Content uploaded by Lee E Brown
Author content
All content in this area was uploaded by Lee E Brown on Oct 14, 2017
Content may be subject to copyright.
NEUROMUSCULAR ADAPTATIONS TO UNILATERAL VS.
BILATERAL STRENGTH TRAINING IN WOMEN
CI
´NTIA E. BOTTON,
1
REGIS RADAELLI,
1
EURICO N. WILHELM,
1
ANDERSON RECH,
1
LEE E. BROWN,
2
AND RONEI S. PINTO
1
1
Exercise Research Laboratory, Physical Education School, Federal University of Rio Grande do Sul, Porto Alegre, RS, Brazil;
and
2
Center for Sport Performance, Human Performance Laboratory, California State University, Fullerton, California
ABSTRACT
Botton, CE, Radaelli, R, Wilhelm, EN, Rech, A, Brown, LE, and
Pinto, RS. Neuromuscular adaptations to unilateral vs. bilateral
strength training in women. J Strength Cond Res 30(7):
1924–1932, 2016—Considering the bilateral deficit, the sum
of forces produced by each limb in a unilateral condition is
generally greater than that produced by them in a bilateral con-
dition. Therefore, it can be speculated that performing unilateral
strength exercises may allow greater training workloads and
subsequently greater neuromuscular adaptations when com-
pared with bilateral training. Hence, the purpose of this study
was to compare neuromuscular adaptations with unilateral vs.
bilateral training in the knee extensor muscles. Forty-three rec-
reationally active young women were allocated to a control,
unilateral (UG) or bilateral (BG) training group, which per-
formed 2 times strength training sessions a week for 12 weeks.
Knee extension one repetition maximum (1RM), maximal iso-
metric strength, muscle electrical activity, and muscle thick-
ness were obtained before and after the study period.
Muscle strength was measured in unilateral (right + left) and
bilateral tests. Both UG and BG increased similarly their uni-
lateral 1RM (33.3 614.3% vs. 24.6 611.9%, respectively),
bilateral 1RM (20.3 66.8% vs. 28.5 612.3%, respectively),
and isometric strength (14.7 611.3% vs. 13.1 612.5%,
respectively). The UG demonstrated greater unilateral isomet-
ric strength increase than the BG (21.4 610.5% vs. 10.3 6
11.1%, respectively) and only the UG increased muscle elec-
trical activity. Muscle thickness increased similarly for both
training groups. Neither group exhibited pretesting 1RM bilat-
eral deficit values, but at post-testing, UG showed a significant
bilateral deficit (26.5 67.8%) whereas BG showed a signifi-
cant bilateral facilitation (5.9 69.0%). Thus, performing unilat-
eral or bilateral exercises was not a decisive factor for
improving morphological adaptations and bilateral muscle
strength in untrained women. Unilateral training, however,
potentiate unilateral specific strength gains.
KEY WORDS resistance exercise, muscle thickness, deficit
INTRODUCTION
Strength exercises are often performed in 2 different
fashions, with 1 limb at a time (unilateral) or both
limbs simultaneously (bilateral). Previous studies
have shown that it is possible to produce greater
force in the sum of unilateral compared with bilateral (2,8).
The mechanisms explaining differences between these con-
ditions are not completely understood. However, the most
consistent explanation is that there is a neural limitation
during bilateral exercise, blunting maximal force produc-
tion (16,23). This phenomenon is known as the bilateral
deficit and is well documented by cross-sectional studies
in different muscle groups (21), populations, (14) and test
conditions (2).
Because of the bilateral deficit, it is possible to use greater
loads when exercise is performed unilaterally, which may be
a strategy to optimize strength gains. However, a limited
number of studies have compared the chronic neuromuscu-
lar adaptations of unilateral and bilateral strength training
(5,11,21,22). Some previous studies (5,11) have demon-
strated that despite either unilateral or bilateral training,
there are strength gains in the specific trained condition,
and an increase in the untrained condition. In contrast,
Taniguchi (21) showed that strength gains occurred only
after bilateral training.
A cross-sectional study reported that subjects who trained
at least 1 year unilaterally demonstrated bilateral deficit on
the knee extensor muscles compared with subjects bilaterally
trained (8). However, longitudinal studies (12–26 weeks)
with untrained people have found that unilateral training
had a slight nonsignificant increase in the bilateral deficit,
possibly due to a similar percentage change in the bilateral
and unilateral strength (5,21). On the other hand, bilateral
training resulted in significant decrease of bilateral deficit
(5,11,21) as a consequence of a greater magnitude of increase
in bilateral compared with unilateral strength.
Address correspondence to Cı
´ntia Ehlers Botton, cintiabotton@yahoo.
com.br.
30(7)/1924–1932
Journal of Strength and Conditioning Research
Ó2015 National Strength and Conditioning Association
1924
Journal of Strength and Conditioning Research
the
TM
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
The divergent results among few studies found in the
literature may be explained by different populations (males,
females, or mixed groups), period of training (6–26 weeks),
and type of resistance machine used during training (isoiner-
tial or isokinetic). For example, Ha
¨kkinen et al. (5) investi-
gated the effect of 12 weeks of isoinertial knee extension
training on middle-aged and older men and women, while
Janzen et al. (11) performed 26 weeks of knee extension
training in postmenopausal women. Differently of the afore-
mentioned studies (5,11), Taniguchi (21) performed 6 weeks
of isokinetic knee extension training in young men and
women. In addition, Kuruganti et al. (13) investigated the
effect of isokinetic bilateral training on the bilateral deficit,
but unilateral training was not studied. Moreover, previous
studies (21,22) have focused primarily on strength measure-
ment while investigating differences between training con-
ditions. Thus, there is a paucity of research regarding
unilateral vs. bilateral training differences related to morpho-
logical and neural adaptations.
Because unilateral training enables one to exercise with
greater load per trained limb, it may be speculated that
unilateral strength training would elicit greater skeletal muscle
adaptations compared with bilateral exercises. Therefore, the
purpose of this study was to investigate neuromuscular
adaptations to unilateral vs. bilateral training, and their
influence on the bilateral deficit in young women, because
the few studies in this area were performed with middle-aged
and elderly subjects and nonhomogeneous gender.
METHODS
Experimental Approach to the Problem
To compare the effects of unilateral and bilateral training,
subjects were required to visit the laboratory on 3 separate
days before study (pre) and twice after 12 weeks (post) with 4
days of interval between visits. On the first visit, muscle
thickness measurements were taken and subjects were famil-
iarized with the dynamic and isometric strength tests. On the
second day, anthropometric data were obtained and subjects
performed a maximal isometric strength test with simultaneous
collection of the electromyographic (EMG) signal. The
maximaldynamicstrengthwasmeasuredonthethirdvisit.
Thereafter, subjects were randomized on a control group or
training group, which exercised 2 times per week for 12 weeks.
Post-testing took place 3 to 5 days after the last training session
and was identical to pretesting. Intraclass correlation coeffi-
cients were calculated with data collected on day one and 2 for
the isometric test; and day one and 3 for dynamic test.
Subjects
Forty-three young women (18–30 years) volunteered for
this study. They were randomly assigned to either unilateral
group (UG) (n= 14: 24.8 61.4 years; 60.8 66.4 kg; 163.0 6
6.5 cm), bilateral group (BG) (n= 15: 24.3 63.7 years; 57.0 6
4.8 kg; 160.2 65.8 cm), or control group (CG) (n= 14: 22.7 6
2.8 years; 58.0 65.7 kg; 163.6 66.2 cm). Sample size was
based on standard deviations and differences between means
from Ha
¨kkinen et al. (5), with an alpha level of 0.05 and
power of 80%, and resulted in a minimum of 12 subjects
per group.
All subjects had not been involved in a systematic strength
training program for at least 3 months before this study, and 6
participants had never practiced strength training before.
Participants were not taking medications other than oral
contraceptives and were instructed to avoid changes in their
diet and recreational physical habits (e.g., sports, jogging, and
walking) throughout the study period. Only 6 women did not
take oral contraceptives (2 for each group). They were
informed of possible risks and discomforts of participation
and all gave their written informed consent before any testing.
This study was conducted according to the Declaration of
Helsinki and all procedures were approved by the Ethics
Committee of the Federal University of Rio Grande do Sul.
Procedures
Maximal Dynamic Strength. Knee extension 1 repetition
maximum (1RM) tests for bilateral and unilateral (right and
left) conditions were assessed on the same test day on the
same isoinertial knee extension machine used for training
(Taurus, Porto Alegre, RS, Brazil). The same researchers, with
identical equipment and subject positioning, conducted all
pre- and post-testing. Subjects were carefully familiarized with
the testing procedures and performed a warm-up of 10
repetitions of bilateral knee extensions with a light resistance.
The bilateral extension was always tested first, followed by the
unilateral extension test for the right and left leg in random
order. Ten minutes of interval was given between the bilateral
and unilateral tests. The cadence was fixed at 2 seconds for
the concentric phase and 2 seconds for the eccentric phase
with the aid of an analog metronome (Quartz, Los Angeles,
CA, USA). Subjects started the test at 908of knee flexion (08=
knee fully extended) and moved to full extension which was
individualized for each subject and controlled by a delimiter
device. The maximal load was found within 3 attempts for
each testing condition, with 5-minute interval between trials.
Intraclass correlation coefficients for bilateral and unilateral
right and left were 0.96, 0.95, and 0.93, respectively.
Maximal Isometric Strength. Isometric knee extension test for
bilateral and unilateral (right and left) conditions were
performed on an isokinetic dynamometer (Cybex Norm,
Ronkonkoma, NY, USA), calibrated according to the
manufacturer’s instructions. The unilateral tests were per-
formed with the standard lever arm supplied by the manu-
facturer, whereas a custom made lever arm was used for
bilateral testing (Figure 1). Subjects were seated with their
hips flexed at 858(08= hip fully extended). For both tests, the
dynamometer’s axis of rotation was aligned with the lateral
femoral condyle of the tested limb, while straps secured the
torso and pelvis. Subjects performed 10 repetitions of con-
centric knee extension at 1208per second as a warm-up.
Thereafter, they completed two 5-second knee extension
Journal of Strength and Conditioning Research
the
TM
|
www.nsca.com
VOLUME 30 | NUMBER 7 | JULY 2016 | 1925
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
maximal isometric actions at a 608angle (08= knee fully
extended), with 3 minutes rest between attempts. All iso-
metric tests on bilateral and unilateral (right and left) con-
ditions were performed on the same day in randomized
order with 30 minutes rest between bilateral and unilateral
tests. Subjects were instructed and verbally encouraged to
produce maximal torque throughout each trial. Peak torque
values were determined by the dynamometer software
(Humac norm 2009, Version 9.6.2). The greatest isometric
peak torque value for each condition was used for further
analyses. Intraclass correlation coefficients for bilateral and
unilateral right and left were 0.78, 0.86, and 0.76, respectively.
Muscle Electrical Activity. Electromyographic (EMG) activity
was recorded from the vastus lateralis (VL) and rectus femoris
(RF) muscles of the right and left limbs during maximal
isometric strength testing. Before electrode placement, skin
preparation was performed, including shaving excess hair and
cleaning the skin with isopropyl alcohol (to reduce impedance
below 2,000 kV). Bipolar configuration electrodes (20-mm
interelectrode distance) were positioned along the estimated
direction of the muscle fibers on the muscle belly according to
SENIAM (www.seniam.org). Electrode position was carefully
mapped using a transparent sheet to ensure replication posi-
tioning at post-testing.
Electromyographic signals were recorded using an electro-
myographic system (Miotool, Miotec-Equipamentos Biome
´-
dicos), amplified by a factor of 100 and digitized at a sampling
frequency of 2,000 Hz. The EMG signal was Butterworth
filtered using cut-off frequencies of 20 Hz and 500 Hz for
lower and upper band-pass respectively. After filtering, the
EMG signal from the highest isometric torque (Nm) attempt
was selected across a one-second torque–time curve plateau,
and the root mean square (RMS) value for each muscle was
calculated. For total muscle electrical activation (EMG
T
), the
RMS values of each muscle of each limb were summed.
Muscle Thickness. Quadriceps femoris muscle thickness was
obtained using a B-Mode ultrasonographic apparatus (Nemio
XG, Toshiba, Japan), with a 7.5 MHz linear array probe (38-
mm length). Before muscle thickness evaluation, each subject
rested for 15 minutes in a supine position with their legs
extended and relaxed to allow fluid shifts to occur (1). The
probe was coated with a water-soluble transmission gel to
provide acoustic contact without depressing the dermal sur-
face. Great care was taken to apply minimal pressure during
scanning to avoid compression of the muscles. Muscle thick-
ness of the RF, VL, vastus medialis (VM), and vastus interme-
dius (VI) were measured at the same sites described in
previous studies (4,12,15,19). All images were digitized and
later analyzed in Image-J software (National Institutes of
Health, version 1.37; USA). Subcutaneous adipose tissue-
muscle interface and the muscle-bone interface were identified
in each image, and the distance between them was accepted as
muscle thickness. Overall quadriceps femoris muscle thickness
(MT QUA
sum
) was calculated from the sum of the four
muscles ( RF + VL + VM + V I) (3,17,19) for each limb followed
by the sum of the right and left. Post-testing measures were
performed 3–5 days after the last training session to avoid any
potential exercise-induced swelling. The same researcher per-
formed all measurements before and after training. These
measurements have demonstrated high reproducibility in our
laboratory and in previous studies (17,19).
Bilateral Index. Bilateral index (BI) was calculated using an
equation proposed previously (8), with peak torque values
(BI
PT
), 1RM values (BI
1RM
) and RMS values (BI
EMG
)
obtained in the bilateral and unilateral (right and left) con-
ditions. The equation is:
BI ð%Þ¼100xBilateral
Unilateral Right þUnilateral Left2100
Positive values indicate that bilateral condition was greater
than the sum of the unilateral conditions (bilateral
Figure 1. Modified lever arm to perform bilateral testing on the
dynamometer.
Unilateral vs. Bilateral Training
1926
Journal of Strength and Conditioning Research
the
TM
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
facilitation), and negative values indicate that bilateral
condition was less than the sum of the unilateral conditions
(bilateral deficit).
Strength Training Program
Training occurred across 12 weeks, consisting of 2 sessions
per week on nonconsecutive days (total of 24 training
sessions). Rest between sessions normally ranged from 48
to 72 hours within a week. The UG performed the knee
extension exercise with 1 leg at a time whereas BG
performed with 2 legs simultaneously. All training sessions
were monitored and supervised by at least 1 experienced
investigator. Apart from the knee extension, subjects per-
formed the following exercises: bilateral knee flexion, bench
press, lateral pull-down, hip abduction, hip adduction,
crunch, biceps flexion, and triceps extension. These exercises
were performed in the same way by both groups and were
included to keep participants interested and motivated.
Participants started all training sessions by the knee exten-
sion exercise.
Training intensity was controlled using the repetition
maximum (RM) method as in previous studies (6,18,19),
thus the heaviest possible weight was used for the designated
number of repetitions for each condition. The intensity was
the same for both training groups. Training for weeks 1–3
was 2 sets of 12–15RM; weeks 4–6 was 3 sets of 9–12RM;
weeks 7–9 was 3 sets of 7–10RM; and weeks 10–12 was 4
sets of 5–8RM. Interset rest interval was 1 minute for weeks
1–3, 2 minutes for weeks 4–9 and 3 minutes for weeks 10–12.
When subjects were able to perform more than the desired
number of repetitions, the load was increased for the next
session in increments from 1.0 to 5.0 kg. All subjects at-
tended more than 80% of the training sessions.
Training Loads. To compare differences in knee extension
training loads between groups, the load used during the last
training session of each mesocycle was considered for
analysis. Loads were expressed as an absolute value (kg).
Loads of each limb for UG were summed for comparison.
Statistical Analyses
All data are presented as means 6SD. Normality, homoge-
neity, and sphericity for outcome measures were tested using
the Shapiro–Wilk, Levene, and Mauchly tests, respectively.
TABLE 1. Maximum dynamic and isometric strength values pre- and post-training, in unilateral and bilateral tests
between groups (means 6SD).*
UG BG CG
Pre Post Pre Post Pre Post
1RM BIL (kg) 39.0 67.3 46.6 67.2†z35.7 67.6 45.5 68.0†z36.7 68.1 37.0 69.6
1RM UNI (kg) 38.0 67.8 50.2 68.3†z34.9 66.8 43.1 67.3†39.1 610.0 39.2 610.2
PT BIL (Nm) 286.6 648.3 327.6 657.3†z274.2 659.1 307.7 658.0†278.5 657.3 265.7 650.7
PT UNI (Nm) 323.7 660.8 390.0 665.8†z311.0 662.4 342.5 672.3†324.0 655.0 323.9 656.9
*UG = unilateral group; BG = bilateral group; CG = control group; 1RM = one repetition maximum; BIL = bilateral test condition;
UNI = unilateral test condition (right + left); PT = peak torque.
†Significantly greater than pre values (p#0.05).
zSignificantly greater than CG at post (p#0.05).
TABLE 2. Total electromyographic activation values, pre- and post-training, in unilateral and bilateral test condition
(means 6SD).*
UG BG CG
Pre Post Pre Post Pre Post
EMG
T
BIL (mV)
476.0 6110.5 591.4 6136.4 565.8 6264.8 610.9 6208.6 576.7 6380.6 596.69 6357.3
EMG
T
UNI (mV)
523.9 699.0 731.2 6152.7†680.9 6265.2 748.0 6288.0 715.8 6401.7 693.9 6256.5
*UG = unilateral group; BG = bilateral group; CG = control group; EMG
T
= total electromyographic activation, sum of vastus
lateralis and rectus femoris muscles of the right and left limbs; BIL = bilateral test condition; UNI = unilateral test condition (right + left).
†Significantly greater than pre values (p#0.05).
Journal of Strength and Conditioning Research
the
TM
|
www.nsca.com
VOLUME 30 | NUMBER 7 | JULY 2016 | 1927
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Two-way analysis of variance (ANOVA) with repeated
measures was used to examine interactions between time x
group for 1RM, isometric peak torque, EMG
T
, and MT
QUA
sum
values. Whenever a significant interaction was
observed, a paired t-test was used to determine within-
group differences, and a one-way ANOVA to determine
between-group differences for unilateral and bilateral condi-
tions. Analysis of variance also examined interactions
between group 3condition for percent change values for
1RM, isometric peak torque, and EMG
T
measures. When-
ever a significant interaction was observed, a paired t-test
was used to determine within-group differences, and
a one-way ANOVA to determine between-group differences.
A one-sample t-test was used to determine if bilateral index
was significantly different from zero at baseline and post-
testing for BI
1RM
,BI
PT
and BI
EMG
. A 2-way (time 3group)
ANOVA with repeated measures was used to determine
differences in the time course of workloads. Tukey post
hoc tests were used when necessary, to verify differences
between groups. Test-retest reliability of force measurements
(dynamic and isometric) were calculated at baseline for typ-
ical error = standard-deviation of the difference between day
1 and day 2 measurements/O2; and coefficient of variation
(%) = (typical error of the difference between day 1 and day
2/means of day 1 and day 2) * 100 (7). Significance level was
set a priori at p#0.05. All statistical procedures were
performed using the Statistical Package for the Social Scien-
ces (SPSS) version 18.0 software (IBM SPSS, Inc., Chicago,
IL, USA).
RESULTS
There were no significant (p$0.05) differences between
groups at baseline for 1 RM, isometric peak torque, EMG
T
,
MT QUA
sum,
BI
1RM
,BI
PT
,orBI
EMG
.
Maximum Strength (Dynamic and Isometric)
There were significant (p#0.05) time 3group interactions
for 1RM and isometric peak torque where UG and BG sig-
nificantly increased after training but CG showed no change.
Both UG and BG showed greater bilateral 1RM values than
CG at post. However, only UG showed greater values than
CG in the unilateral 1RM test at post. For isometric peak
torque only UG showed greater values than CG for both test
conditions at post (Table 1).
The typical error for 1RM measures was 1.44 kg (3.92%),
1.03 kg (5.70%), and 0.90 kg (5.16%) for bilateral, unilateral
right and left tests, respectively. Typical error for peak torque
measures was 38.50 Nm (10.03%), 15.15 Nm (7.12%), and
20.49 Nm (9.48%) for bilateral, unilateral right and left tests,
respectively.
For 1RM percentage changes, there was a significant (p#
0.05) interaction for group 3condition. The UG and BG
TABLE 3. Sum of muscle thickness values of quadriceps muscles, pre- and post-training (means 6SD).*
UG BG CG
Pre Post Pre Post Pre Post
MT QUA
sum
(mm) 141.0 621.9 152.8 619.7†138.7 613.7 151.0 614.5†151.5 618.3 149.2 618.2†
*UG = unilateral group; BG = bilateral group; CG = control group; MT QUA
sum
= sum of the quadriceps muscles thickness of the
right and left limbs.
†Significantly different of the pre values (p#0.05).
TABLE 4. Percentage values of bilateral index at pre- and post-training (means 6SD).*
UG BG CG
Pre Post Pre Post Pre Post
BI
1RM
(%) 3.7 614.4 26.5 67.8†2.6 69.2 5.9 69†25.4 613.7 24.7 613.3
BI
PT
(%) 210.5 610.7†215.7 67.7†211.5 69.9†29.3 611.3†213.8 6210.7†217.8 66.7†
BI
EMG
(%) 29.5 67.9†218.8 69.2†217.4 614.6†215.3 623.4†221.7 611.6†24.8 622.5
*UG = unilateral group; BG = bilateral group; CG = control group; BI
1RM
= bilateral index with 1 repetition maximum values; BI
PT
=
bilateral index with peak torque values; BI
EMG
= bilateral index with maximal electromyographic activation values.
†Significantly different from zero (p#0.05).
Unilateral vs. Bilateral Training
1928
Journal of Strength and Conditioning Research
the
TM
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
increased unilateral 1RM (33.3 614.3% vs. 24.6 611.9%,
respectively) and bilateral 1RM (20.3 66.8% vs. 28.5 6
12.3%, respectively) similarly and both groups showed signifi-
cantly (p#0.05) greater values than the CG in unilateral (0.1 6
5.5%) and bilateral (0.8 65.0%). Moreover, UG showed
a greater (p#0.05) increase in unilateral than in bilateral
whereas the BG was not different. In contrast, for peak torque
percentage values, there was only a significant (p#0.05) main
effect for group. For the unilateral test, UG increased peak
torque values 21.4 610.5% which was significantly greater
(p#0.05) than the BG (14.7 611.3%). In the bilateral test,
groups increased similarly (p$0.05) (UG 14.7 611.3% vs. BG
13.1 612.5%). Both BG and UG showed significantly (p#
0.05) greater values than the CG in both tests.
Muscle Electrical Activity
There was a significant (p#0.05) group 3time interaction
for muscle electrical activity in unilateral only. The UG
showed a significant (p#0.05) increase in maximal EMG
activity values at post training in unilateral (Table 2), show-
ing greater (p#0.05) percentage change (39.9 618.3%) in
comparison to the BG (12.0 621.7%) and CG (5.1 6
21.4%). For bilateral, there was no significant (p$0.05)
difference between UG (25.4 618.0%), BG (15.7 6
31.8%), and CG (6.9 620.7%). The BG and CG showed
no significant increase (p$0.05) on muscle electrical activity
at post training for unilateral and bilateral conditions.
Muscle Thickness
A significant (p#0.05) group 3time interaction was
observed for muscle thickness. The training groups signifi-
cantly (p#0.05) increased MT QUA
sum
at post, while the
CG significantly (p#0.05) decreased it (Table 3). There was
no difference on muscle thickness between training groups
(p$0.05) at post training.
Bilateral Index
At baseline, there were no significant (p$0.05) BI
1RM
values
for any group. At post training BI
1RM
values of the training
groups were significantly different from zero (p#0.05),
indicating bilateral deficit and bilateral facilitation for UG
and BG, respectively (Table 4). For BI
PT,
all groups showed
values significantly lower than zero (p#0.05), and post
training values did not show any significant changes. For
BI
EMG
values, there was a significant difference from zero
(p#0.05) at baseline and post training for both training
groups (Table 4). There was large variability of BI
1RM
between subjects within the same group (Figure 2).
Training Loads
A significant time 3group interaction (p#0.05) for training
load was observed. Both training groups significantly
increased (p#0.05) at every mesocycle, but the UG trained
with a significantly greater (p#0.05) loads in the last mes-
ocyle (Figure 3).
DISCUSSION
The aim of this study was to compare the effects that
unilateral and bilateral strength training have on muscle
strength, neural, and morphological adaptations of the knee
extensors in young women after 12 weeks of training. This
Figure 2. Individual percentage bilateral index with 1 repetition maximum values (BI
1RM
) before and after training. UG = unilateral group; BG = bilateral group;
CG = control group.
Figure 3. Absolute loads used during the training period (mean 6SD).
UG: unilateral group; BG: bilateral group; *Significantly greater than the
previous mesocycle (p#0.05); dSignificantly greater than BG workload
(p#0.05).
Journal of Strength and Conditioning Research
the
TM
|
www.nsca.com
VOLUME 30 | NUMBER 7 | JULY 2016 | 1929
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
study investigated the responses in a homogenous popula-
tion, and is the first to consider only young women for
training. In general, we found that the 2 forms of training
resulted in increases in quadriceps muscle strength and
thickness after training. Strength increases were not
restricted to the specific training condition, but interestingly
and for the first time, the lateral specificity was shown only
for UG in both strength and muscle activation changes. The
training loads were similar between groups for almost all
training periods, but in the last mesocycle, the UG trained
with greater workload than the BG. Moreover, training
specificity seems to influence the bilateral index differently
when considering isometric or dynamic strength. The BI
1RM
seems to be reduced by bilateral training and raised by uni-
lateral training, whereas BI
PT
values were not altered as
a function of the intervention.
The outcomes of this study relative to maximum strength
gains showed that both UG and BG demonstrated signifi-
cant increases in trained and untrained conditions. In
relation to dynamic strength, our results are in agreement
with others (5,11,13) that have found increases in both bilat-
eral and unilateral strength, after knee extension training
performed bilaterally or unilaterally. Even when only bilat-
eral isokinetic training was performed, Kuruganti et al. (13)
found knee extension strength increases in both conditions
for men and women. However, our results do not support
some previous findings (21), that demonstrated increases in
isokinetic knee extensor strength in men and women only
for BG in the specific trained condition. These authors con-
clude that one of the reasons for this may be the lower initial
physical activity level of subjects included in the BG, thus
demonstrating greater potential for adaptation. Participants
in both training groups of this study had not being involved
with strength training for a similar time (10.2 66 months for
UG and 13 611.6 months for BG) between training groups
and 2 participants in each group had never undertook
strength training routines.
Although the 2 groups increased strength in both test
conditions, Ha
¨kkinen et al. (5) and Janzen et al. (11) showed
a greater percentage change for each group in their specific
trained condition. Our results partially corroborate with
these findings, since only UG showed greater 1RM gains
in the specific trained condition. The BG strength changes
were similar between bilateral and unilateral tests, suggesting
no lateral specificity. It is possible that differences in study
populations, level of physical activity of the subjects, selec-
tion criteria and randomization of the sample may have
influenced these study differences (5,11). To our knowledge
this study is the first that studied only young women.
Ha
¨kkinen et al. (5) investigated adults and older, men and
women, while Janzen et al. (11) studied post-menopausal
women. From the results found in our work, we could sug-
gest that there is no restriction in strength gains only in the
trained condition, but that positive lateral specificity seems
to occur with unilateral training. Unlike the results found for
1RM, maximum isometric strength gains in the unilateral
test condition were greater for UG than BG. All groups
began training with greater peak torque values in unilateral
than bilateral, different than 1RM pre values. Therefore, UG
training maximized unilateral strength gains without ham-
pering bilateral adaptations.
The specificity of isometric strength gains for UG can be
explained by an increase in quadriceps muscle activation.
The percentage of increase was greater for UG in the
unilateral test than the BG, which was similar to that
observed in strength changes. Only UG showed significant
increases in muscle activation in their specific training
condition. Muscle activation was not measured during
maximum dynamic tests which was the specific training
condition. Therefore, we cannot rule out neural mechanisms
involved with maximal isoinertial strength, although it can
be speculated that a transfer of strength and muscular
activation gains may have occurred in the nonspecific test
(isometric) condition.
Regarding morphological responses, few studies have
compared the effects of unilateral and bilateral training on
skeletal muscle adaptations (5,11,13). Some previous inves-
tigations have hypothesized that unilateral training might
allow the use of heavier loads, resulting in greater gains than
bilateral training (11). In our current study, both BG and UG
significantly increased muscle thickness similarly which cor-
roborates previous studies (5,11) examining knee extensor
muscles. Furthermore, the absolute training loads were
greater in UG than BG only in the last mesocycle; so it is
possible to suggest that for longer periods of training, the
difference between loads will be more apparent and could
optimize gains from unilateral training. In a 26-week study
by Janzen et al. (11), UG trained with heavier loads than BG
at 3 time points (pre, mid, and post-training). However, the
authors performed their analysis with percent loads relative
to baseline 1RM values. Moreover, the UG began training
with greater loads.
The bilateral index in the present study was measured for
both 1RM and isometric peak torque. For BI
PT
, both training
groups presented a bilateral deficit at baseline, however nei-
ther group showed bilateral deficit or facilitation before
training in BI
1RM
. According to Jakobi e Chilibeck (10), iso-
metric muscle actions can best represent the bilateral deficit,
because they are more stable than dynamic actions. In addi-
tion, it is possible that during the isoinertial 1RM test, the
difference between loads for unilateral and bilateral is not
pronounced. Accordingly, Janzen et al. (11) and Ha
¨kkinen
et al. (5) did not find significant knee extension 1RM bilateral
deficit at pretraining using a isoinertial equipment. These
results highlight the importance of considering the specific
test designed to measure bilateral deficit. Hence, it seems
that bilateral deficit occurs in maximal isometric and isoki-
netic tests (2), but may be limited when derived from
isoinertial resistance equipment (5,11), suggesting an
action-specific behavior.
Unilateral vs. Bilateral Training
1930
Journal of Strength and Conditioning Research
the
TM
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
Performing bilateral strength exercises has been suggested
to reduce the BI (5,11,21,22) because of the greater magni-
tude of increase in bilateral strength, whereas unilateral
training may increase the bilateral deficit according to
a cross-sectional study (8). Our findings regarding BI
1RM
are in accordance with this specificity concept, because the
BG showed bilateral facilitation, whereas bilateral deficit was
evident after training in UG. However, there were no
changes in the bilateral deficit with isometric peak torque
values. Since BI
PT
was large for all groups at pre, it is possible
that the 12 weeks of training was not enough to generate
measurable BI
PT
changes. In addition, the isometric test dif-
fers from specific training condition.
Ha
¨kkinen et al. (5) reported that bilateral training signifi-
cantly increased the ratio of bilateral and unilateral force by
7% after 12 weeks of strength training in the knee extensors
of adults and elderly men and women, while unilateral train-
ing showed a not significant 2% change in 1RM. In the
present study, the percent change of the UG was approxi-
mately 10%, whereas BG showed an increase of approxi-
mately 3%. These divergent results between studies could
be related to variability in the BI values among individuals
of the same group, which can be observed in Figure 2. More-
over in a previous study by Hakkinen et al. (5), there is no
information about individual behavior of the subjects. It is
possible that none of the subjects had a bilateral deficit
before training, unlike our investigation where some subjects
showed a bilateral deficit or bilateral facilitation at baseline.
Considering individual variability in the BI, UG had
uniform changes in the BI
1RM
after training, showing
a reduction in variability, unlike the heterogeneous response
observed in BG. It is possible that less between-subjects var-
iability in unilateral adaptations might have resulted in the
lateral specificity found after unilateral training. Moreover,
Ha
¨kkinen et al. (5) and Janzen et al. (11) did not show any
effect of unilateral training on BI, probably because the per-
centage of change of unilateral and bilateral strength were
similar for UG but not BG, while in present study the BG
had percentage values more similar between conditions that
resulted in only 3% of change in the BI.
The bilateral deficit is probably a result of neural
limitations (16), so it would be expected that lower torque
values in bilateral tests were accompanied by lower levels of
activation. The bilateral index with muscle activation values
was performed only on data obtained during the maximal
isometric test. At pre, all groups showed bilateral deficit for
muscle activation in accordance with the results found in
BI
PT
. At post, both training groups still demonstrated a bilat-
eral deficit, while the CG demonstrated none. Other cross-
sectional studies have not find proportional lower values
between torque and muscle activation (8,9,20).
It should be noted as potential limitation of this study that
it is not possible to extrapolate our results to other muscle
groups and different exercises because only the knee
extension movement was investigated. Furthermore, the
current study was conducted on young women, but no
control for the menstrual cycle variations was done. Thus,
the generalization of the current findings should be done
with caution to other populations and other strength
exercises.
PRACTICAL APPLICATIONS
The use of unilateral or bilateral exercises does not seem to
be decisive in improving neuromuscular adaptations to
strength training in untrained young women. However, if
the training aim is to optimize the increase in force produced
for each lower limb separately, the unilateral training may be
recommended. The increase of BD post unilateral training
means that force in each limb was maximized and more
force is produced for each one separately than simulta-
neously. This may imply in improved performance in daily
activities and in athletic performance during unilateral
movements, such as kicking and running. Although unilat-
eral training enabled the use of heavier training loads, there
were no greater superior gains in muscle mass compared
with bilateral training for 12 weeks. It is worth noting,
however, that longer training periods may have resulted in
greater neuromuscular adaptations in the UG, since differ-
ences in training loads were only evident in the last weeks of
training.
ACKNOWLEDGMENTS
The authors acknowledge the subjects who volunteered for
this study. The authors also to acknowledge CNPq and
CAPES for their funding support for this study. There are no
conflicts of interest among authors or external funding
sources to disclose. The results of this study do not
constitute endorsement by the authors or the National
Strength and Conditioning Association.
REFERENCES
1. Berg, HE, Tedner, B, and Tesch, PA. Changes in lower limb muscle
cross-sectional area and tissue fluid volume after transition from
standing to supine. Acta Physiol Scand 148: 379–385, 1993.
2. Botton, CE, Radaelli, R, Wilhelm, EN, Silva, BGC, Brown, LE, and
Pinto, RS. Bilateral deficit between concentric and isometric muscle
actions. Isokinet Exerc Sci 21: 161–165, 2013.
3. Cadore, EL, Izquierdo, M, Alberton, CL, Pinto, RS, Conceicao, M,
Cunha, G, Radaelli, R, Bottaro, M, Trindade, GT, and Kruel, LF.
Strength prior to endurance intra-session exercise sequence
optimizes neuromuscular and cardiovascular gains in elderly men.
Exp Gerontol 47: 164–169, 2012.
4. Chilibeck, PD, Stride, D, Farthing, JP, and Burke, DG. Effect of
creatine ingestion after exercise on muscle thickness in males and
females. Med Sci Sports Exerc 36: 1781–1788, 2004.
5. Ha
¨kkinen, K, Kallinen, M, Linnamo, V, Pastinen, UM, Newton, RU,
and Kraemer, WJ. Neuromuscular adaptations during bilateral
versus unilateral strength training in middle-aged and elderly men
and women. Acta Physiol Scand 158: 77–88, 1996.
6. Hanssen, KE, Kvamme, NH, Nilsen, TS, Ronnestad, B,
Ambjornsen, IK, Norheim, F, Kadi, F, Hallen, J, Drevon, CA, and
Raastad, T. The effect of strength training volume on satellite cells,
myogenic regulatory factors, and growth factors. Scand J Med Sci
Sports 23: 728–739, 2013.
Journal of Strength and Conditioning Research
the
TM
|
www.nsca.com
VOLUME 30 | NUMBER 7 | JULY 2016 | 1931
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
7. Hopkins, WG. Measures of reliability in sports medicine and
science. Sports Med 30: 1–15, 2000.
8. Howard, JD and Enoka, RM. Maximum bilateral contractions are
modified by neurally mediated interlimb effects. J Appl Physiol 70:
306–316, 1991.
9. Jakobi, JM and Cafarelli, E. Neuromuscular drive and force
production are not altered during bilateral contractions. J Appl
Physiol 84: 200–206, 1998.
10. Jakobi, JM and Chilibeck, PD. Bilateral and unilateral contractions:
Possible differences in maximal voluntary force. Can J Appl Physiol
26: 12–33, 2001.
11. Janzen, CL, Chilibeck, PD, and Davison, KS. The effect of unilateral
and bilateral strength training on the bilateral deficit and lean tissue
mass in post-menopausal women. Eur J Appl Physiol 97: 253–260,
2006.
12. Korhonen, MT, Mero, AA, Alen, M, Sipila, S, Hakkinen, K,
Liikavainio, T, Viitasalo, JT, Haverinen, MT, and Suominen, H.
Biomechanical and skeletal muscle determinants of maximum
running speed with aging. Med Sci Sports Exerc 41: 844–856, 2009.
13. Kuruganti, U, Parker, P, Rickards, J, Tingley, M, and Sexsmith, J.
Bilateral isokinetic training reduces the bilateral leg strength deficit
for both old and young adults. Eur J Appl Physiol 94: 175–179,
2005.
14. Kuruganti, U and Seaman, K. The bilateral leg strength deficit is
present in old, young and adolescent females during isokinetic knee
extension and flexion. Eur J Appl Physiol 97: 322–326, 2006.
15. Miyatani, M, Kanehisa, H, and Fukunaga, T. Validity of
bioelectrical impedance and ultrasonographic methods for
estimating the muscle volume of the upper arm. Eur J Appl Physiol
82: 391–396, 2000.
16. Ohtsuki, T. Decrease in human voluntary isometric arm strength
induced by simultaneous bilateral exertion. Behav Brain Res 7:
165–178, 1983.
17. Pinto, RS, Correa, CS, Radaelli, R, Cadore, EL, Brown, LE, and
Bottaro, M. Short-term strength training improves muscle quality
and functional capacity of elderly women. Age 36: 365–372, 2014.
18. Pinto, RS, Gomes, N, Radaelli, R, Botton, CE, Brown, LE, and
Bottaro, M. Effect of range of motion on muscle strength and
thickness. J Strength Cond Res 26: 2140–2145, 2012.
19. Radaelli, R, Botton, CE, Wilhelm, EN, Bottaro, M, Lacerda, F,
Gaya, A, Moraes, K, Peruzzolo, A, Brown, LE, and Pinto, RS. Low-
and high-volume strength training induces similar neuromuscular
improvements in muscle quality in elderly women. Exp Gerontol 48:
710–716, 2013.
20. Schantz, PG, Moritani, T, Karlson, E, Johansson, E, and Lundh, A.
Maximal voluntary force of bilateral and unilateral leg extension.
Acta Physiol Scand 136: 185–192, 1989.
21. Taniguchi, Y. Lateral specificity in resistance training: The effect of
bilateral and unilateral training. Eur J Appl Physiol Occup Physiol 75:
144–150, 1997.
22. Taniguchi, Y. Relationship between the modifications of bilateral
deficit in upper and lower limbs by resistance training in humans.
Eur J Appl Physiol Occup Physiol 78: 226–230, 1998.
23. Vandervoort, AA, Sale, DG, and Moroz, J. Comparison of motor
unit activation during unilateral and bilateral leg extension. J Appl
Physiol Respir Environ Exerc Physiol 56: 46–51, 1984.
Unilateral vs. Bilateral Training
1932
Journal of Strength and Conditioning Research
the
TM
Copyright © National Strength and Conditioning Association Unauthorized reproduction of this article is prohibited.
