (SLBJ), and single-leg
lateral jumps did not show any correla-
tion with COD and sprint performance
(58). Thus, the evidence to date seems to
be conflicting, which is supported in a
recent systematic review (17). However,
with these studies being associative in
design, we cannot truly determine any
cause and effect relationship; this can
onlybedonebyassessingchangesin
asymmetry after training interventions.
Unilateral and bilateral training inter-
ventions have been examined to
clearly elucidate the effects of such
training modalities on physical perfor-
mance (37,78). Although the volume of
training interventions reporting pre-
and post asymmetry values are rela-
tively rare, such training interventions
have been recently investigated, with a
focus on bilateral and unilateral
strength exercises, plyometrics, bal-
ance, and core training
(2,3,16,22,41,42), in a wide variety of
athletic populations. Given the eviden-
tial increase in studies conducted on
this topic in recent years, understand-
ing the effects of asymmetry on differ-
ent measures of physical performance
would be useful to determine whether
practitioners should be striving for
some level of symmetry in their ath-
letes. To the best of our knowledge,
no systematic reviews have been car-
ried out on this topic. Thus, the pri-
mary aim of this systematic review
was to examine if training interventions
are effective in reducing interlimb
asymmetries across a range of physical
qualities. The second aim was to inves-
tigate which types of training interven-
tions are more effective in reducing
these between-limb imbalances in the
athletic population.
METHODS
LITERATURE SEARCH
METHODOLOGY
A systematic search was conducted
between December 25 and December
31, 2020, in 3 different databases: MED-
LINE, CINAHL, and SPORTDiscus.
The PRISMA guidelines were used to
conduct this systematic review (Figure 1)
(70). The literature search was related to
specific search terms, combined with
“asymmetries”. In addition, truncation
function was used (i.e., “asymmetr*”) to
expand the literature search of the fol-
lowing: “training,” “intervention,”
“strength,” “jump,” “change of direction,”
“single leg,” “unilateral,” and “bilateral.”
An additional hand-made search was
carried out to find other possible articles,
consulting the reference lists of the stud-
ies selected.
ELIGIBILITY CRITERIA
Studies were included if they fulfilled
the following eligibility criteria:
(a) minimum of a 6-week training
intervention, (b) comparison groups
were applied in the study design (i.e.,
control group vs. intervention group or
intervention A vs. intervention B), (c)
adult or youth athletic populations for
both genders, (d) uninjured subjects
only, and (e) published in English.
STUDY SELECTION
One reviewer (F.B.) removed dupli-
cates, and screened titles and abstracts
for eligibility, which was then agreed
with a second reviewer (C.B.). Next,
the remaining full text articles were
included or excluded according to the
eligibility criteria.
DATA EXTRACTION
Initially, one reviewer (F.B.) indepen-
dently extracted data from the studies
included in this systematic review.
Demographic data (age and gender)
and type of sports population were ini-
tially extracted. Also, training interven-
tion modalities, physical tests used for
interlimb asymmetries, duration/fre-
quency of interventions, and the results
of the training interventions (i.e.,
means, standard deviations, sample
sizes, percentage values and effect sizes
[ES]) on interlimb asymmetries were
extracted. This information was cor-
roborated and agreed by a second
reviewer (C.B.).
METHODOLOGICAL QUALITY AND
RISK OF BIAS ASSESSMENT
To appraise study methodological
quality, the Physiotherapy Evidence
Database (PEDro) scale was used
(61). Each item aimed to assess the
level of evidence of the included stud-
ies. Studies were graded based on the
checklist items, as either: “X” 5criteria
not met or “O”5criteria satisfied. The
total score of each study included was
reported in Table 1.
Risk of bias was assessed first by cre-
ating funnel plots. This enabled for the
visualisation of the spread of the stan-
dardized effect estimates, relative to
their standard error. Follow-up analy-
sis (i.e., both qualitative and quantita-
tive analysis of the funnel plots) was
only conducted where the number of
entries within the analysis was equal to
or exceeded 10 (79), and in these cir-
cumstances, we conducted Egger’s
The Effects of Training Interventions
VOLUME 00 | NUMBER 00 | JANUARY 2022
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regression test (p,0.05 indicating the
presence of asymmetry and thus
potential risk of small study bias)
(35). This analysis was conducted using
jamovi (jamovi, Version 1.6.23.0), an
open-source statistical software, which
is developed on top of the R statistical
language.
STATISTICAL ANALYSIS
Meta-analysis. Review Manager
(RevMan 5.2; Cochrane Collaboration,
Oxford, United Kingdom) and Micro-
soft Excel (Microsoft Office 16; Micro-
soft Corporation, 2018) were used to
carry out the meta-analysis. Effect
sizes (Hedges g) of the same interlimb
asymmetry test (e.g., jump height from
the SLCMJ) before and after training
interventions were calculated as the
standardized mean difference (SMD)
with mean 6SD and 95% confidence
intervals (47). The calculation of Hed-
ges gwas computed using the current
formula (26):
g5Mean post 2Mean pre
SD pooled :
Additionally, articles that reported
the same interlimb asymmetry tests
comparing an intervention versus a
control group were analyzed, as men-
tioned above. The Cohen scale was
used to interpret ESs, where ,0.2 5
trivial, 0.20–0.49 5small, 0.50–0.79 5
moderate, and .0.8 5large (25).
Subgroup analyses. Subgroup
analyses were performed using the sub-
group analysis function in Review Man-
ager (RevMan 5.2; Cochrane
Collaboration, Oxford, United King-
dom). First, analyses were conducted
comparing the asymmetry test scores
before and after the training programs
in the intervention groups only. Second,
The SMD was calculated as the differ-
ence from pre- to post-intervention for
each variable in both groups (i.e., inter-
vention vs. control). Owing to the fact
that analyses were conducted to exam-
ine percentage changes, the SD was set
as the pretesting score, in line with
recent suggestions (13). If articles re-
ported symmetry scores, percentage val-
ues were converted to asymmetry scores,
noting that this does not affect the
SD value.
Heterogeneity. Heterogeneity
between studies was evaluated with I
2
statistics (relative heterogeneity) (50) and
between-study variance with the
tau-square (t
2
)(absolute heterogeneity)
(51). The magnitude of heterogeneity
for results was classified according to
Figure 1. Flow diagram showing the identification and selection of studies on the available body of literature for the current review.
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the following scale: ,25% 5low, 25–
75% 5medium, and .75% 5high (50).
If the pvalueforthechi-squarewas,0.1,
this indicated the presence of heteroge-
neity, with a t
2
value of .1suggestingthe
presence of substantial statistical hetero-
geneity (51). A pvalue of ,0.1 indicated
whether statistically significant subgroup
differences were present (65,75).
RESULTS
STUDY DESCRIPTION
The initial search strategy provided
30,735 articles plus 2 additional articles
were identified after consulting the refer-
ence lists of the articles selected for final
analysis. After removing duplicates, and
filtering articles through appropriate
sports-related journals (a function that
can be applied in the databases), the final
search provided 3,212 articles.
Following the initial searches, 3,031
articles were excluded after reviewing
the title. After the evaluation of 181
abstracts, 160 articles were excluded.
Finally, the full-text manuscripts of the
remaining 21 studies were carefully as-
sessed. Eleven articles were excluded
because they did not meet the inclusion
criteria; thus, 10 studies fulfilled the
eligibility criteria and were therefore
included in the final analysis. Two stud-
ies were ruled out because the interlimb
asymmetry test was not comparable to
other studies (i.e., single-leg hamstring
bridge performed at failure and incre-
mental unilateral squat) (44,74). Eight
studies were determined to be selected
for the meta-analysis. Subsequently,
studies were grouped based on the in-
terlimb asymmetry test reported (e.g.,
SLCMJ), to examine all studies investi-
gating the effects of training interven-
tions on asymmetry, for any given
test. Following this, studies using an
intervention versus a control group
were also compared for the same inter-
limb asymmetry tests. Study design, in-
terlimb asymmetry tests and metrics,
training interventions, results, and qual-
ity scoring data are reported in Table 2.
META-ANALYSIS
Single-leg broad jump. When
investigating the effects of different
training interventions on SLBJ asymme-
try scores, 10 studies were found (Fig-
ure 2). Medium levels of heterogeneity
were observed among studies (I
2
554%;
t
2
50.17). Compared with the
pre-training intervention, the asymmetry
index in SLBJ decreased with a small
effect after the training protocols with
no significant difference between the 2
time points (ES: 0.22; 95% CI, 20.12
to 0.57).
When investigating the effects of train-
ing interventions compared with the
control groups, 5 studies were found
(Figure 3). Medium levels of heteroge-
neity were observed among studies (I
2
544%; t
2
50.12). Examining the
results, the asymmetry index showed
a small reduction in favor of the train-
ing intervention groups with no signif-
icant difference between the 2 groups
(ES: 0.37; 95% CI, 20.10 to 0.83).
Single-leg countermovement
jump. When investigating the effects
of different training interventions on
SLCMJ asymmetry scores, 8 studies
were found (Figure 4). High levels of
heterogeneity were observed among
studies (I
2
584%; t
2
50.76). Com-
pared with the pretraining interven-
tion, the asymmetry index in SLCMJ
decreased with a moderate effect after
the training protocols with no signifi-
cant difference between the 2 time
Table 1
PEDro score of each study
PEDro scale Item 1 Item 2 Item 3 Item 4 Item 5 Item 6 Item 7 Item 8 Item 9 Item 10 Item 11 Total score
Sannicandro et al.
(2014)
XOXOXXXXOO O 5
Dello Iacono et al.
(2016)
OOXOXXXXOO O 6
Madruga-Perera
et al. (2020)
OOXOXXXXOO O 6
Hammami et al.
(2020)
XOOOXXXXOO O 6
Gonzalo-Skok et al.
(2019)
XOOOXXXXOO O 6
Pardos-Mainer et al.
(2019)
XOOOXXXXOO O 6
Gonzalo-Skok et al.
(2016)
XOXOXXXXOO O 5
Pardos-Mainer et al.
(2020)
OOXOXXXXOO O 6
The Effects of Training Interventions
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Table 2
Summary of studies that have used training interventions on interlimb asymmetries
References Subjects and
study design
Tests and metrics Training intervention Results
Sannicandro
et al.
(2014)
Tennis players (n
523), mean
age 513 y
RCT (n511
balance Tr gr;
n512 control
gr)
SLBJ: Jump
distance (cm)
SL lateral jump:
Jump distance
(cm)
6 weeks (2 sessions per week)
Balance gr: skipping (6 35), single leg bound (4
36), forward bound (3 34), low rows on
inflatable disk, medicine ball chest pass (4 3
10), lateral raise on BOSU ball (3 310), balance
on skimmy cushion (2 34), two-legged
successive jump (4 34), step-up on BOSU ball
(4 310), squat on unstable surface (3 310),
step-up on BOSU ball (4 38)
AI Balance gr Control gr
SLBJ 9.0 63.6% to 3.7
61.5% (p,
0.05)
9.0 63.7% to 9.3 65.2%
(p.0.05)
SL lateral
jump
10.8 65.9% to 3.2
61.4% (p,
0.05)
13.2 610.1% to 13.0 6
8.1% (p.0.05)
Dello Iacono
et al.
(2015)
Male soccer
players (n5
20), mean age
519 y
RCT (n510 core
stability gr; n
510 control
gr)
SLCMJ: Peak
vertical ground
reaction force
6 weeks (5 sessions per week)
Core stability gr: seated torso + eccentric phase,
kneeling superman exercise, hip extension,
Nordic hamstring, walking lunge (3 310),
frontal balance, lateral balance, sprint and stop
in delimitated space (3 34)
AI Core stability gr Control gr
SLCMJ 5.4 60.11% to 1.6
60.2% (ES 5
2.01; p,0.05)
4.8 60.3% to 7.2 60.1%
(ES 51.28; p,0.05)
Madruga-
Parera et
al. (2020)
Male handball
players (n5
20), mean age
516 y
RCT (n517
isoinertial gr; n
517 cable-
resistance gr)
SLBJ: Jump
distance (cm)
SLCMJ: Jump
height (cm)
SL lateral jump:
Jump distance
(cm)
1808COD:
seconds
8 weeks (2 sessions per week)
Isoinertial and cable-resistance gr: forward lunge,
lateral squat, lateral lunge, single-leg hop,
acceleration (3 312 RPE 6/8), SLCMJ, crossover
step, acceleration, 1808turn (3 38 RPE 8/9)
AI Isoinertial gr Cable-resistance gr
SLBJ 5.97 65.29% to
7.07 65.84% (ES
50.15; p.0.05)
7.97 63.95% to 5.03 6
4.16% (ES 520.40;
p.0.05)
SLCMJ 14.81 68.82% to
6.05 62.93% (ES
520.70; p,
0.05)
9.44 67.66% to 6.79 6
3.80% (ES 520.32;
p,0.05)
SL lateral
jump
7.55 67.38% to
5.27 63.53% (ES
520.34; p.
0.05)
8.18 68.69% to 8.15 6
5.53% (ES 520.00;
p.0.05)
COD 1.96 61.81% to
2.51 61.56% (ES
50.16; p.0.05)
2.88 62.21% to 2.82 6
2.19% (ES 520.02;
p.0.05)
(continued)
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Table 2
(continued)
Hammami et
al. (2020)
Weightlifters (n
520), mean
age 511 y
RCT (n510
eccentric gr; n
510 control
gr)
SLBJ: Jump
distance (cm)
6 weeks (2 sessions per week)
Eccentric gr: eccentric glute-hamstring raise,
glute-hamstring rise, single-leg Romanian
deadlift, hip thrust, good morning (from 3 310
60%1RM to 5 312 80%1RM)
SI Eccentric gr Control gr
SLBJ 105.1 66.8% to
103.9 63.8% (p
.0.05)
106.1 67.3% to 106 6
5.1% (p.0.05)
Gonzalo-
Skok et al.
(2019)
Male soccer
players (n5
45), mean age
515 y
RCT (n515
same vol both
legs starting
with weaker
[SVW]; n515
Double vol
weaker leg
starting with
weaker [DVW];
n515 same
vol both legs
starting with
stronger [SVS])
SLBJ: Jump
distance (cm)
SLCMJ: Jump
height (cm)
10 weeks (1 session per week)
All gr: eccentric unilateral lateral squat (from 6
repetitions speed:force ratio 1 to 10 repetitions
speed:force ratio 3)
AI SVW DVW SVS
SLBJ 4.3 63.4% to 4.0
62.7% (ES 5
0.06)
3.3 62.8%
to 4.2 6
1.7% (ES
520.51)
3.4 64.2%
to 5.8 6
5.5% (ES
520.58)
SLCMJ 10.9 69.8% to 6.4
64.1% (ES 5
0.23)
8.3 67.9%
to 6.2 6
3.7% (ES
50.08)
6.8 65.4%
to 5.0 6
4.6% (ES
50.24)
Pardos-
Mainter et
al. (2020)
Female soccer
players (n5
37), mean age
516 y
RCT (n519
strength and
power gr; n5
18 control gr)
SLBJ: Jump
distance (m)
SLCMJ: Jump
height (cm)
1808COD:
seconds
8 weeks (2 sessions per week)
Strength and power gr: the diver, single-leg box
step-up, forward lunge, backward lunge (from
6to832 10%BW), one-leg hip thrust,
eccentric box drops, Russian belt posterior and
anterior chain (from 6 to 10 32), plank, lateral
plank, lumbar bridge (from 15 to 20 s)
AI Strength and
power gr
Control gr
SLBJ 6.32 65.41% to
5.26 65.08% (ES
520.26; p.
0.05)
4.09 63.93% to 4.48
63.33% (ES 50.07;
p.0.05)
SLCMJ 11.5 68.99% to
11.5 611.3% (ES
520.21; p.
0.05)
12.5 610.2% to 15.7
610.1% (ES 50.22;
p.0.05)
COD 4.42 63.14% to
2.55 61.93% (ES
520.30; p.
0.05)
2.62 62.29% to 2.14
61.55% (ES 520.07;
p.0.05)
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VOLUME 00 | NUMBER 00 | JANUARY 2022
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points (ES: 0.53; 95% CI, 20.16
to 1.23).
When investigating the effects of train-
ing interventions compared with the
control groups, 3 studies were found
(Figure 5). High levels of heterogeneity
were observed among studies (I
2
5
94%; t
2
53.38). Examining the results,
the asymmetry index showed a large
reduction in favor of the training inter-
vention groups with no significant dif-
ference between the 2 groups (ES: 1.78;
95% CI, 20.72 to 4.28).
Single-leg lateral lump. When
investigating the effects of different
training interventions on single-leg lat-
eral jump asymmetry scores, 3 studies
were found (Figure 6). High levels of
heterogeneity were observed among
studies (I
2
574%; t
2
50.44). Com-
pared with the pre-training interven-
tion, the asymmetry index in
single-leg lateral jump decreased with
a moderate effect after the training pro-
tocols with no significant difference
between the 2 time points (ES: 0.62;
95% CI, 20.25 to 1.50).
When investigating the effects of train-
ing interventions compared with the
control groups (Figure 7), no studies
were found.
Change of direction speed.
When investigating the effects of dif-
ferent training interventions on COD
speed asymmetry scores, 4 studies
were found (Figure 7). Medium levels
of heterogeneity were observed among
studies (I
2
545%; t
2
50.09). Com-
pared with the pre-training interven-
tion, the asymmetry index in COD
decreased with a small effect after the
training protocols with no significant
difference between the 2 time points
(ES: 0.23; 95% CI, 20.22 to 0.68).
When investigating the effects of train-
ing interventions compared with the
control groups, 2 studies were found
(Figure 8). Medium levels of heteroge-
neity were observed among studies (I
2
549%; t
2
50.12). Examining the
results, the asymmetry index showed
a large reduction in favor of the training
intervention groups with a significant
Table 2
(continued)
Gonzalo-
Skok et al.
(2016)
Male basketball
players (n5
22), mean age
516 y
RCT (n511
power gr; n5
11 control gr)
SLBJ: Jump
distance (cm)
6 weeks (2 sessions per week)
Power gr: leg press using maximal power (5 35)
SI Power gr Control gr
SLBJ 94.3 63.6% to
95.9 62.3% (ES
50.39)
94.9 64.4% to 95.5 64.3%
(ES 50.13)
Pardos-
Mainer et
al. (2019)
Female soccer
players (n5
36), mean age
512 y
RCT (n519
intervention
gr; n517
control gr)
SLBJ: Jump
distance (m)
SLCMJ: Jump
height (cm)
1808COD:
seconds
10 weeks (2 sessions per week)
Intervention gr (FIFA11+): plank, lateral plank (15
s x2), chest pass, Nordic hamstring exercise,
forward bend, figure-of-eight, jump over line
(10 32), zig-zag shuffle, bounding
AI FIFA11+ gr Control gr
SLBJ 92.9 64.74% to
94.3 65.09% (ES
50.26; p.0.05)
91.9 66.37% to 93.5
65.31% (ES 50.15;
p.0.05)
SLCMJ 90.0 65.46% to
87.3 68.67% (ES
520.62; p.
0.05)
93.1 64.35% to 91.3
66.87% (ES 520.76;
p.0.05)
COD 96.8 61.84% to
97.7 61.79% (ES
50.49; p.0.05)
97.7 62.13% to 96.2 6
2.36% (ES 520.44;
p.0.05)
AI 5asymmetry index; cm 5centimeter; COD 5change of direction; ES 5effect size; Gr 5group; m 5meter; p 5p value; RCT 5randomized controlled trial; RM 5repetition maximum;
RPE 5rate of perceived exertion; SI 5symmetry index; SL 5single leg; SLBJ 5single-leg broad jump; SLCMJ 5single-leg countermovement jump.
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difference between the 2 groups (ES:
0.82; 95% CI, 0.15–1.50).
METHODOLOGICAL QUALITY AND
RISK OF BIAS ASSESSMENT
The PEDro scale scores for the
included studies within the meta-
analyses ranged from 5 to 6 (mean 6
SD: 5.75 60.46) (Table 1). These
scores indicate that studies ranged in
quality from fair to good.
Funnel plots are presented in Figure 9
(for all analyses with greater than 2 data
points). Risk of small study bias (includ-
ing publication bias) as denoted by Eg-
ger’s regression test indicated the
prevalence of small study bias for SLBJ
(p50.037). Because of an insufficient
number of data entries for the remaining
analyses, we were unable to explicitly
conduct follow-up analysis and thus draw
inferences from the funnel plots concern-
ing potential risk of small study bias.
DISCUSSION
The main aim of this systematic review
was to examine if training interventions
are effective in reducing interlimb asym-
metries across different physical quali-
ties. The second aim was to investigate
which types of training interventions are
more effective in reducing these
between-limb imbalances. Results sug-
gested that training interventions have
small to moderate effects on the reduc-
tion of asymmetry in SLBJ, SLCMJ,
COD speed, and SL lateral jump indexes
from pre- to post-training; however, no
statistically significant differences were
reported. Furthermore, a small to large
reduction in interlimb asymmetry
indexes was found in the intervention
groups compared with the control
groups, with statistically significant dif-
ference found only in COD speed. How-
ever, caution should be taken when
interpreting these findings (owing to
the potential risk of small study/publica-
tion bias as a result of the small sample
of studies included). Readers should note
2 important points in relation to this
section: (a) changes in asymmetry are
a result of changes in raw scores that
create the asymmetry value in the first
instance; thus, we have also discussed
changes in asymmetry relative to the test
in question and (b) many studies used
more than 1 test in their methodology.
However, because we have divided this
section up by test type (as per the
Figure 2. Effects of different training interventions on single-leg broad jump asymmetry index.
Figure 3. Comparison of intervention versus control groups on single-leg broad jump asymmetry index.
The Effects of Training Interventions
VOLUME 00 | NUMBER 00 | JANUARY 2022
8
Copyright © National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.
statistics used in the meta-analysis), we
have summarized the associated meth-
ods for each study at the earliest oppor-
tunity but do refer back to the same
study in subsequent sections.
SINGLE-LEG BROAD JUMP
Pre and post intervention. The
findings of the present meta-analysis
suggest that collectively, training inter-
ventions have a small effect in reducing
distance asymmetry values in the SLBJ
test from pre- to post-training interven-
tion, yet no statistically significant dif-
ference was found (ES: 0.22; 95%
CI, 20.12 to 0.57) (Figure 2). In the
study by Sannicandro et al. (77), they
used a combination of bilateral and uni-
lateral bodyweight and plyometric exer-
cises, twice a week for 6 weeks, which
showed large reductions in distance
asymmetries (ES: 1.85). However, and
somewhat surprisingly, it should be
noted that the raw broad jump scores
were not provided, impeding any
possible conclusion regarding improve-
ments in unilateral jump performance.
Importantly though, no significant
improvements were evident in linear
or COD speed times (p.0.05) for
either the intervention or control group.
Thus, despite large reductions in dis-
tance asymmetry, this did not translate
to improvements in independent mea-
sures of performance, which may be
because of the training age of the par-
ticipants (i.e., 12 years) and the duration
of the intervention (i.e., 6 weeks).
Despite these limitations, this does
question the relevance of perfect sym-
metry for enhanced athletic perfor-
mance, a concept which has been
suggested in a recent longitudinal asso-
ciative study (12).
Gonzalo-Skok et al. (43) used a com-
bination of bilateral strength and
power exercises, twice a week for 6
weeks. They found a moderate reduc-
tion in distance asymmetries (ES: 0.51)
and moderate improvements in the
raw jump scores after the training pro-
gram (ES: 0.64–0.65). Additionally,
findings revealed small to moderate
improvements in repeated sprint ability
(RSA) (ES: 0.49), repeated COD per-
formance (ES: 0.60), and DJ followed
by 2 hops for distance (measuring jump
distance only) (ES: 0.23–2.29) from
pre- to post-training intervention,
whereas the control groups showed
negligible or even small harmful effects
in such tests (i.e., RSA (ES: 20.03),
repeated COD (ES: 20.9), and DJ
(ES: 20.11 to 20.04)). Therefore, it
seems that reductions in interlimb
asymmetries were coupled with
improvements in the raw jump scores,
compared with the control groups.
However, the intensity of the training
intervention (i.e., RPE or %RM), which
was not reported, the duration of the
training program (i.e., session lasted
10–20 minutes), and the age of the
participants (i.e., 16-years of age) limit
the confidence in the results given.
Figure 4. Effects of different training interventions on single-leg countermovement jump asymmetry index.
Figure 5. Comparison of intervention versus control group on single-leg countermovement jump asymmetry index.
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In the study led by Pardos-Mainer
et al. (71), they adopted a strength
training program based on core and
lower limbs exercises, twice a week
for 8 weeks, finding small effects in
reducing between-limb imbalances
(ES: 0.20). This may be explained by
the fact that the raw broad jump scores
indicated significant improvements on-
ly on the left leg (ES: 0.58) after the
training intervention. Interestingly,
despite the fact that modifications on
asymmetries were small, linear sprints
(i.e., 10, 20, 30, 40-m linear sprint)
(ES: 21.29 to 21.02), V-Cut
(ES: 20.58), bilateral broad jump (BJ)
(ES: 0.29), and countermovement
jump (CMJ) (ES: 0.55) performances
were significantly improved after the
training intervention. Interestingly,
interaction analysis between the inter-
vention and control groups showed
significant changes in the strength
and power training group in linear
sprints, apart from 10 m (ES: 21.30
to 21.16) and V-Cut (ES: 20.62).
The small effects in asymmetries may
be attributed to the intensity selected
to accomplish the training program,
which was 10% of the BW and the fact
that the population was based on ado-
lescent female soccer players (i.e., 16
years). However, given the reductions
in asymmetry were only small, it seems
likely that any improvements in phys-
ical performance were independent of
any changes in asymmetry.
Pardos-Mainer et al. (72) also exam-
ined the effects of the FIFA11+
warm-up protocol, performed twice a
week for 10 weeks. Results showed
small non-significant reductions in
hop distance asymmetries (ES: 0.28)
after the training protocol (calculated
from the SLBJ test), even though the
raw broad jump scores significantly
improved between time points (ES:
0.34 to 0.40). This means that improve-
ments in horizontal jumps did not
translate to substantial reduction in
asymmetries. Interestingly, the inter-
vention did not yield improvements
in bilateral BJ scores (ES: 0.07) and
even induced a significant negative per-
formance in the V-Cut test (ES: 0.59).
However, DJ (ES: 0.61) and CMJ (ES:
0.48) performances were significantly
improved. Thus, these controversial
results may be because of the training
age of participants (i.e., 12 years).
Indeed, youth respond differently com-
pared with adults to the training stim-
uli, based on the age of growth
maturation (57). Moreover, the current
literature reports that the FIFA11+
protocol appears efficient in reducing
injuries, but the effects on asymmetries
and performance are still limited (21).
Conflicting results in distance asym-
metries (ES: 20.48 to 0.10) were found
in the eccentric training protocols con-
ducted by Gonzalo-Skok et al. (45),
where different volumes of training
were applied to the weaker or stronger
legs, previously identified. The training
protocol consisted of lateral squats
using flywheel technology only and
was performed once per week for 10
weeks. It should be noted that the raw
broad jump scores reported trivial to
small changes across groups
(ES: 20.21 to 0.32), and the only inde-
pendent test used to assess physical
performance was the bilateral CMJ,
which reported small improvements
across the groups (ES: 0.27 to 0.48).
Thus, and once again, the link between
reductions in asymmetry and improve-
ments in the tests responsible for the
asymmetry index seems tenuous.
Although a true explanation for this
is challenging, it is likely that the use
of a single exercise, performed only
once per week, may have not been
enough to elicit larger improvements
in jump performance and/or greater
reductions in asymmetry. Thus, future
Figure 6. Effects of different training interventions on single-leg lateral jump asymmetry index.
Figure 7. Effects of different training interventions on change of direction speed asymmetry index.
The Effects of Training Interventions
VOLUME 00 | NUMBER 00 | JANUARY 2022
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research should consider the efficacy of
such training interventions, when aim-
ing to concurrently enhance physical
performance and impact interlimb
asymmetry.
Hammami et al. (46) used a strength
training program, twice a week for 6
weeks, which showed small effects in
reducing distance asymmetries (ES:
0.21). The authors did not calculate
the pre- to post-interaction of raw
broad jump scores, limiting any possi-
ble consideration concerning the actual
changes in single-leg jump scores.
Interestingly, significant improvements
in linear sprints (i.e., 10 and 30 m), agil-
ity, and BJ (p,0.05) were evident,
although no ES values were reported
for these improvements. However,
results should be interpreted with cau-
tion because of the age of participants
(i.e., 11 years), which arguably would
have had a low training age and there-
fore are likely to improve in testing
from most forms of training. Further-
more, given the age of the participants,
this impedes any possible translation to
adults (57).
Finally, Madruga-Perera et al. (59),
used an 8-week training intervention
performed twice a week in handball
players, finding trivial to moderate
effects in reducing distance asymme-
tries in the isoinertial (ES: 0.15) and
cable-resistance (ES: 20.40) training
groups. Despite this, both groups
improved significantly their raw broad
jump scores (ES: 1.13 to 1.67). Despite
this, athletic performance tests (i.e., V-
Cut (ES: 20.74 to 20.15) and repeated
COD (ES: 21.35 to 20.22)) signifi-
cantly improved, in both groups,
whereas 20-m linear sprint showed
no meaningful changes (ES: 20.30 to
0.24). When considering the efficacy of
the training program, it is worth noting
that the intervention started with sim-
ple COD maneuvers but then got more
complex because the intervention went
on by integrating the use of sport-
specific handball movements (in an
attempt to replicate game demands).
Additionally, velocity of a handball
throw was used to determine the effec-
tiveness of the training interventions in
a more sport-specific test, which
showed significant improvements in
throwing velocity (ES: 0.88) but for
the isoinertial group only. Thus, with
some athletic performance tests show-
ing meaningful improvements, the
sport-specific test shows improve-
ments and no clear changes in asym-
metry; the link between reductions in
asymmetry and athletic performance
seems to be minimal.
Intervention Versus control
groups. When examining the effects
of different training interventions
compared with control groups for
the SLBJ asymmetries, the findings
suggest a small effect in reducing the
magnitude of imbalance in favor of
the intervention groups, yet no statis-
tically significant difference was found
(ES: 0.37; 95% CI, 20.10 to 0.83) (Fig-
ure 3). However, findings may be dis-
torted by the results reported by
Sannicandro et al. (77) because of
the large effect reported (ES: 1.48).
Surprisingly, 2 studies did not calcu-
late or report the raw broad jump
scores (46,77), limiting our under-
standing of the effects on the raw jump
data. For the remaining studies, either
no significant or small improvements
in the raw broad jump scores were
foundinthecontrolgroups
(43,71,72). Thus, it appears that train-
ing interventions likely have greater
effects on the raw jump scores, which
create the asymmetry value compared
with the control group. However, as
previously discussed, the efficacy of
some of the training programs could
be questioned. As a final point for con-
sideration, horizontal hopping tests
(which report hop distance only) were
recently considered a poor indicator
for horizontal jumping performance
(54). Therefore, more research is
required to effectively examine the
strategy of horizontal jumps to pro-
vide a more in-depth understanding
of how the jump is performed and sub-
sequently the interlimb differences of
these strategy metrics.
SINGLE-LEG
COUNTERMOVEMENT JUMP
Pre and post intervention. The
findings of the present meta-analysis
suggest that collectively, training inter-
ventions have a moderate effect in
reducing height or vertical ground reac-
tion force asymmetry values in the
SLCMJ test from pre- to post-training
intervention, yet no statistically signifi-
cant difference was found [ES: 0.53;
95% CI, 20.16 to 1.23) (Figure 4).
In the study by Gonzalo-Skok et al.
(45), eccentric training protocols
(described in previous section) induced
small to moderate changes in jump
height asymmetry (ES: 0.33 to 0.58)
for all 3 groups (e.g., 10.9–6.4%; 8.3–
6.2%; and 6.8–5.0%). These relatively
small changes can likely be explained
by only trivial to moderate changes in
raw jump scores after the training
protocol (ES: 0.07 to 0.82). In addi-
tion, the analysis of physical
Figure 8. Comparison of intervention versus control group on change of direction speed asymmetry index.
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Figure 9. Contour-enhanced funnel plots displaying the spread of the standardized effect estimates, relative to their standard error.
The Effects of Training Interventions
VOLUME 00 | NUMBER 00 | JANUARY 2022
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performance, using the CMJ, showed
small improvements after the training
protocol (ES: 0.27 to 0.48). There-
fore, it seems that such improvements
in CMJ may be influenced by a reduc-
tion in asymmetries, given the simi-
larity of movement. However, it is
still questionable the transfer from
the lateral squats to the vertical jump-
ing actions.
Madruga-Perera et al. (59) reported
small to large reductions in jump height
asymmetries when comparing the isoi-
nertial (ES: 1.30) and cable-resistance
(ES: 0.43) training intervention. Interest-
ingly, the raw jump scores significantly
improved between time points (ES: 0.74
to 1.41) in both groups. Therefore, it
should be plausible to presume that the
training intervention selected, stimulated
more adaptations in vertical jumping
compared with horizontal jumps (as out-
lined in the previous section). Moreover,
significant improvements were found in
both groups in V-Cut (ES: 20.74
to 20.15) and repeated COD tests
(ES: 21.35 to 20.22), although no sig-
nificant changes were found in 20-m lin-
ear sprint (ES: 20.30 to 0.24). Therefore,
it should be investigated why linear
sprint was not influenced by the
enhanced symmetry index in vertical
jumping actions, although COD tests
showed positive adaptations.
In the study by Dello Iacono et al. (31),
they used a combination of core, sprint,
and strength exercises (e.g., hip exten-
sion, Nordics, and lunges), finding an
extremely large effect (ES: 22.55) in
reducing asymmetries in vertical
ground reaction force. Surprisingly
though, the raw jump scores were
not reported, and the very low SD rel-
ative to the mean asymmetry data is also
surprising, given what previous research
has reported (5,7,8,14,18,19,39). Interest-
ingly, the authors also examined the
effects of the training intervention to
reduce asymmetries on isokinetic peak
torque in flex/ext 1.05 rad $s
21
(ES:
0.61 to 0.75) and flex/ext 3.14 rad $
s
21
(ES: 0.71 to 0.95), revealing signifi-
cant improvements after the training
protocol. However, it is strange that this
study was classified as a “core training
intervention,” given the inclusion of the
lower-body exercises, which are likely to
have had a much greater effect on
changing raw scores and asymmetry val-
ues, than any core-based exercise, owing
to the fact that the current literature has
shown a negligible effect of core stability
on athletic performance (24).
Somewhat surprisingly, Pardos-Mainer
et al. (72) reported a small increase in
jump height asymmetry (ES: 20.36),
when the FIFA11+ protocol was adop-
ted. This may be attributed to the fact
that the raw jump scores showed sig-
nificant improvements only on the left
leg (ES: 0.54), which likely explains the
increase in the interlimb imbalance.
Interestingly, even though asymme-
tries increased, the bilateral vertical
jump tests (i.e., DJ (ES: 0.61) and
CMJ (ES: 0.48)), mentioned above,
showed significant improvements
between time points, whereas no sig-
nificant changes were found in hori-
zontal jumps (i.e., BJ) (ES: 0.07) or
even a negative significant perfor-
mance in V-Cut test (ES: 0.59). Thus,
it seems that an increase in asymme-
tries may not be related to any reduc-
tions in vertical jump performance,
when performed bilaterally, as shown
recently (12).
In the study by Pardos-Mainer et al. (71),
they examined the effects of a combined
strength and power training intervention
on jump height asymmetry, and no
effects (ES: 0.00) were found after the
training. The findings of the raw jump
scores revealed a significant improve-
ment only on the right leg (ES: 0.58).
Therefore, it can be assumed that such
a training intervention is not sufficient at
driving muscular modifications to reduce
asymmetries in vertical jumps. In fact,
10% of body mass is probably not
enough to promote substantial changes
(80,81).However,andsurprisingly,all
the physical tests conducted (mentioned
above), namely, 10, 20, 30, 40-m linear
sprints (ES: 21.29 to 21.02), V-Cut (ES:
20.58), BJ (ES: 0.29), and CMJ (ES:
0.55) showed significant improvements
after the training intervention. Thus, no
significant changes in jump height asym-
metries appears not to be associated with
changes in physical performance. There-
fore, it is still arguable to establish if
reductions in asymmetries are essential
to improve athletic performance.
Intervention versus control
groups. When examining the results
of different training interventions com-
pared with control groups for the
SLCMJ asymmetries, the findings sug-
gest a large effect in reducing the mag-
nitude of asymmetry in favor of the
intervention group, yet no statistically
significant difference was found (ES:
1.78; 95% CI, 20.72 to 4.28)] (Figure 5).
However, this finding should be inter-
preted with caution because of the
extremely large effects found in Dello
Iacono et al. (31), which have likely
skewed the results. Moreover, the lack
of the raw jump scores in the control
group makes a true comparison hard to
do. Interestingly, the literature is not
conclusive regarding the benefits of
core training on athletic performance
(24). The other studies (71,72) showed
that the raw jump scores of the control
groups did not change significantly
(p.0.05), confirming therefore that
when muscular stimuli were applied,
reductions in between-limb imbalances
occurred. Therefore, it is plausible to
assume that training interventions that
use isoinertial or flywheel technology
can elicit reductions in vertical jump
asymmetries.
SINGLE-LEG LATERAL JUMP
Pre and post intervention. The
findings of the present meta-analysis
suggest that collectively, training inter-
ventions have a moderate effect in
reducing distance asymmetry values
in the SL lateral jump from pre- to
post-training intervention, yet no sta-
tistically significant difference was
found (ES: 0.62; 95% CI 20.25 to
1.50) (Figure 6).
In the aforementioned study by
Madruga-Perera et al. (59) led on hand-
ball players, they found trivial to small
reductions (ES: 0.00 to 0.38) in dis-
tance asymmetries based on the afore-
mentioned isoinertial or cable-
resistance training programs.
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Interestingly, the findings revealed that
the raw scores significantly improved
only in the non-dominant leg (ES: 0.34
to 0.49) in both groups after the train-
ing intervention. Additionally, the V-
Cut (ES: 20.74 to 20.15) and repeated
COD (ES: 21.35 to 20.22) signifi-
cantly improved, whereas 20-m linear
sprint did not improve (ES: 20.30 to
0.24). Despite the limitations regarding
the age of participants (i.e., 16 years)
and the training program selected (i.e.,
mix of exercises using or simulating ball
in the hands), it appears that reduction
in asymmetries in SL lateral jumps may
have an impact on COD performance.
Although a somewhat anecdotal expla-
nation, it is likely that the notion of
specificity plays a part here, given that
these jumps occur in the frontal plane
and COD movements also have fre-
quent movement patterns in this direc-
tion (34).
Sannicandro et al. (77) found large
effects (ES: 1.71) in reducing distance
asymmetries. However, as discussed
above, the raw jump scores were not
reported, impeding any possible con-
clusion regarding the improvements
in lateral jumps. Furthermore, and con-
trary to the previous study, the physical
tests conducted (i.e., 10, 20-m linear
sprint and Foran test) did not show
any significant change from pre- to
post-training intervention (p.0.05).
Thus, although the study included
bodyweight and plyometrics exercises,
in addition to balance training (per-
formed in the frontal plane), which ap-
peared to be enough to stimulate
meaningful reductions in interlimb
asymmetries, it was not enough to
drive positive adaptations in linear or
COD speed tests, which is arguably
more important for a sport like ten-
nis (55).
CHANGE OF DIRECTION SPEED
Pre and post intervention. The
findings of the present meta-analysis
suggest that collectively, training inter-
ventions have a small effect in reducing
time asymmetry values in the COD
speed test from pre- to post-training
intervention, yet no statistically
significant difference was found (ES:
0.23; 95% CI 20.22 to 0.68) (Figure 7).
In the study by Pardos-Mainer et al.
(72), previously described, they re-
ported small changes in reducing time
asymmetries (ES: 0.49) after the train-
ing program based on the FIFA11+
protocol. Surprisingly, despite the
reductions in asymmetries, the raw
COD scores showed an increase in
time (i.e., slower performance),
although these changes were not sta-
tistically significant (ES: 20.01 to 0.32),
as well as the V-Cut test, which did
reveal a significant increase in time
(i.e., slower performance) (ES: 0.59).
In addition, although the COD speed
tests showed a reduction in perfor-
mance, the vertical jumps tests con-
ducted (i.e., DJ (ES: 0.61) and CMJ
(ES: 0.48)) significantly improved.
Likely, the training intervention, previ-
ously described (i.e., bodyweight in
nature), did not elicit such modifica-
tions to promote changes in COD
actions, which based off previous liter-
ature, is likely to require greater levels
of muscular strength (27) and improve-
ments in technique (34).
Moderate effects in reducing time
asymmetries (ES: 0.70) were also
found by Pardos-Mainer et al. (71),
adopting a strength and power training
program (previously described). In
contrast to the previous study, the
raw COD scores showed significant
improvements (i.e., faster performance)
only on the left leg (ES: 20.52). Con-
sequently, 10, 20, 30, 40-m linear sprint
(ES: 21.29 to 21.02), V-Cut
(ES: 20.58), BJ (ES: 0.29), and CMJ
(ES: 0.55) showed significant improve-
ments from pre- to post-training inter-
vention. In this case, a reduction in
asymmetries did translate to improve-
ments in physical performance. The
reasons may be attributed to the mus-
cle stimuli given by performing sprints,
DJs, and hip thrust exercises; which are
determinants in promoting muscle
adaptations during COD movements
(69). However, it should be acknowl-
edged that minimum load (i.e., 10%
body weight) was provided during
strength exercises. Thus, it is still
questionable if reductions in time
asymmetries have a consistent impact
on physical performance.
In the aforementioned study by
Madruga-Perera et al. (59), a trivial
effect was found in the cable-
resistance group (ES: 0.03) in reducing
time asymmetry, and a small effect in
increasing asymmetries for time during
COD tests in the isoinertial group
(ES: 20.32). Surprisingly, the raw
COD scores revealed a significant
improvement (i.e., faster performance)
(ES: 20.83 to 20.65) from pre- to
post-training intervention in both
groups. Additionally, V-Cut
(ES: 20.74 to 20.15) and repeated
COD (ES: 21.35 to 20.22) signifi-
cantly improved, whereas 20-m linear
sprint did not improve significantly
(ES: 20.30 to 0.24). These results sug-
gest that improvements in athletic per-
formance did not strictly correlate with
reductions in asymmetries. Therefore,
which are the determinants to reduce
interlimb asymmetries in COD speed
and which are the determinants to
improve performance in COD require
further research. Although somewhat
anecdotal, it is feasible that the training
protocol did not stimulate eccentric
peak force and cutting technique,
which are determinants in reducing
COD speed asymmetries (23,30,32).
Intervention versus control
groups. When examining the results
of different training interventions com-
pared with control groups for the
COD speed asymmetries, there is a
large significant effect in reducing the
asymmetry index in favor of the train-
ing intervention group (ES: 0.82; 95%
CI, 0.15, 1.50) (Figure 8). This may be
explained by the muscular stimuli
occurred in the training interventions
compared with the control groups (71)
but also by the fact that no significant
changes were evident in the raw scores
for the control groups (p.0.05). Inter-
estingly, despite the large effects found
by Pardos-Mainer et al. (72), both the
intervention and the control group did
not significantly improve their raw
COD scores (p.0.05), indicating that
The Effects of Training Interventions
VOLUME 00 | NUMBER 00 | JANUARY 2022
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asymmetry changes are independent to
changes in raw score performance.
Therefore, future research should
investigate if reductions in asymmetries
translate to improvements in COD
speed performance. Furthermore, it
should be acknowledged that the met-
ric of total time in COD speed tests is
not overly sensitive in detecting
between-limb imbalances because of
the movement variability to complete
such a physical test (9). Therefore, the
use of COD speed as interlimb asym-
metry tests should be analyzed care-
fully, especially if only using the
outcome measure of time (33). In con-
trast, time can still be considered a sta-
ble and reliable measure if monitoring
performance during COD tasks (19).
PRACTICAL RECOMMENDATIONS,
DIRECTIONS FOR FUTURE
RESEARCH, AND LIMITATIONS
Practical recommendations.
With regard to interlimb asymmetry
changes from pre- to post-training
and the comparison between interven-
tion and control groups, the current
analysis indicates that large standard
deviations are often evident when
compared with the mean asymmetry
scores (Table 2). Therefore, the inher-
ent variability found in interlimb asym-
metry and some equivocal results
reported may be explained by this.
Simply put, with such large within-
and between-group variation in asym-
metry often present, it is likely that this
precluded any meaningful differences
from being reported between time
points (5). Consequently, with asym-
metry evidently being less “sensitive
to change” than raw test scores, it does
question the usability of this as a metric
during the ongoing monitoring process
(10,20). Indeed, it is worth noting that
interlimb asymmetry raw scores did
not show significant changes consis-
tently. For this reason, it is difficult to
find substantial change in asymmetry
indexes. However, if practitioners still
believe that asymmetry can inform the
decision-making process, then an indi-
vidual analysis is likely needed to deter-
mine true change and avoid the noise
associated with group mean
asymmetry data (5). In fact, interlimb
asymmetries would be considered
meaningful only if the changes of the
scores are greater than the pre-testing
individual coefficient of variation (CV)
value, which measures the errors in the
physical tests (5).
Moreover, it should be acknowledged
that changes in interlimb asymmetry
scores from pre- to post-training inter-
ventions were not consistently associ-
ated with significant improvements in
such physical tests. From a perfor-
mance perspective, owing to the con-
flicting results obtained, it is still
unclear whether reductions in inter-
limb asymmetries may have a signifi-
cant impact in physical or sporting
performance and is something that
future research should endeavour to
investigate.
Directions for future research.
First, further research should focus on
alternative study designs. For example,
(a) increasing the sample size and
recruit homogeneous populations (i.e.,
youth, adults, male subjects, female sub-
jects) in well-designed randomized con-
trolled trials (82), (b) using control
groups to compare efficacy of training
interventions, (c) adopting consistent
and reliable metrics and physical tests
to measure interlimb asymmetries (15),
(d) more long-term interventions with
consistent repeated measures over time
(12), (e) examining whether changes in
interlimb asymmetries are associated
with changes in physical performance,
and (f) also analyzing the direction of
asymmetries (11,52,63) to determine if
fluctuations in limb dominance are evi-
dent, which has been noted in recent
test-retest design research (18,52).
From a training intervention perspective,
researchers should be encouraged to
carefully select the intensity and volume
of training interventions to promote spe-
cific muscular adaptations (e.g., strength
and power training) (28,29,66,80,81,83–
85). For instance, Sannicandro et al. (77)
and Dello Iacono et al. (31) reported sets
and repetitions of training interventions.
However, both articles did not state the
intensity used (i.e., RPE, RIR, %RM, or
BW), impeding therefore, our ability to
establish accurately if the adequate mus-
cle stimuli were provided. Similarly,
Pardos-Mainer et al. (72) adopted the
FIFA11+ protocol, based on bodyweight
exercises, which is probably insufficient
to drive significant muscular strength
gains to modify interlimb asymmetries.
Indeed, it is worth noting that muscular
adaptations occur only if the stimuli are
correctly administered (40). This may be
a reason for no significant changes in the
raw asymmetry scores and, likely, in
some of the physical performance tests
used. Thus, changes in interlimb asym-
metries without any significant changes
in raw physical performance scores may
be considered questionable.
Finally, all the studies included in this
systematic review adopted outcome mea-
sures when reporting asymmetry (e.g.,
jump height or distance). However, it is
worth noting that such metrics are not
always sensitive to change when detect-
ing side-to-side differences (15). In addi-
tion, and to use jump tests as an example,
outcome measures provide no informa-
tion on jump strategy (i.e., how the jump
was performed). In contrast, metrics such
as peak or mean force, and propulsive or
braking impulse have been shown to pro-
vide useful information for practitioners
(15,86) and may be more useful at mon-
itoring discrete changes in asymmetry,
although further research is needed to
fully corroborate this suggestion.
Limitations. This review is not with-
out limitations. Owing to the paucity of
intervention studies on this topic that
used a control group, practitioners
should be mindful of the true “cause
and effect” relationships being inferred
from studies, which only report
within-group differences. This is re-
flected in the moderate to large cases
of heterogeneity observed, and the pres-
ence of potential small study bias (inclu-
sive of publication bias) in our findings.
However, it is important to recognize
that including a control group in elite
or professional sport settings can be con-
sidered a luxury, and it is unlikely to be
available. This does not mean that the
research is not worth conducting, but it
Strength and Conditioning Journal | www.nsca-scj.com 15
Copyright © National Strength and Conditioning Association. Unauthorized reproduction of this article is prohibited.
does mean that practitioners may wish
to consider alternative methods of estab-
lishing whether changes in asymmetry
are meaningful. As such, the previous
literature has highlighted the importance
of measuring the CV alongside any re-
ported interlimb differences (36). Simply
put, if the between-limb imbalance is
greater than the test variability (CV),
the magnitude of asymmetry can be con-
sidered “real” (36). Consequently, if such
analyses identified persistent real asym-
metries to be present each time testing
was undertaken, it may provide useful
information on whether targeted training
interventions were needed to enhance
the weaker limb’s capacity (10,63).
PRACTICAL APPLICATIONS
This systematic review showed that the
effects of training interventions on inter-
limb asymmetries in sports population
are controversial. Cumulatively, it appears
that different training protocols have a
small effect in reducing interlimb asym-
metries in the SLBJ and the COD speed,
whereas a moderate effect has been
found in the SLCMJ and the SL lateral
jump after a minimum of a 6-week train-
ing intervention. However, it should be
acknowledged that small to large effects
in reducing between-limb imbalances
were found in favor of the training groups
compared with the control groups. Sim-
ply put, the data from this systematic
review pointed out that training interven-
tions can reduce interlimb asymmetries
to a certain extent only, and presence of
potential small study bias (inclusive of
publication bias) should also be noted.
Therefore, it is suggested that if practi-
tioners wish to decrease interlimb asym-
metries, unilateral and bilateral training
intervention should be considered. How-
ever, further research is required to exam-
ine the effects of interlimb asymmetry
reductions on physical performance.
DECLARATIONS
Conflicts of Interest and Source of Funding:
The authors report no conflicts of interest
and no source of funding.
Data availability statement: The data
that support the findings of this study
are available from the corresponding
author upon reasonable request.
Francesco
Bettariga is a
postgraduate
physiotherapist,
who completed
the MSc in
Strength and
Conditioning at
Middlesex
University.
Anthony
Turner is an
associate profes-
sor in Strength
and Conditioning
and the Research
Degrees Coordi-
nator for sport at
the London Sport
Institute, Mid-
dlesex University.
Sean Maloney
owns Maloney
Performance and
works as a visit-
ing lecturer in
strength and
conditioning for
organisations,
including Mid-
dlesex University
and the University of Bedfordshire.
Luca Maestroni
is a sports phys-
iotherapist cur-
rently working in
Italy and PhD
candidate at the
London Sport
Institute, Mid-
dlesex University.
Paul Jarvis is a
PhD student and
associate lecturer
in Sport &
Exercise Science
at the London
Sport Institute,
Middlesex
University.
Chris Bishop is
a senior lecturer
in Strength and
Conditioning at
the London Sport
Institute, Mid-
dlesex University,
where he is the
program leader
for the MSc in
Strength and Conditioning.
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