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Biology of Sport, Vol. 39 No3, 2022 485
Effects of unilateral vs. bilateral training on performance
INTRODUCTION
Resistance training is one of the most widely used methods of en-
hancing athletic performance[1–3]. One of the challenging problems
faced by practitioners is the issue of how to optimally choose the
exercises to maximize training effects when prescribing resistance
training programs. Typically, exercises in the weight room can be
categorized into either unilateral or bilateral. Aunilateral exercise is
aweight bearing movement mainly or completely involving one limb
(e.g. single leg squat, Bulgarian split squat and single leg jump),
whereas, abilateral exercise is aweight bearing movement executed
evenly and simultaneously by both limbs (e.g. back squat, deadlift
and countermovement jump). Traditionally, bilateral exercises are
selected as the primary exercises for athletic development[4, 5] due
to their effects on improving strength and power[6–9]. In contrast,
Effects of unilateral vs. bilateral resistance training interventions
on measures of strength, jump, linear and change of direction
speed: asystematic review and meta-analysis
AUTHORS: Kai-Fang Liao1,2, George P. Nassis3,4, Chris Bishop5, Wei Yang1, Chao Bian1,
Yong-MingLi1,6
1School of Physical Education and Sport Training, Shanghai University of Sport, Shanghai, China
2 Department of Strength and Conditioning, Guangdong Vocational Institute of Sport, Guangzhou, China
3 Physical Education Department, College of Education (CEDU), United Arab Emirates University, Al Ain, Abu
Dhabi, United Arab Emirates
4 Department of Sports Science and Clinical Biomechanics, Faculty of Health Sciences, SDU Sport and Health
Sciences Cluster, University of Southern Denmark, Odense, Denmark
5 School of Science and Technology, London Sport Institute, Middlesex University, London, UK
6 China Institute of Sport Science, Beijing, China
ABSTRACT: Exercises can be categorized into either unilateral or bilateral movements. Despite the topic
popularity, the answer to the question as to which (unilateral or bilateral) is superior for a certain athletic
performance enhancement remains unclear. To compare the effect of unilateral and bilateral resistance training
interventions on measures of athletic performance. Keywords related with
unilateral
,
bilateral
and
performance
were used to search in the Web of Science, PubMed databases, and Google Scholar and ResearchGate™
websites. 6365articles were initially identied, 14 met the inclusion criteria and were included in the nal
analysis, with overall article quality being deemed
moderate
. The quantitative analysis comprised 392subjects
(aged: 16 to 26years). Sub-group analysis showed that unilateral exercise resistance training resulted in alarge
effect in improving unilateral jump performance compared to bilateral training (ES = 0.89 [0.52, 1.26]). In
contrast, bilateral exercise resistance training showed asmall effect in improving bilateral strength compared
to unilateral (ES=-0.43[-0.71, -0.14]). Non-signicant differences were found in improving unilateral strength
(ES=0.26[-0.03, 0.55]), bilateral jump performance (ES = -0.04 [-0.31, 0.23]), change of direction (COD)
(ES=0.31[-0.01, 0.63]) and speed (ES = -0.12 [-0.46, 0.21]) performance. Unilateral resistance training
exercises should be chosen for improving unilateral jumping performance, and bilateral resistance training
exercises should be chosen for improving bilateral strength performance.
CITATION: Liao KF, Nassis GP, Bishop C et al. Effects of unilateral vs. bilateral resistance training interventions
on measures of strength, jump, linear and change of direction speed: asystematic review and
meta-analysis Biol Sport. 2022;39(3):485–497.
Received: 2021-01-11; Reviewed: 2021-03-21; Re-submitted: 2021-05-15; Accepted: 2021-05-21; Published: 2021-07-03.
unilateral exercises have been commonly considered to be ‘more
supplementary’ for injury prevention[10]. However, many key sport-
specic skills involved with the basic lower-body movements (e.g.
running, changing direction, jumping, kicking) are executed com-
pletely or predominantly unilaterally. Under the specicity of training
adaptation and to maximize the transfer of training[11], it could be
argued that unilateral exercises similar to sport-specic skills, may
be the best choice to improve athletic performance[10, 12], and
prioritized as akey exercise in such training programs. But the answer
to the question of which one (unilateral vs. bilateral) is better for
athletic performance enhancement remains unclear.
Specicity of training exercises is crucial for transference of train-
ing-induced adaptations to the target performance[13]. Young[5]
Review Paper
DOI: https://doi.org/10.5114/biolsport.2022.107024
Key words:
Exercise selection
Unilateral exercises
Bilateral exercises
Performance
Specicity
Corresponding author:
Yongming LI
1School of Physical Education
& Sport Training, Shanghai
University of Sport,
Changhai Road 399, Yangpu
District, Shanghai 200438,
P.R.China
2China Institute of Sport
Science, Beijing 100061, China
Fax: +86 (0) 21 65507122
Tel.: +86 (0) 21 65507122
E-mail: liyongming@sus.edu.cn
ORCID:
Kai-Fang Liao
0000-0001-6893-1491
George P. Nassis
0000-0003-2953-3911
Chris Bishop
0000-0002-1505-1287
Wei Yang
0000-0002-1564-6862
Chao Bian
0000-0002-0451-7459
Yong-Ming Li
0000-0001-9654-0199
486
Kai-Fang Liao et al.
studies have found no differences between methods[25, 31]. There
is also confusion about which method is superior for the improvement
of change of direction (COD) speed performance, with some favoring
unilateral training methods[32] and others favoring bilateral meth-
ods[26]. Furthermore, there is also conicting evidence on the effects
of these training methods on bilateral strength, jump and speed
performance[24–26, 28, 31, 32]. Recently, ameta-analysis carried
by Moran etal.,[33] concluded that there was no difference between
the effect of unilateral and bilateral resistance training on horizontal
movement speed (ES=0.17, p=0.30), but noted that the effect
size was pooled by 7short sprints, 2CODs, 1ve alternated leg
bounding and 1stair climb outcomes from 11included studies.
Given this review focused purely on the effects on horizontal speed,
it is still unclear how both bilateral and unilateral exercises transfer
to other key physical attributes, such as strength, jumping and COD
speed. Therefore, the purpose of this systematic review and meta-
analysis aimed to compare the effects of unilateral vs. bilateral re-
sistance training on improving athletic performance. The hypothesis
was that the effect of unilateral and bilateral resistance training would
follow the principle of specicity. In other words, the unilateral resis-
tance training would be better for improving the unilateral perfor-
proposed that exercise should be as specic as possible to optimize
the transfer of training. In addition, Bosch[14] proposed that intra-
muscular and inter-muscular coordination, outer movement resem-
blance and energy production are the key factors to evaluate and
predict the specicity of the training methods. Acompelling body of
empirical evidence also supports exercise type specicity with regards
to the range of motions, velocities, postures, and patterns[13, 15–18].
Actually, except for the obvious mechanical differences, unilateral
and bilateral exercises also differ in intra- and inter-muscular aspects
such as interhemispheric mutual activation[19], postural stabili-
ty[20], relationship of force and velocity[21], psychological state[22]
and lumbar load[23]. Based on those distinctions, unilateral and
bilateral resistance training are expected not to transfer equally.
According to the training principle of specicity, unilateral resis-
tance training should improve unilateral performance measures bet-
ter compared with bilateral resistance training, and vice versa. How-
ever, current ndings are conicting with respect to which is better
for the improvement of measures of athletic performance[24–30].
Some studies support the notion that unilateral resistance training
(e.g. Bulgarian split squat) improves unilateral strength more than
bilateral exercise training (e.g. back squat)[24, 26], and some
FIG. 1. Flow chart illustrating the different phases of the search and study selection
Biology of Sport, Vol. 39 No3, 2022
487
Effects of unilateral vs. bilateral training on performance
mance, and the bilateral resistance training would be better for im-
proving the bilateral performance.
MATERIALS AND METHODS
Literature research
This systematic review was conducted under the Preferred Reporting
Items for Systematic Reviews and Meta-Analysis Protocol (PRISMA).
One author searched the related articles from Web of Science
(1980–2020) and PubMed (1949 to 2020). The following keywords
inclusive of three main terms as unilateral, bilateral and performance
were used and combined under Boolean’s language with the opera-
tors AND and OR. Term 1: unilateral, split squat, single leg, one
leg, step up, lunge, Bulgarian split squat. Term 2: bilateral, back
squat, deadlift, double leg, two legs, hang clean, hang snatch.
Term 3: performance, strength, resistance, speed, power, jump,
agility, change of direction (COD), endurance. If studies were not
available in relevant electronic databases, then further searches were
conducted in Google Scholar and Research Gate™ websites. Finally,
additional studies were identied by checking the reference list of
the selected articles. The nal search date for literature was October1,
2020.
Inclusion Criteria
Studies were eligible if they met the following criteria: (1) imple-
mented both unilateral and bilateral resistance training interventions;
(2) the duration of training was longer than 4weeks; (3) the training
intensity was moderate to heavy; (4) included healthy participants
aged from 16 to 40years old in both genders; (5) had measured
athletic performance (Speed, strength, COD, power test etc.) before
and after the training intervention; (6) presented full data (mean
andSD) that allowed effect sizes to be calculated; (7) the manuscripts
were written in English and were published in apeer-reviewed journal.
Literature selection
The rst author imported all records into Endnote software (X9.3.3)
and deleted any duplicates. Then, the rst and fourth authors checked
the title and abstract separately to exclude any unrelated articles,
with the remaining full texts screened against the inclusion criteria.
There was no disagreement between the 2authors on that aspect.
Risk of bias assessment
The fourth and fth authors independently assessed the selected
studies. In case of disagreement on certain item scores, the item
scores would be given after discussion. Considering the most risk of
bias assessment scales such as Delphi scale, PEDro scale and Co-
chrane scale are designed for medical research, studies about train-
ing interventions usually get very low score under these method-
ological scales[34]. We preferred the scale (Table1) modied by
Brughelli et al[34] and Hooren etal.[35]. This scale is deemed
more suitable for sport science research, and includes 10items, with
each item rated as: 0=clearly no/not reported, 1 =maybe, and
2=clearly yes. The articles were rated poor with a total score
lower than 10, moderate with ascore between 10 and 15, good
with ascore>15, and excellent with ascore equal to 20.
Coding of the studies
The rst author extracted the data from the selected literatures with
astandard table. The code included: (1) participants: age, gender,
identity and training experience; (2) interventions: frequency, dura-
tion, exercises, intensity and volume; (3) measurements and results:
pre and post test outcome, with means and standard deviation.
Statistical analysis
The review manager software (5.3) was used for the meta-analytic
comparison if more than one outcome were evaluated for acertain
kind athletic performance measure between unilateral and bilateral
resistance training (e.g. Strength, speed), subgroup analyses were
performed.
Chi2 and I2 were calculated to test the heterogeneity. For I2 values
of 25, 50, and 75% represent low, medium, and high heterogeneity,
respectively[36]. For Chi2 with large value and p<0.1 show evi-
dences of heterogeneity. If p>0.1 and I2<50%, the xed effects
model was applied. Otherwise, the random effects model was applied
and provoked further investigation through a subgroup analysis of
moderator variables (Training experience, identity, gender, training
frequency, training modalities, training weeks). In order to identify
the presence of highly inuential studies, asensitivity analysis was
executed by removing one study at atime. Studies were considered
as inuential if removal resulted in achange of heterogeneity (p) from
signicance (p<0. 1) to non-signicance (p>0.1).
TABLE 1. Risk of bias assessment scale[34, 35]
No. Items Scores
1Clear inclusion criteria 0–2
2Clear description of the participants’ training
experience
0–2
3Random allocation of the participants to groups 0–2
4 Clearly dened intervention 0–2
5 Similarity test at baseline for all groups 0–2
6 Use of acontrol group that did not perform
resistance training
0–2
7 Clearly dened outcome variables 0–2
8 Adequate familiarization period 0–2
9 Appropriate between-group statistical analysis 0–2
10 Point measures of variability 0–2
Total 0–20
488
Kai-Fang Liao et al.
moderate,≥0.8 as large. Values are reported with 95% condence
intervals to describe the range of the true effect. If the absolute
value of aggregated effect and 95% condence interval are above
zero, effect size can be considered as clear evidence: Apositive effect
size indicated that the effect of unilateral exercises training was bet-
ter for the improvement of athletic performance than bilateral exercise
training, and anegative effect size indicates the opposite, i.e. bilat-
eral better than unilateral.
Given that unilateral athletic performance tests in included stud-
ies are generally divided into the left and right leg separately, and
5of included studies have used multiple measures of the same
athletic performance such as 10m sprint, 20m sprint for speed
performance (table 2), which may introduce statistical dependency
into the meta-analytic data set and enlarge the type Ierror due to
the same participants contributing to two or more effect sizes[39],
Standardized mean differences (SMD) was calculated with the
following algorithm[37]:
[(Mpost,unilateral– Mpre, unilateral)
– (Mpost,bilateral– Mpre, bilateral)/pooled SDpre]
This algorithm was selected as it has been recommended for
effect size calculation of independent pre-/post- study designs in
meta-analysis based on simulation results. The algorithm in speed
and COD outcomes was adjusted as[Mpre – Mpost], of which the
smaller values represent better results compared with other outcomes.
SMD was used because the studies all evaluate the same outcome
but test it in various methods such as maximum power of leg press
(n.m) and 1RM of leg press (kg) in strength measures. The absolute
values of effect sizes were rated with the following criterion given by
Cohen[38]: <0.2 as trivial, 0.2–0.49 as small, 0.5–0.79 as
TABLE 2. Multiplicity of included studies
Study Athletic performance Multiplicity of Outcome Selected for
meta-Analysis
Botton etal. [29] Bilateral strength Isokinetic knee extension
Knee extension 1RM √
Unilateral strength Isokinetic knee extension
Knee extension 1RM √
Ramirez-Campillo
etal. [40]
Bilateral strength Knee extension 1RM √
Knee exion 1RM
Unilateral Jump Countermovement jump √
Horizontal crossover triple jumps
Horizontal triple jumps
Squat jump
Bilateral Jump Countermovement jump √
Squat jump
Horizontal jump
Horizontal triple jumps
Stern etal. [25] Unilateral Jump Drop jump
Countermovement jump √
Horizontal jump
Bilateral Jump Drop jump
Countermovement jump √
Speed 10m √
30m
Gonzalo-Skok. [41] Unilateral jump Countermovement jump √
Lateral jump
Horizontal jump
Change of Direction 10m shuttle √
20m shuttle
25m shuttle
Speed 10m √
20m
25m
Javier Nunez et al.
[42]
Change of Direction 5m–90º–5m
5m shuttle √
Biology of Sport, Vol. 39 No3, 2022
489
Effects of unilateral vs. bilateral training on performance
we selected asingle most relevant effect size to deal with the mul-
tiplicity according to adecision rule as following: 1) the most used
test in included studies; 2) the right leg test; 3) in accordance with
training practice: a) multiple joint movements>Single joint move-
ments; b) Dynamic movements>isometric movements/isokinetic
movements; 4) shorter links with the true athletic performance: Jump-
ing performance: single jump>multiple jumps; change of direction:
preference to the greater angle and shorter sprint distance.
RESULTS
Search results
The initial search resulted in 6365records. After excluding 2791
duplicates, 3574 studies were selected to be screened by title and
abstract. When applying the inclusion criteria, 3498 papers were
subsequently excluded. The remaining 76articles were read in full,
with 62rejected because they were acute studies (n=21), not
including both unilateral and bilateral training groups (n=19), re-
views (n=10), participants with an age outside the criteria bound-
aries (n=5), not English (n=3), not peer-reviewed (n=3), mul-
tiple publication (n=1). This left 14studies to be included in the
nal analysis.
Risk of bias assessment
In accordance with the modied scale[34, 35], the scores of 14in-
cluded articles ranged from 12 to 18, the mean score was 15.
Therefore, the overall quality was moderate. The majority of the
studies were rated as moderate (n=9studies[24, 27, 29–32, 40,
42, 43]), and 5studies were rated as good[25, 26, 28, 41, 44].
13studies had allocated the participants in arandom manner. Most
of the studies got high score in item 4and 7, but only 28.6% of the
studies used acontrol group. 78.6% of the studies did not clearly
describe the inclusion criteria and training experience of the partici-
pants (Table3).
Studies’ Characteristics
The total number of subjects was 392 (Table4) and the participants’
age ranged from 16 to 26 years. Nine studies solely included
males[24–26, 28, 32, 40–42, 44]. Three studies included both
males and females[27, 31, 43], and 2 studies included females
only[29, 30]. All male athletes[24–26, 28, 32, 40, 41] were from
team sports, including soccer[25, 32, 40, 41], basketball[24, 26]
and rugby[28]. Twelve studies had training frequency of twice per
week[24–32, 40–42], and two studies had performed training
3times per week[43, 44]. The training duration ranged from 5to
12weeks.
The training modalities can be categorized into resistance training
and jump training. Among them, 8studies purely utilized resistance
training as the intervention modality[24, 26, 28, 29, 41–44], 1study
purely utilized jump training[30], 5studies combined resistance
with jumping training exercises together[25, 27, 31, 32, 40]. 3kinds
of muscle actions were applied among the included studies, 11stud-
ies used concentric overload training; 2studies used eccentric over-
load training[41, 42]; 1study used isokinetic training[43].
The unilateral multi-joint resistance exercises included step
up[26,31], Bulgarian split squat[24, 25, 28, 31, 32], single leg
deadlift[44], lunge and eccentric lunge[31, 41, 42]. Unilateral
single-joint resistance exercises included single knee extension and
exion[27, 29, 40, 43]. Bilateral multi-joint resistance exercises
included back squat [24–26, 28], deadlift [44], eccentric
squat[41, 42] and front squat[31]. Single-joint exercises included
knee extension and exion[27, 29, 40, 43]. Jump exercises in-
cluded squat jump, broad jump, countermovement jump and differ-
ent variants of box jumps.
The intensity of the resistance training ranged from 45–90% 1RM,
and the volume, which was equal in both groups, ranged from 3to
15repetitions. The intensity of jumps was maximum without exter-
nal loading, with repetitions and sets ranging from 3to 15 and 1to8,
respectively.
The outcome indicators of the 14included studies were various.
The strength-related tests included: 1) 1RM test of back
squat[25, 26, 28, 31, 44], Bulgarian split squat[25, 28, 31, 44],
step up[26], knee extension[29, 40], knee exion[40] and leg
press[27]; 2) maximal power test of 10s Wingate[30], Margaria-
kalamen stair-climb test[31], isokinetic knee extension[43], back
squat and Bulgarian split squat[24]. The jump-related tests (unilat-
eral and bilateral) included squat jump and countermovement
jump[24, 27, 30, 31, 40–42], broad jump [25, 41], drop
jump[25, 27, 40], 3steps and 5 steps jump [30, 40]. The
COD-related tests included the 50° test[26], the Vtest[24], 180°
shuttle[24, 41, 42], the pro-agility test[28], the Ttest[32, 40],
the 505 test[25] and the Illinois test[32]. The speed-related tests
included 10m sprint running[25, 28, 32, 41], 20m sprint run-
ning[26, 41], 25m[24, 41] and 40m sprint running[25, 28].
After dealing with the multiplicity, the nal included outcomes for
analysis were showed in table 2.
TABLE 3. The results of risk of bias assessment
Studies Items
1 2 3 4 5 6 7 8 9 10 Score
Bogdanis etal.[27] 1 2 2 2 0 0 2 2 2 0 13
Appleby etal.[26] 1 1 2 2 2 2 2 2 2 2 18
Gonzalo-Skok etal.[24] 1 2 1 2 0 0 2 0 2 2 12
Speirs etal.[28] 2 2 2 2 2 0 2 2 2 2 18
Botton etal.[29] 1 0 2 2 0 2 2 1 2 2 14
Fisher etal.[32] 1 2 2 2 1 0 2 0 2 2 14
Makaruk etal.[30] 1 2 2 2 2 2 2 0 2 0 15
McCurdy etal.[31] 1 2 2 2 2 0 2 2 2 0 15
Taniguchi etal.[43] 1 0 0 2 1 2 2 1 2 1 13
Ramirez-Campillo etal.[40] 1 2 2 2 1 0 2 1 2 2 15
Stern etal.[25] 1 2 2 2 2 0 2 1 2 2 16
Krajewski etal.[44] 2 2 2 2 2 0 2 1 2 2 17
Javier Nunez et al.[42] 1 1 0 1 2 0 2 2 2 1 12
Gonzalo-Skok etal.[41] 2 2 2 2 2 0 2 2 2 2 18
490
Kai-Fang Liao et al.
TABLE 4. The characteristics of the included studies
Study
Participants Training Program Outcome Measure
Identity
Training
Experience
(years)
Age
(years) Gender N
Frequency
(times/
week)
Weeks Sets Reps Intensity
(%1RM)
Unilateral
Exercises
Bilateral
exercises Performance test
Bogdanis
etal.[27]
College
students None 19.8
±2.9 F/M 15 2 6 2–3
3–4
10
3–8 60–90%
6 Jumps U
KE U
KF U
6 Jumps
B
KE B
KF B
CMJ U
CMJ B
Iso-LP BMP
Iso-LP UMP
DJ B
Gonzalo-Skok
etal.[24]
Basketball
Players >2 16.9
±2.1 M 22 2 6 3 >10%
MP
80–100%
MP Bul squat Back
squat
Back squat MP
Bul Squat MP
CMJ B
25m Sprint
Vtest
15m shuttle
Botton
etal.[29]
Healthy
active
females
None 24.8
±1.4 F43 2 12 2–4 5–15 5–15RM KE U KE B
KE B1RM
KE U1RM
Iso-KE BMP
Iso-KE UMP
Makaruk
etal.[30]
College
students None 20.6
±1.3 M 54 2 12 2–8 4–15 5 Jumps U 5 Jumps
B
10sWingate
SJ U
SJ B
Ramirez-Campillo
etal.[40]
Soccer
Players >2 17.6
±0.5 M 18 2 8 1–2
3
3–5
10 70%
4 Jumps U
KE U
KF B
4 jumps
B
KE B
KF B
KE B1RM
KF B1RM
Ttest
CMJ B
CMJ U
SJ B
SJ U
H3J
HCMJ
H3MJ
Fisher etal.
[32]
Soccer
Players >2 19.8
±1.5 M 20 2 6 1–3
3
6–10
6 80%
5 Jumps U
Bul squat
5 Jumps
B
Back
squat
T test
Illinois test
10m sprint
Appleby
etal.[26]
Basketball
Players >2 22.4
±4.1 M 33 2 6 6–8 4–8 45–85% Step up Back
squat
Back squat 1RM
Step up 1RM
20m sprint
Speirs
etal.[28]
Rugby
Players >1 18.1
±0.5 M 18 2 5 4 3–6 >75% Bul squat Back
squat
Bul squat 1RM
40m sprint
Pro test
Krajewski
etal.[44]
Healthy
active
males
no 26.4
±5.5 M 15 3 4 3 4–6 60–90% Bul squat
Deadlift U
CMJ U
Back
squat
Deadlift
CMJ
Back squat 1RM
Bul squat 1RM
Javier Nunez
etal.[42]
Healthy
active
males
-22.8
±2.9 M 27 2 6 4 7 0.05–0.1
kg/m2ECC lunge ECC
Squat
CMJ
5m shuttle
5m 90º 5m
10m sprint
Gonzalo-Skok
etal.[41]
Football
Players 1–3 20.5
±2 M 48 2 8 6 6–10 0.27
kg/m2
6 ECC
Lunges
ECC
squat
CMJ
CMJ U
HCMJ U
Lateral jump U
25m sprint
20m sprint
10m sprint
10m shuttle
20m shuttle
25m shuttle
Biology of Sport, Vol. 39 No3, 2022
491
Effects of unilateral vs. bilateral training on performance
Study
Participants Training Program Outcome Measure
Identity
Training
Experience
(years)
Age
(years) Gender N
Frequency
(times/
week)
Weeks Sets Reps Intensity
(%1RM)
Unilateral
Exercises
Bilateral
exercises Performance test
Taniguchi
etal.[43]
College
students None 20
±1.1 F/M 18 3 6 3 6 KE Isok U KE Isok
B
Isok-KE U
Isok-KE B
Stern
etal.[25]
Soccer
Players >2 17.6
±1.2 M 23 2 6 4
4
6
3–6 75–85% Bul squat
3Jumps U
Back
squat
3Jumps
B
Back squat 1RM
CMJ B
CMJ U
Broad jump B
Broad jump U
Drop jump B
Drop jump U
10m sprint
30m sprint
505
McCurdy
etal.[31]
College
students None 20.7
±2.6 F/M 38 2 8 3
3–6
5–15
5–15 50–87%
2 Jumps U
Bul squat
2 Jumps
B
Back
squat
Back squat 1RM
Bul squat 1RM
CMJ B
CMJ U
F: female; M: male; U: unilateral; B: bilateral; KE: knee extension; KF: knee exion; Bul squat: Bulgarian split squat; Iso: isometric;
Isok: isokinetic; LP: leg press; CMJ: countermovement jump; MP: maximum power; DJ: drop jump; SJ: squat jump; H3J: horizontal
3jumps; HCMJ: horizontal countermovement jump; ECC: eccentric
TABLE 4. Continue
Quantitative analysis
The performance tests in included studies can be categorized into
6subgroups, namely: unilateral strength, bilateral strength, unilat-
eral jump, bilateral jump, COD and speed performance tests. After
sensitive analysis, the I2<50% in all subgroups suggested anon-
signicant heterogeneity, and the xed effects models were used to
aggregate the SMDs. The total heterogeneity (Chi2= 70.25,
p=0.03; I2=39%) of 44 effects was medium. The overall effect
size was non-signicant and classied as trivial effect (ES=0.09;
p=0.15; 95% CI: -0.03, 0.22) (Figure2). The heterogeneity for
inter subgroup differences was large (I2=86.1%).
Unilateral strength performance
After dealing with the multiplicity (Table2), 9outcomes were se-
lected for meta-analysis. The heterogeneity was low (Chi2=10.30,
p=0.24; I2=22%). In xed effects model, overall effect was not
signicant (p=0.07). Pooled effect size and 95% CI were 0.26
(-0.03, 0.55) and classied as asmall effect.
Bilateral strength performance
After dealing with the multiplicity, 9outcomes were selected to do
meta-analysis. The heterogeneity test was not statistically signicant
(Chi2=6.77, p =0.56; I2=0%). In xed effects model, overall
effect was signicant (p=0.004). Pooled effect size and 95%CI
were -0.43 (-0.71, -0.14) and classied as asmall effect.
Unilateral jump performance
After dealing with the multiplicity, 6outcomes were selected for me-
ta-analysis. The original heterogeneity was high (I2=60%), after
alternated sensitive analysis, one study with eccentric overload train-
ing different from others was excluded, and the adjusted heterogene-
ity was low (Chi2=4.85, p =0.30, I2 =18%). In xed effects
model, the pooled effect size and 95% CI was 0.89 (0.52, 1.26) with
statistical signicance (p<0.0001) and classied as alarge effect.
Bilateral jump performance
After dealing with the multiplicity, 8outcomes were selected for
meta-analysis. The heterogeneity was not statistically signicant
(Chi2=1.68, p=0.98; I2=0%), in xed effects model, the pooled
effect size and 95%CI was -0.12 (-0.40, 0.15) with non-signicance
(p=0.79) and classied as atrivial effect.
COD performance
After dealing with the multiplicity, 7outcomes were selected for
meta-analysis. The heterogeneity was not statistically signicant
(Chi2=5.83, p=0.44; I2=0%). The pooled effect size and 95%CI
was 0.31 (-0.01, 0.63) with non-signicance (p=0.06) and clas-
sied as asmall effect.
Speed performance
After dealing with the multiplicity, 6outcomes were selected for
meta-analysis. The heterogeneity was not statistically signicant
492
Kai-Fang Liao et al.
FIG. 2. Forest plot comparing the effects of bilateral and unilateral exercises training on athletic performance. Bul s: Bulgaria split
squat; LP: leg press; ISO:isokinetic; MP:Maximum power; KE: knee extension; U: unilateral; B:bilateral; L: left; R: right; CMJ:
countermovement jump; SJ: squat jump; Tmod: Tmodied test.
Biology of Sport, Vol. 39 No3, 2022
493
Effects of unilateral vs. bilateral training on performance
(Chi2=2.38, p=0.67; I2=0%). in xed effects model, the pooled
effect size and 95%CI was -0.12 (-0.46, 0.21) with non-signicance
(p=0.82) and classied as atrivial effect.
DISCUSSION
This systematic review with meta-analysis aimed to compare the
training effects of unilateral vs. bilateral resistance training on mea-
sures of athletic performance. The results showed that: 1) both uni-
lateral and bilateral resistance training improve athletic performance;
2) with respect to all performance measures, overall effect showed
no difference between unilateral resistance training and bilateral
resistance training; 3) based on the subgroup analysis, the unilat-
eral resistance training showed alarge effect on improving unilat-
eral jumping performance, but non-signicant difference on improv-
ing unilateral strength performance in comparison to bilateral
exercises. In contrast, bilateral resistance training exhibited asmall
effect on improving bilateral strength performance, but non-signicant
differences on improving bilateral jumping performance in compari-
son to unilateral training. Finally, COD and speed performance im-
provement showed no differences between unilateral and bilateral
resistance training; however COD, which involves unilateral propuslive
force production, did show asmall effect in favor of unilateral resis-
tance training.
Strength performance
The outcome of bilateral strength performance subgroup supported
our hypothesis that bilateral resistance training followed the training
principle of specicity, which demonstrated asmall effect (ES=-0.43)
on improving bilateral strength performance in comparison with uni-
lateral resistance training. It is clear that the muscle cross-sectional
area and neuromuscular adaptation are the main factors for develop-
ing maximum strength[45]. There is evidence that unilateral and
bilateral strength traning had asimilar impact on muscle mass[46],
girth[29] and cross-sectional area[47]. However, Helme etal.,[48]
found that almost 15% of the load was placed on the rear leg during
the Bulgarian split squat. It could be inferred that the rear leg might
be contributed to the concentric phase, which means that the lead
leg might decrease force development. Anderson et al.,[49] and
McCurdy et al[50] found that the bilateral squat activated the knee-
joint agonists (e.g., quadriceps) greater than the Bulgarian split squat,
while the Bulgarian split squat showed greater antagonists such as
the hamstrings, hip abductors and trunk musculature. Cumulatively,
it could be speculated that bilateral strength training may produce
greater knee agonist neuromuscular adaptation owing to higher load
other than the muscle growth.
Four studies in this systematic review showed that the bilateral
group mitigated the bilateral force decit (BLD), but unilateral resis-
tance training increased the BLD in measures of knee extension,
isometric leg press and squat with one leg and both legs, respec-
tively[24, 27, 29, 43]. BLD is described as the sum of the maximum
forces exerted by the left and right limbs unilaterally as being
greater than the simultaneous exertion of both limbs bilaterally[51].
More recently, Bishop etal. found that acombination of bilateral and
unilateral resistance training had superior effects on unilateral jump
performance compared to bilateral. Consequently, it stands to reason
that if greater improvements in unilateral jump performance are
evident (compared to bilateral), it will have an effect on the BLD
outcome (remembering that the BLD is aproduct of both unilateral
and bilateral scores, presented as asingle ratio number)[52]. In line
with the denition and the ndings by Bishop etal., the results of
these 4studies might be explained by the reason of specicity. Sim-
ply put, that unilateral resistance training is likely to enhance unilat-
eral performance measures more than bilateral performance mea-
sures, and vice versa. Given the inherent differences in study design
of these 4studies (e.g.different levels of stability requirements in the
chosen methods but still with similar effects on the BLD), it is spec-
ulated that neuromuscular factors induced by the mechanism of BLD
may be more of acontributing factor to changes than stability.
The results of unilateral strength performance subgroup indicated
that unilateral resistance training had asmall effect (ES=0.26) on
improving unilateral strength. However, there were not statistically
signicant between-group differences. Our nding did not correspond
well with included studies which found clear evidence that unilat-
eral resistance training was better for improving unilateral strength
performance[24, 26]. The possible explanation was that these stud-
ies did not adjust each intervention group’s mean changes between
pre and post tests for analysis as in the current study, which may
enlarge the probability to make the false inference due to the selec-
tion effect. For example, McCurdy etal.,[31] found that 8weeks’
unilateral resistance training was more effective in improving 1RM
of aBulgarian split squat than the bilateral resistance training. How-
ever, after adjusting the pretest difference, both groups exhibited
similar effects on the 1RM of Bulgarian split squat. In addition, the
unilateral exercises may stimulate the stabilizing muscles in the core
and knee to agreater extent than bilateral exercises[50, 53], which
are likely to be benecial for improving stability and force transference
through the kinetic chain. However, the greater agonist neuromus-
cular adapation of bilateral resistance training may counteract the
superiority of stability and specicity of unilateral exercises in unilat-
eral strength tests. Therefore, it is up to coaches to determine what
the athlete needs and program accordingly.
Jump performance
The larger improvement in unilateral jump performance induced by
unilateral rather than bilateral resistance training corresponded with
the training principle of specicity. Five studies were included in the
unilateral jump subgroups, all of which applied jumping exercises as
intervention modalities[25, 27, 30, 31, 40] and found that the
effect of unilateral jumping training were better (ES: 0.24–1.66) than
bilateral jumping training. These ndings might result from the dif-
ferences in stability and neuromuscular adaptation of agonists. In
unilateral jumping, the smaller supporting surface triggers ahigher
494
Kai-Fang Liao et al.
push relative higher load, which may result in greater range of motion
and deeper squat depth for producing greater force, subsequently
increase the contact time. Thus, it is conceivable that the unilateral
jump group might be unable to adapt in the shorter exertion time to
develop similar force as the bilateral jump group in the measure of
bilateral jump test[58]. In contrast, bilateral jump exercises similar
with the test modality may have apriority of familiarization and be
adapted to exert force in shorter time.
Change of direction (COD) and speed performance
COD speed and sprint have been suggested to be largely unilateral
exercises in previous research[12, 59]. However, our results indi-
cated that the effect size in the speed subgroup showed no difference
between unilateral and bialteral resistance training, similar to recent
ndings[33]. However, COD was likely to be in favor of unilateral
resistance training with asmall effect.
Four of the 7studies in the COD subgroup that adopted both the
Bulgarian split squat versus back squat as the modalities of resistance
intervention[24, 25, 28, 32] supported use of the Bulgarian split
squat over the back squat. McCurdy etal. found that the Bulgarian
split squat could produce greater activation in glutes maximus and
hamstring compared with back squat[50]. Another 2studies in this
subgroup used eccentric overload as the intervention modality also
supported the unilateral group as being the more favourable method
on enhancing COD performance. Chaabene etal.,[60] obtained
amoderate to very large correlation (r=0.45–0.89) between ec-
centric strength and the COD performance through a systematic
review. The concentric force of push, the eccentric force during de-
celerating, and the ground contact time are the key factors affecting
the ability to COD effectively[61,62]. Alarger eccentric braking
force can shorten the deceleration time, increase the elastic force of
the muscles and connective tissue, and generate agreater subsequent
push force, which collectively results in a shorter ground contact
time, greater acceleration, and thereby improving overall COD per-
formance[62]. Accordingly, the unilateral exercises such as Bulgar-
ian split squat emphasizes the use of eccentric force in apredomi-
nantly unilateral manner, which could explain the greater improvement
on COD performance.
The current data also demonstrated that bilateral resistance train-
ing had atrivial effect on improving linear speed performance. Vari-
ous factors account for the improvement of speed performance.
Empirical evidence shows that the rate of force development in short
time and large propulsive forces in initial acceration stage are the
main attributes of speed performance. Wilkau etal.[63] found that
the contact time, vertical force and peak propulsive forces (r=–0.64,
r=0.57 and r=0.66, respectively) contributed most to the step
velocity. In ameta analysis of 15studies, Seitz etal.[64] reported
astrong relationship between squat strength and sprint performance
(r=-0.77; p=0.001). In the current study, the strength subgroup
showed that bilateral exercise training had alarger effect in improv-
ing bilateral strength with statistical signicance. Collectively, the
co-activation in the stabilizing muscle groups (e.g. hamstrings), and
helps maintain the head, arms, trunk and lower limbs in the same
direction during both landing and taking off. As aresult, the stabil-
ity of the lower limbs are improved, and the absorption of the reac-
tion force of the lower limbs decreases during the landing phase[54].
Bogdanis etal.[27] found that unilateral jumping training was more
effective at improving unilateral squat jump performance and
RFD0–50ms and RFD0–100ms (Rate of force development) in unilat-
eral isometric seated leg extension in comparison to bilateral jump
training. Turki etal. [55] also found that the electromyographic
level of the vastus intermedius and gastrocnemius in unilateral body
weight vertical jump was 10–25% higher than that in bilateral body
weight vertical jumps. According to the force-velocity relationship,
unilateral jumping without asecond limb to ‘spread the load’ needs
to push agreater relative load with alonger time to take off. It could
be inferred that unilateral jumping might make the muscles generate
more force, and more strongly stimulate the muscles such as the
extensors of the ankle, knee and hip to result in greater neuromus-
cular adaptation when considering individual limbs. It should be
noted that 1study not included for aggregating effect size after
sensitive analysis purely used eccentric overload resistance exer-
cises as intervention modalities and found bilateral training group
improved unilateral countermovment jump better than the unilat-
eral exercise group[41]. Although speculative, the results may be
attributed to the eccentric force improvement in bilateral group which
help to stablize the posture during testing. In addition, the training
status of subjects will also have an effect on results. For example, it
is plausible that high-level athletes might exhibit less improvements
in bilateral jump performance (compared to non-athletes), as there
is arguably areduced ‘window of opportunity’ for enahnced adapa-
tation[56]. However, it could be suggested that improvements in
unilateral jump performance may be more likely for all, owing to the
use of unilateral resistance training methods being employed less
frequently. This concept is partly supported by the ndings of Bish-
op etal.[53] who showed sigicant improvements in unilateral jump
height (but not bilateral), after an 8-week combined bilateral and
unilateral strength training programme. Collectively, these evidences
may explain why unilateral resistance training had alarger effect on
improving unilateral jumping performance in comparison with bilat-
eral resistance training.
The overall effect size of bilateral jumping subgroup was trivial in
favor of bilateral exercises training, but the value was not statisti-
cally signicant. The results may be the contribution of the mechanism
of bilateral decit and differences in kinetics[57]. When both legs
contract simultaneously, the nerve activation of the left and right
interhemisphere may be mutually inhibited, and there was alack of
bilateral activation while training unilaterally, so it would be ex-
pected that muscles would not achieve maximum voluntary contrac-
tion in unilateral jump group. Furthermore, the contact time for bi-
lateral and unilateral jumps (CMJ) was 178–190ms and >250ms
respectively[21]. As above explained, the unilateral jump needs to
Biology of Sport, Vol. 39 No3, 2022
495
Effects of unilateral vs. bilateral training on performance
improvement of bilateral strength may explain the trivial effect in
favor of bilateral exercises training on improving speed.
It should be noted that the overall effect of each training method
(unilateral vs bilateral) on COD and speed were not statistically sig-
nicant. This could be attributed to the studies’ small sample size,
the lack of specicity of training methods with regards to the tests
measures, and/or the low transference of jump and resistance train-
ing adaptations to sprint and COD performance measures [65].
However, Hopkins etal.[66] stated the smallest worthwhile enhance-
ment (SWE) by 10% will help the athletes to win the game. SWE
was calculated by the coefcient of the variablility (CV) of within-
athlete’s performance from competition to competition. With respect
to short running events, it was found SWE was even lower to
0.3–0.5%[67]. This indicated that although the effect sizes were
rather small or not signicance between unilateral vs. bilateral resis-
tance training, when inter-athlete variability was counted, the small
difference in improving performance caused by the exercise selection
were within arange that was meaningful for elite athletes. Thus,
these ndings still have signicant implications for understanding
how to choose unilateral or bilateral resistance exercise for optimiz-
ing speed performance.
It should be acknowleged that some limitations exist in our paper.
Firstly, the lower number (<10) of studies in subgroup analyses
means it was not possible to do further meta-regression analysis.
Thus, as is often the case in science, more research is likely needed
to provide greater clarity between the two training methods. Sec-
ondly, all included studies for this meta-analysis applied lower body
exercises as training intervention, thus, whether the present ndings
based on lower limbs studies can be applied to the upper limbs re-
mains unclear. Future research should attempt to investigate the
difference between bilateral and unilateral exercises training for the
upper limbs. Thirdly, the included participants’ age ranged 16 to
26years, which means that our results may not be extrapolated to
those participants with age out of this range. But it still can provide
references for these practitioners.
CONCLUSIONS
This study set out to establish the different effects between unilateral
and bilateral resistance training on strength, jump, linear and COD
speed performance. The most interesting ndings was that unilateral
resistance training was superior for enhancing unilateral jump perfor-
mance, and bilateral resistance training was superior for enhancing
bilateral strength performance. But both had no signicant difference
in enhancing unilateral strength, bilateral jump, linear and COD speed
performance. The results suggest that both kinds of training should
likely be considered by practitioners. Specically, bilateral exercises
should be chosen for enhancing bilateral strength performance, and
unilateral exercises should be chosen for enhancing unilateral jump
performance. With regards to other performance outcomes (e.g., lin-
ear and COD speed), practitioners can probably choose any kind of
exercises based on their interest and the availability of facilities.
Funding
This work was supported by the Winter Olympics Founda-
tion[2018FF0300901] and China Institute of Sport Science Basic
Foundation[Basic 17–30] grants to Yong-ming Li.
Conict of interest
The authors declare that they have no competing interests.
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