J Appl Physiol 99: 1880 –1884, 2005.
First published July 14, 2005; doi:10.1152/japplphysiol.00559.2005.
Training with unilateral resistance exercise increases contralateral strength
Joanne Munn,1 Robert D. Herbert,1 Mark J. Hancock,1 and Simon C. Gandevia2
1
School of Physiotherapy, The University of Sydney, Lidcombe; and 2Prince of Wales
Medical Research Institute, University of New South Wales, Randwick, Australia
Submitted 11 May 2005; accepted in final form 10 July 2005
cross education; training volume; training speed; elbow flexors; neural
adaptations
to others performing a motor task can affect performance of the
observer (11), so it is important to control for this influence
when determining the effect of unilateral training on contralateral strength.
Although there is no consensus on the mechanism that
produces contralateral strength adaptations, it has been suggested that the magnitude of strength gained on the contralateral side is related to the strength gain on the trained side (31).
A review by Zhou (31) explored this premise by examining
results of previous studies, but the relationship between
strength gain on the trained and untrained sides has not been
assessed in an individual study. Given that training speed (7,
24) and the volume of training (26) are believed to influence
the magnitude of strength adaptations on the trained limb, it is
possible that these parameters could also affect adaptations of
contralateral strength.
This study aimed to determine the effect of unilateral resistance training on the strength of the untrained limb. It was
designed to overcome limitations of previous studies, including
inadequate sample size, inappropriate statistical analysis, and
failure of control subjects to attend training sessions. A second
aim was to determine whether the number of training sets and
contraction speed of unilateral progressive resistance exercise
affects contralateral strength. Lastly, we sought to determine
whether contralateral increases in strength scaled with ipsilateral strength gains.
METHODS
Subjects
that unilateral strength training increases
strength in the homologous muscle group of the contralateral
limb (for reviews, see Refs. 6, 20, 31). Recently, a metaanalysis of randomized studies concluded that unilateral training produces a small but statistically significant effect on the
strength of the homologous muscles on the contralateral side
(an increase of 8% of initial strength of the untrained limb)
(20). This review pooled data from 13 studies in an attempt to
overcome the problem of underpowered individual studies.
Although the review only considered studies that were randomly controlled, the conclusion in this review (20) is weakened because its conclusions were based on data pooled from
studies that were heterogenous with respect to the training
method, the muscle group trained, and the training duration
(20). Another limitation in most unilateral training studies is
that there has been no control for the potential influence of the
training environment on contralateral strength. Control subjects are typically not exposed to other subjects training because they typically do not attend training sessions. Exposure
IT IS WIDELY BELIEVED
Address for reprint requests and other correspondence: J. Munn, School of
Physiotherapy, The Univ. of Sydney, PO Box 170, Lidcombe NSW, 1825
Australia (e-mail: [email protected]).
1880
One hundred fifteen untrained and apparently healthy subjects (21
men and 94 women) with a mean age of 20.6 yr (SD 6.1) [mean height
of 168.1 cm (SD 9.1) and weight of 64.2 kg (SD 11.6)] participated in
the study. Subjects had no known musculoskeletal or neurological
impairment that might affect performance of the upper limb, and their
responses to a standardized questionnaire (3) suggested that they
could safely participate in physical activity. Subjects had not been
involved in regular strength training for the upper limbs in the 6 mo
before commencement of the study. Strength was measured bilaterally, and details of the unilateral increase in strength have been
reported (21). Written, informed consent was obtained from all participants before commencement of the study, which had been approved by the local Human Research Ethics Committee.
Measurements
After a warm-up, strength of the elbow flexor muscles was measured on the right and left arms of all subjects using a one-repetition
maximum (RM) test. High reliability of RM testing for the elbow
flexors (coefficients ⬎ 0.90) have been reported previously (5, 25).
Subjects were seated holding a loaded bar with the test arm resting on
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Munn, Joanne, Robert D. Herbert, Mark J. Hancock, and Simon
C. Gandevia. Training with unilateral resistance exercise increases contralateral strength. J Appl Physiol 99: 1880 –1884, 2005. First published
July 14, 2005; doi:10.1152/japplphysiol.00559.2005.—Evidence that
unilateral training increases contralateral strength is inconsistent,
possibly because existing studies have design limitations such as lack
of control groups, lack of randomization, and insufficient statistical
power. This study sought to determine whether unilateral resistance
training increases contralateral strength. Subjects (n ⫽ 115) were
randomly assigned to a control group or one of the following four
training groups that performed supervised elbow flexion contractions:
1) one set at high speed, 2) one set at low speed, 3) three sets at high
speed, or 4) three sets at low speed. Training was 3 times/wk for 6 wk
with a six- to eight-repetition maximum load. Control subjects attended
sessions but did not exercise. Elbow flexor strength was measured with a
one-repetition maximum arm curl before and after training. Training with
one set at slow speed did not produce an increase in contralateral strength
(mean effect of ⫺1% or ⫺0.07 kg; 95% confidence interval: ⫺0.42– 0.28
kg; P ⫽ 0.68). However, three sets increased strength of the untrained
arm by a mean of 7% of initial strength (additional mean effect of 0.41
kg; 95% confidence interval: 0.06 – 0.75 kg; P ⫽ 0.022). There was a
tendency for training with fast contractions to produce a greater increase
in contralateral strength than slow training (additional mean effect of 5%
or 0.31 kg; 95% confidence interval: ⫺0.03– 0.66 kg; P ⫽ 0.08), but there
was no interaction between the number of sets and training speed. We
conclude that three sets of unilateral resistance exercise produce small
contralateral increases in strength.
EFFECT OF UNILATERAL TRAINING ON CONTRALATERAL STRENGTH
Training Procedures
Subjects in both the training and control groups attended 18
training sessions over a 6- to 7-wk period. Where training was missed,
subjects were encouraged to make up the session to ensure that all
subjects completed the same total number of sessions. Just over
one-half of all subjects required a training session to be rescheduled.
All training sessions were supervised by a physical therapist.
Training groups. After a light warm-up, subjects performed unilateral elbow flexion contractions to failure (i.e., until they could no
longer complete a lift) with an estimated 6- to 8-RM load (⬃80% 1
RM). When greater than eight lifts could be performed in a training
set, the training load was increased by the researchers for the subsequent training session. Average training speed for subjects assigned to
high-speed training (1-s concentric and 1-s eccentric phase) was
⬃140°/s and for low-speed training (3-s concentric and 3-s eccentric
phase) was ⬃50°/s. Cadence was set by a tape recording that also
instructed subjects to keep the nontraining arm relaxed and controlled
the duration of the rest interval between sets (2-min maximum).
Control group. Control subjects attended the same gymnasium at
the same times as training subjects. Control subjects sat with the
assigned training arm resting on the bench but did not lift a weight.
They also listened to a tape recording, which instructed them to keep
both arms relaxed for 4 min (the average time taken in the other
groups).
After the training period, elbow flexor strength, skinfold thickness,
and arm circumference were remeasured for both limbs as previously
described. This testing was performed between 2 and 7 days after
training was completed.
Data Analysis
Data for the untrained arm were analyzed with factorial analysis of
covariance using a linear regression approach. The purpose of this
analysis was to determine the effect of number of sets and speed of
contraction on outcomes for the untrained arm. The regression model
was:
outcome ⫽ a ⫹ b共initial value兲 ⫹ c共trained兲
⫹ d共sets兲 ⫹ e共speed兲 ⫹ f共sets ⫻ speed兲
where outcome is the final measure of strength, arm girth, or biceps
skinfold thickness; initial value is the initial measure of strength, arm
J Appl Physiol • VOL
girth, or biceps skinfold thickness; trained indicates whether the
subject trained (dummy coded as 0 or 1 for control and trained
subjects, respectively); sets indicates whether the subject trained with
one or three sets (dummy coded as 1 for subjects who trained with 3
sets and 0 for all other subjects); speed indicates whether the subject
trained at high or low speeds (dummy coded as 1 for subjects who
trained fast and 0 for all other subjects); and a, b, c, d, e, and f are the
coefficients that describe the magnitude of each of these effects.
Outcome data were analyzed by intention to treat (14). Thus outcome
measures were obtained for all subjects, and data were analyzed
according to the groups to which subjects were randomized, regardless of compliance with the training protocol. Significance for all
statistical analysis was set at P ⬍ 0.05.
RESULTS
Of the 115 subjects, two discontinued participation following randomization but before commencement of training (1 in
the control group, and 1 in the 3 sets low-speed training group)
because they did not wish to adhere to training protocols. One
subject in the one-set low-speed training group completed only
4 wk of training (12 sessions) because she developed neck
pain, and one other subject, who completed all training requirements, admitted to having trained the contralateral arm outside
the research gymnasium despite being aware of the study
requirements not to do so. Thus a total of 111 subjects completed the study according to the training protocol (n ⫽ 22
control group, n ⫽ 22 training with 3 sets at high speed, n ⫽
23 training with 3 sets at low speed, n ⫽ 22 training with 1 set
at high speed, and n ⫽ 22 training with 1 set at low speed).
However, in keeping with the intention to treat principle,
outcome data are reported for all 115 subjects.
For the trained arm, one set of slow exercise increased initial
strength by 25% [mean increase of 1.38 kg; 95% confidence
interval (CI): 0.75–2.02 kg; P ⬍ 0.001], and three sets produced a greater effect and increased initial strength by 48%
(additional mean effect of 1.28 kg; 95% CI: 0.65–1.92 kg; P ⬍
0.001). Training at the higher speed resulted in an 11% greater
strength increase than slow training (additional mean effect of
0.64 kg; 95% CI: 0.01–1.27 kg; P ⫽ 0.046) (21). Pre- and
posttraining data for the untrained arm for each training group
are reported in Table 1. The change in strength for the untrained arm of each individual subject who completed training
is illustrated in Fig. 1.
Data analysis with factorial analysis of covariance showed
that there was no detectable effect of training on contralateral
strength if subjects performed only one set of exercise at the
slower speed (mean increase of ⫺0.07 kg; 95% CI: ⫺0.42–
0.28 kg; P ⫽ 0.68). However, when three sets of exercise were
performed, there was a significant increase in contralateral
strength compared with one set (additional mean effect of 0.41
kg; 95% CI: 0.06 – 0.75 kg; P ⫽ 0.02). This was equivalent to
a 7% increase in strength (% of pooled initial strength).
The effect of training speed on contralateral strength was not
statistically significant, but there was a tendency for training at
the higher speed to have a greater effect on strength of the
contralateral limb than training at the lower speed (additional
mean effect of 0.31 kg, or ⬃5% of initial strength; 95% CI:
⫺0.03– 0.66 kg; P ⫽ 0.08). There was no interaction between
training speed and number of sets (additional mean effect of
⫺0.41 kg; 95% CI: ⫺0.90 – 0.07; P ⫽ 0.10). Unilateral training had no effect on the girth or skinfold thickness of the
untrained arm.
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a bench, starting from full elbow extension and supination. The
nontest arm remained relaxed behind the back. Instructions were
given to ensure correct technique and to minimize compensatory
movements that may assist in performing the lift. The following
strategies were adopted to try to ensure maximal efforts. Subjects
were reminded to perform with maximal effort on each lift, were
loudly exhorted by the researcher, were able to discount an attempt if
he/she believed it was submaximal, and were informed that a prize
would be awarded to the strongest person (factored for gender,
weight, and height) (8). The testing procedure was repeated to determine a 1 RM to the nearest 0.25 kg. A 2- to 3-min rest interval was
given between each effort.
Skinfold thickness over the biceps site was measured in a standardized way with Harpenden calipers (23). Upper arm circumference was
measured with the shoulder flexed to ⬃90°, at a point where circumference was greatest. The subject clenched the fist and attempted to
maximally tense the elbow flexors while maintaining the forearm at
⬃45° to the upper arm (23). (This task requires cocontraction of the
elbow extensors.)
After baseline measurements, subjects were randomly allocated to
one of five equally sized groups: control, training with three sets at
high speed, training with three sets at low speed, training with one set
at high speed, or training with one set at low speed. The training arm
was also randomly assigned for all training and control subjects.
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EFFECT OF UNILATERAL TRAINING ON CONTRALATERAL STRENGTH
Table 1. Pre- and posttraining measures of strength, girth, and biceps skinfold for the
contralateral arm of control and trained subjects
Control (n ⫽ 23)
1 RM, kg
Girth, cm
Skinfold, mm
One Set, High Speed
(n ⫽ 23)
One Set, Low Speed
(n ⫽ 23)
Three Sets, High Speed
(n ⫽ 23)
Three Sets, Low Speed
(n ⫽ 23)
Pretraining
Posttraining
Pretraining
Posttraining
Pretraining
Posttraining
Pretraining
Posttraining
Pretraining
Posttraining
6.0 (2.2)
29.2 (3.0)
8.4 (4.3)
6.2 (2.5)
29.7 (2.8)
8.1 (3.2)
6.2 (4.7)
29.4 (4.6)
7.7 (3.8)
6.7 (5.0)
29.4 (4.2)
7.2 (3.3)
5.3 (2.0)
28.8 (2.9)
8.2 (2.7)
5.5 (2.0)
29.2 (2.9)
8.2 (2.3)
5.5 (2.2)
29.0 (3.2)
7.8 (3.4)
6.0 (2.3)
29.3 (3.2)
7.5 (3.0)
5.5 (2.7)
28.9 (3.2)
7.4 (4.0)
5.6 (2.0)
29.7 (3.2)
7.8 (3.9)
Data are means (SD). RM, repitition maximum.
DISCUSSION
This study investigated the contralateral effects of ipsilateral
strength training using a rigorous design. There was true
random allocation of a moderately large sample of subjects to
exercise and control groups, subjects in the control group
attended the training facility but did not train, and estimates of
the effects of training on contralateral homologous muscles
were based on analysis by intention to treat of contrasts
between trained subjects and untrained controls.
Fig. 1. One-repetition maximum (RM) elbow flexor strength pre- and posttraining (kg) illustrated by training group for the contralateral arm of each
subject. Percentage values indicate mean change in contralateral strength for
each group expressed as a percentage of mean initial strength for that group.
J Appl Physiol • VOL
In the present study, 6 wk of training with three sets of
dynamic resistance exercise produced a mean increase in
contralateral strength of 7% (95% CI: 1–13%) in subjects
without recent history of training. This effect is similar to that
derived from a recent meta-analysis (mean of 8%; 95% CI:
4 –12%) (20) and to the effect reported by Shaver (27), who
also trained elbow flexors dynamically using three sets (mean
increase of 9%; 95% CI: 4 –14%). In the present study, although there was a significant increase in mean contralateral
strength, 10 subjects from the sample showed decreased contralateral strength at follow up (Fig. 2). Interestingly, for most
of these 10 subjects, increases in strength on the trained side
were relatively small compared with the majority of the sample
(Fig. 2).
In a recent review, Zhou (31) suggested that contralateral
strength gains were related to the magnitude of strength gained
in the trained limb. This hypothesis is supported by our data,
which show that the relationship between strength gains on the
trained and untrained limb is highly significant. There was a
nonsignificant trend for training at higher speeds to produce a
greater contralateral effect than training at lower speeds (P ⫽
0.08). In the present study, the effect of training at higher
Fig. 2. Strength changes for the elbow flexors of the trained and contralateral
limb in trained subjects after 6 wk of unilateral resistance training, expressed
as a percentage of pretraining strength. Many data points are superimposed.
Contralateral arm increase in strength (%) ⫽ 0.54 ⫻ trained arm strength
increase (%) ⫺ 7.0; r ⫽ 0.532; P ⬍ 0.001.
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The relationship between ipsilateral and contralateral strength
gains for trained subjects is shown in Fig. 2. The regression
was contralateral strength increase (%) ⫽ 0.54 ⫻ ipsilateral
strength increase (%) ⫺ 7.0 (r ⫽ 0.532, P ⬍ 0.001). This
indicates that there is a highly significant relationship between
the magnitude of strength gained for the trained and untrained
limbs.
EFFECT OF UNILATERAL TRAINING ON CONTRALATERAL STRENGTH
ACKNOWLEDGMENTS
The authors thank Dr. Cath Dean and Nicole Munn for assistance with
training procedures during the study and the Allied Health Staff at Ramsay
Professional Services Baringa Hospital, Coffs Harbour, NSW for their support.
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speeds on strength of the trained arm was 11% greater than the
effect of training at lower speeds (and 36% greater than for
controls) (21). The trend seen in favor of training at higher
speeds on contralateral strength increases may be related to the
greater magnitude of strength gained in the trained arm when
training at faster speeds (21).
There were no changes in the circumference of the untrained
arm. Although the arm circumference measure used here may
not be as sensitive to small changes in muscle size as other
measures, such as those made with ultrasound (28), it is
unlikely that the contralateral increases in strength resulted
from muscle hypertrophy. Several studies, using a range of
methods, have shown that increases in contralateral strength
with unilateral training are not accompanied by contralateral
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of the contralateral side following unilateral resistance training
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limb. There may also be neural changes at other levels involving spinal (15, 16) and supraspinal premotor networks (4).
In summary, three sets of progressive dynamic resistance
exercise produce small contralateral increases in strength, and
these are graded according to the ipsilateral strength increases.
There is a trend toward greater contralateral training effects
when the training load is lifted quickly. It has not been
established whether this small increase in contralateral strength
occurring with unilateral training is functionally important.
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