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The effect of unilateral training on contralateral limb strength in young, older, and patient populations: a meta-analysis of cross education


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Background: Cross education is the contralateral strength gain following unilateral training of the ipsilateral limb. This phenomenon provides an ideal rehabilitation model for acute or chronic rehabilitation; however, previous cross education meta-analyses have been limited to a handful of studies. Objectives: The present meta-analysis aimed to (1) be as inclusive as possible, (2) compare cross education in young able-bodied, older able-bodied, and patient populations, (3) compare cross education between training modalities, and (4) detail the impact of methodological controls on the quantification of cross education. Methodology: A review of English literature identified studies that employed unilateral resistance training and reported contralateral strength results. Studies were separated to examine the effect of population, training modality, limb, sex, and familiarization on the magnitude of cross education. The percent strength gain and effect size were calculated for ipsilateral and contralateral limbs. Results: A total of 96 studies fit the predetermined inclusion criteria and were included in the analysis. The included studies were further divided into 141 units employing separate unilateral training paradigms. These were separated into young, able-bodied (n = 126), older, able-bodied (n = 9), and neuromuscular patients (n = 6). Cross education was an average of 18% (standardized mean difference (SMD) = 0.71) in young, able-bodied participants, 17% (SMD = 0.58) in healthy able-bodied participants, and 29% (SMD = 0.76) in neuromuscular patients. Conclusion: Cross education was present in young, older, and patient populations and similar between upper and lower limbs and between males and females. Electromyostimulation training was superior to voluntary training paradigms.
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Physical Therapy Reviews
ISSN: 1083-3196 (Print) 1743-288X (Online) Journal homepage:
The effect of unilateral training on contralateral
limb strength in young, older, and patient
populations: a meta-analysis of cross education
Lara A. Green & David A. Gabriel
To cite this article: Lara A. Green & David A. Gabriel (2018): The effect of unilateral training
on contralateral limb strength in young, older, and patient populations: a meta-analysis of cross
education, Physical Therapy Reviews, DOI: 10.1080/10833196.2018.1499272
To link to this article:
© 2018 The Author(s). Published by Informa
UK Limited, trading as Taylor & Francis
Published online: 29 Oct 2018.
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The effect of unilateral training on contralateral limb strength in young,
older, and patient populations: a meta-analysis of cross education
Lara A. Green and David A. Gabriel
Department of Kinesiology, Brock University, St. Catharines, ON, Canada
Background: Cross education is the contralateral strength gain following unilateral training
of the ipsilateral limb. This phenomenon provides an ideal rehabilitation model for acute or
chronic rehabilitation; however, previous cross education meta-analyses have been limited
to a handful of studies.
Objectives: The present meta-analysis aimed to (1) be as inclusive as possible, (2) compare
cross education in young able-bodied, older able-bodied, and patient populations, (3) com-
pare cross education between training modalities, and (4) detail the impact of methodo-
logical controls on the quantification of cross education.
Methodology: A review of English literature identified studies that employed unilateral
resistance training and reported contralateral strength results. Studies were separated to
examine the effect of population, training modality, limb, sex, and familiarization on the
magnitude of cross education. The percent strength gain and effect size were calculated for
ipsilateral and contralateral limbs.
Results: A total of 96 studies fit the predetermined inclusion criteria and were included in
the analysis. The included studies were further divided into 141 units employing separate
unilateral training paradigms. These were separated into young, able-bodied (n¼126), older,
able-bodied (n¼9), and neuromuscular patients (n¼6). Cross education was an average of
18% (standardized mean difference (SMD)¼0.71) in young, able-bodied participants, 17%
(SMD ¼0.58) in healthy able-bodied participants, and 29% (SMD ¼0.76) in neuromuscu-
lar patients.
Conclusion: Cross education was present in young, older, and patient populations and simi-
lar between upper and lower limbs and between males and females. Electromyostimulation
training was superior to voluntary training paradigms.
Cross education;
cross-transfer; unilateral
strength training;
contralateral strength
transfer; stroke; elderly
Cross education is the strength gain that is found in
the contralateral limb following a unilateral training
program on the homologous limb. Cross education
was first reported in 1894 by Scripture et al. [1]
who determined that task steadiness and muscular
strength could be improved in the contralateral limb
following unilateral training. This phenomenon is of
great importance for clinical applications and
rehabilitation, and requires further mechanistic
investigation. Cross education provides a beneficial
rehabilitation model for unilateral injuries or disor-
ders; including, acute injuries or immobilization
(casting) of a single limb, and neurologic disorders,
such as stroke, affecting the body unilaterally.
Previous research has proposed that cross
education can be explained by two distinct, but not
necessarily mutually exclusive, hypotheses: cross-
activationand bilateral access[2,3]. The
cross-activationhypothesis proposes that unilateral
activity excites both ipsilateral and contralateral
cortical motor areas. With this hypothesis, the uni-
lateral training causes adaptations in both hemi-
spheres, though to a lesser extent in the untrained
hemisphere. Alternatively, the bilateral access
hypothesis suggests that the homologous untrained
muscle can access the unilateral adaptations of train-
ing through interhemispheric communication from
the associated motor areas [2,3].
Previous meta-analyses and systematic reviews
have determined that the average contralateral
strength gain from cross education is approximately
812% [47]. This amount corresponds to approxi-
mately 3560% of the strength increase that is found
in the ipsilateral (trained) limb [4,6,8]. Manca
et al. [7] further separated their estimate of cross
education into lower limb (16.4%) and upper limb
(9.4%). However, these previous reviews of cross
education were limited to 2 [9], 8 [8], 10 [10], 13
[6], 16 [4], and 31 [7] articles. There are several fac-
tors that make the review of cross education compli-
cated and limited, including the name discrepancies
CONTACT Lara A. Green Department of Kinesiology, Brock University, St. Catharines, ON L2S 3A1, Canada.
ß2018 Informa UK Limited, trading as Taylor & Francis Group
confounding the search for studies, and the variety
of training paradigms. However, the primary reason
for the small sample sizesof cross education
reviews is the stringency of inclusion criteria. The
reviews by Munn et al. [6], Carroll et al. [4], Cirer-
Sastre et al. [10], and Manca et al. [7], were limited
to the analysis of randomized controlled studies. In
addition, only studies with full data (means and
standard deviations) for each of the ipsilateral
experimental, contralateral experimental, and con-
trol limbs were included.
The inconsistent terminology and the uninten-
tional examination of cross education using the
contralateral limb as a control limbfor unilateral
training has confounded the analysis of the field.
Cross education of strength has been referred to by
many names including cross-transfer, cross-over, or
contralateral training. Similarly, the cross education
of skill following unilateral practice is typically
referred to as interlateral transfer of learning, bilat-
eral transfer, or intermanual transfer. These studies
generally focus on single session practice, rather
than training, and the transfer of a skill, rather than
strength. Although widely studied, the practice para-
digms and the outcome measurements of the cross
education of skill vary drastically across studies
making them extremely difficult to quantitatively
compare. Therefore, this meta-analysis focuses solely
on the cross education of strength.
Lastly, variability in training paradigms makes it
difficult to compare cross education between studies.
There is a considerable variation in the duration
(number of sessions), volume (contractions per ses-
sion), intensity (load), and modality (type of con-
traction or stimuli) of unilateral training. The
reviews by Carroll et al. [4], Munn et al. [6], and
Manca et al. [7] limited their analyses to studies
employing training intensities greater than 50%
maximal strength for a minimum of 2 weeks. Most
notably, the previous meta-analyses included only
isometric, isokinetic, and dynamic training [4,6,7,
10], specifically excluding alternativetraining via
electromyostimulation (EMS), transcranial magnetic
stimulation, vibration, or acupuncture.
The present analysis prioritized inclusivity over
selectivity to capture the greatest overview of the
field. A review of literature was undertaken to
include as many contralateral strength transfer
studies as possible, including studies that uninten-
tionally examined cross education by using an
untrained contralateral limb as a control for unilat-
eral training. The present analysis included studies
using alternativetraining, specifically EMS training
(or neuromuscular electrical stimulation (NMES)),
since previous meta-analysis have not previously
included non-traditionalforms of strength training.
In order to advance the use of cross education for
rehabilitation purposes, the analysis was not limited
to healthy populations as long as strength was
assessed pre and post intervention.
For the purpose of this analysis the term study will
refer to an article as referenced. The term unit will
refer to a training paradigm within a study, while
the term limb will be the designated trained,
untrained, or control limb of a participant. For
example, one study may have two units within it
where one unit was assigned to one type of training
(e.g. eccentric training, elbow flexion training, low
frequency training, etc.) and another unit was
assigned to a separate training paradigm (e.g. con-
centric training, knee flexion training, high fre-
quency training, etc.).
Literature search
The included studies were collected from an
ongoing review of cross education and unilateral
training literature. Studies were identified using
Google Scholar, PubMed, and Research Gate using
the search terms: cross education, cross-transfer,
interlimb transfer, and contralateral strength gain.
The reference list of each identified study was exam-
ined to include previously noted cross education
studies not identified in the database search. In add-
ition, studies using unilateral training were identi-
fied using search terms including: unilateral strength
training, dominant AND non-dominant control
limb and were examined for the unintentional
observation of cross education where the contralat-
eral limb was designated as a control limb.
Inclusion criteria
The selection of inclusion criteria was designed to
be as inclusive as possible for the broadest
review possible.
Population. All ages, sexes, and abilities were
included in the present review. Units were separated
into three groups: (1) young able-bodied (young)
participants (<50 years of age), (2) older able-bod-
ied (older) participants (>50 years of age), and (3)
neuromuscular disorder (patient) populations.
Training paradigm. All training types aimed at
improving strength were included in the present
study, including EMS training which has been previ-
ously excluded from cross education meta-analyses.
Training modalities (contraction types) were sepa-
rated into the following categories: isometric,
isokinetic, dynamic (including isotonic), EMS, or
other. If two types of voluntary contractions were
performed for training, then the unit was placed in
the othercategory. The EMS category consists of
stimulation alone or superimposed on a voluntary
contraction. Any training intensity (load) was
included as long as it was greater than 0% maximal
strength (i.e. the intention was strength gain, rather
than endurance gain). The criteria for number of
sessions was >5 sessions to include training stimuli
rather than mechanistic examinations.
Outcomes. Studies were included if strength was
measured and reported in any manner including: pre-
training and posttraining means, mean gain, or per-
cent gain. Studies were further separated into units
only where separate training paradigms were
employed, rather than separate outcomes. Where one
training unit had multiple outcomes, the single out-
come that was homologous to the training modality
(i.e. closest in contraction type, joint angle, speed of
contraction, etc.) was selected, with the exception of
EMS, vibration, or electroacupuncture training, where
contraction types were used for training, as well as
testing, the contraction type used most in training was
selected as the outcome measure.
Sample size. The inclusion criterion for unit sample
size was 3 to get an appropriate mean and standard
deviation for effect size calculation. No control group
was required for inclusion in the analysis.
Effect size. Where means and standard deviations
were reported effect size was calculated for each
limb within a unit using The Cochrane
Collaboration Review Manager (RevMan V.5.3) [11].
The standardized mean difference (SMD) and 95%
confidence intervals were calculated using inverse
variance as the statistical method, and random
effects as the analysis model. Statistical significance
(Z-score) was calculated in RevMan to determine if
the effect size is greater than null. Where standard
error (SE) was reported it was converted to standard
deviation (SD) using the following formula includ-
ing group sample size (n):
The effect size was calculated where possible for
the experimental limbs (trained and untrained) and
the control limb(s). If both limbs of the control
group were measured (dominant and non-domin-
ant) then each limb was separately used as a control
for the experimental limb. If only one control limb
was tested then it was included as the control for
both the trained and untrained experimental limbs.
Percent gain. Where means were reported the
percent gain of the trained and/or untrained limb
was calculated according to the following formula:
%Gain ¼PostPre
Pre 100
If only percent gain was reported but not pre-
training or posttraining mean values then the per-
cent gain was included as reported.
Cross-body transfer. The magnitude of cross-body
transfer was calculated to determine how much of
the training effect was transferred to the untrained
limb. The calculation was conducted for each unit
as follows:
Cross-body Transfer ¼Untrained %Gain
Trained %Gain 100
Comparisons. Independent sample t-tests were
performed using SAS 9.4 (SAS Institute Inc., Cary,
NC, USA) with a 0.05 significance level. The magni-
tude of percent gain in the untrained (cross educa-
tion) limb and the trained limb was examined
between (1) upper versus lower limb, (2) males ver-
sus females, and (3) familiarized versus non-fami-
liarized units. The upper limb training consisted of
elbow flexion, wrist flexion and extension, and
handgrip exercises amongst others. The lower limb
training consisted primarily of knee extension and
flexion, and secondarily plantar flexion and dorsi-
flexion exercises. The effect of sex was examined
from units that were composed of only males or
only females. Finally, familiarization was taken as
reported and included anything from a familiariza-
tion contraction or testing procedures familiariza-
tion to an entire familiarization session.
Study and unit characteristics
A total of 113 studies were identified and 96 studies
were included in the analysis (Figure 1). The 17
excluded studies did not fit the following criteria:
no strength measure (4 studies), no strength data
113 S tudi es
17 Studies Excluded:
No strength measure (4)
No strength data (4)
No pre-test data (4)
< 5 training sessions (2)
< 3 participants (3)
96 Studies
(141 units)
87 Studies
Young Able-Bodied
(126 units)
8 Studies
Older Able-Bodied
(9 units)
6 Studies
Patient Population
(6 units)
Figure 1. Flow diagram of the identification and
review process.
reported for untrained limb (4 studies), no pretest
data (4 studies), less than 5 training sessions (2
studies), and less than 3 participants (3 studies).
The remaining 96 studies included a total of 141
units. Of those, 126 units (from 87 studies [1297])
included young, able-bodied participants with a
median age of 23 years and a median sample size of
11 (range 3342) participants. Nine units (from 8
studies [13,27,72,92,93,98100]) included older,
able-bodied participants with a median age of 69
years and a median sample size of 11 (range 614).
The remaining 6 units (from 6 studies [101106])
were conducted using neuromuscular patient popu-
lations with a median sample size of 10 (range
521) participants. The neuromuscular disorder
breakdown is as follows: stroke patients (three stud-
ies), patients with various neuromuscular disorders
(one study), multiple sclerosis (one study), and
osteoarthritis patients (one study).
Outcome measures
The training characteristics are presented in Table 1
for each of the groups. The results of effect size and
percent gain for the number of units that fit each
criterion are reported for the untrained (cross edu-
cation) limb in Table 2 and for the trained limb in
Table 3. Forest plots are presented for the untrained
limb in Figure 2 for the young group and Figure 3
for the older (A) and patient (B) groups.
The average percent gain (above baseline
strength) in the untrained contralateral limb of
young participants following unilateral training in
the ipsilateral limb was 18%, as calculated from 126
units. A review of 86 units with adequate cross edu-
cation data (means and standard deviations of the
untrained limb) resulted in an effect size of 0.71
(95% CI: 0.600.83, p<0.001). The amount of cross
education was similar amongst different training
modalities with the exception of EMS training. EMS
training was employed in 10 units, which demon-
strated an average strength gain of 27%. Six units
reported enough data to calculate effect size which
was large [107] at 1.57 (95% CI: 0.812.33,
p<0.001). This is greater than the small effect size
of 0.10 (95% CI: 0.040.23, p¼0.16) in the control
limb, which corresponded to a mean 2.2% gain.
The average percent gain in the untrained limb
of older participants following unilateral training
was 15%, as calculated from 9 units. A review of 6
units with adequate cross education data resulted in
an effect size of 0.58 (95% CI: 0.220.94, p<0.01).
The modes of training included: dynamic (5), iso-
kinetic (2), isometric (1), and resistance tubing (1).
The amount of cross education in the Patients sub-
group was a 29% strength gain (calculated from 6
units), which corresponded to a large effect size of
0.76 (95% CI: 0.211.31, p<0.01, calculated from 4
Table 1. Median and range of training characteristics.
Training characteristic
Young Older Patients
Median (range)
Number of
training sessions
21 (684) 27 (636) 18 (1627)
Training volume
(sets x reps)
30 (3250) 34 (2040) 33 (2442)
Training intensity
(% maximum)
100 (10100) 100 (70100) 100 (80100)
Table 2. Effect size (standardized mean difference), percent gain, and controlled percent gain for the untrained (contralat-
eral) limb.
N(units) Effect size (95% CI) N(units) % gain Cross-body transfer
Young 86 0.71 (0.60, 0.83) 126 18 70%
Isometric 24 0.73 (0.56, 0.89) 37 15 65%
Isokinetic 23 0.61 (0.41, 0.80) 31 20 70%
Dynamic 27 0.65 (0.43, 0.86) 41 18 71%
EMS 6 1.57 (0.81, 2.33) 10 27 77%
Other 6 0.46 (0.18, 0.74) 7 12 80%
Older 6 0.58 (0.22, 0.94) 9 15 48%
Patients 4 0.76 (0.21, 1.31) 6 29 77%
Control limb 38 0.10 (0.04, 0.23) 48 2.2
CI: confidence interval; EMS: electromyostimulation. , p<0.001, for effect size only. Cross-body transfer is the amount of strength gain transferred
from the ipsilateral limb to the contralateral limb.
Table 3. Effect size (standardized mean difference), percent gain, and controlled percent gain for the trained (ipsilat-
eral) limb.
N(units) Effect size (95% CI) N(units) % gain
Young 81 1.11 (0.96, 1.25) 123 29
Isometric 25 1.11 (0.90, 1.32) 37 25
Isokinetic 20 0.95 (0.79, 1.11) 30 31
Dynamic 23 1.18 (0.89, 1.47) 39 31
EMS 7 1.87 (0.93, 2.82) 10 35
Other 6 0.53 (0.24, 0.81) 715
Elderly 6 1.44 (1.00, 1.87) 931
Patients 2 0.56 (0.11, 1.01)429
Control limb 32 0.13 (0.02. 0.27) 39 3.0
CI: confidence interval; EMS: electromyostimulation. p<0.05,  p<0.001, for effect size only.
units). Five of the studies employed strength training
(resistive exercises) of the less-affected limb, one study
[102] employed kicking and tracking movements of
the less-affected limb while secured to a tilt-table.
The influence of limb, sex, and task familiariza-
tion had no influence on the percent gain of the
untrained or trained limb, or the cross-body trans-
fer, as presented in Table 4.
Figure 2. Forest plot of standardized mean difference (SMD) for each young unit included in the analysis for the untrained
(cross education) limb. Light grey lines indicate cutoff values for small (0.2), moderate (0.5), and large (0.8) effect sizes.
The primary aim of the current meta-analysis was
to prioritize inclusivity for the largest systematic
analysis of cross education. Secondarily, this meta-
analysis aimed to further cross education within the
rehabilitation field by quantifying the presence of
cross education in young and older able-bodied par-
ticipants, as well as in patient populations. By care-
fully identifying the crucial inclusion criteria and
reducing inclusion selectivity this meta-analysis was
able to include data from 96 studies with 141 units
of training groups.
The cross education gain was an 18% increase
from baseline strength in young, able-bodied adults;
a 15% increase in older, able-bodied participants,
and a 29% increase in a patient population consist-
ing of poststroke, neuromuscular disorders, and
osteoarthritis patients. The values of cross education
are higher than the previous and most widely cited
estimates of 8% by Carroll et al. [4], and Munn
et al. [6], and higher than the recent estimate of
12% reported by Manca et al. [7] The cross-body
transfer to the untrained limb ranged from 52% to
80% of the ipsilateral training effect.
The separation of training modalities allowed for
the analysis of cross education and training adapta-
tion from different contraction types with sufficient
sample sizes and statistical power. This identified
the advanced capabilities of EMS training producing
a cross education effect of 27%, of which previous
meta-analyses excluded [4,6,7,10]. Compared to
cross education produced by isokinetic (20%),
dynamic (18%), and isometric (15%) voluntary con-
tractions, it is evident that EMS training produces a
superior transfer of strength. The logistical ease of
EMS training for varying populations and the asso-
ciated voluntary strength gains, make it an ideal
modality for cross education in rehabilitation set-
tings. Additionally, EMS training provides a viable
alternative for patients (e.g. osteoarthritis) where
pain or joint stiffness are limiting factors in conven-
tional strength training protocols [108].
The rehabilitative benefits of cross education are
present, both as a strength gain and a prevention of
strength loss. Andrushko et al. [109] detailed the
preventative effects (sparing of muscle atrophy) of
unilateral limb training during a period of contralat-
eral limb immobilization. Alternatively, the present
meta-analysis has demonstrated the presence of a
strength gain in the contralateral (more-affected)
limb of patient populations, following unilateral
training of the less-affected limb. Dragert and Zehr
[101] reported significant improvements in the
timed-up-and-go (TUG) test following unilateral
dorsiflexion training poststroke, and small but non-
significant improvements in the modified Ashworth
and Berg balance tests. Similarly, Kim et al. [102]
Figure 3. Forest plot of standardized mean difference (SMD) for each older (A) and patient (B) unit included in the analysis for
the untrained (cross education) limb. Light grey lines indicate cutoff values for small (0.2), moderate (0.5), and large (0.8)
effect sizes. DF: dorsiflexion; KE: knee extension; MS: multiple sclerosis; OA: osteoarthritis.
demonstrated significant increases in gait velocity,
cadence, stride length, symmetry, and double sup-
port periods following unilateral kicking movements
of the less-affected limb, poststroke. Manca et al.
[103] compared functional gains following direct
versus contralateral training of the more-affected
versus less-affected limb, respectively. Significant
improvements in timed walking tests were seen in
both groups. However, the direct training group had
larger effects as well as significant improvements on
the TUG test, for which contralateral training group
did not. Taken together, the contralateral strength
gains of cross education are promising for the
rehabilitation of functional movements, specifically
when the more-affected limb is unable to perform
strength training.
There were numerous methodological deficiencies
that were identified by previous meta-analyses
including the need for control group data [6] and
the lack of familiarization [4]. Both of these meth-
odological controls are instituted for the purpose of
minimizing quick jumps in strengththat would
over-estimate the magnitude of cross education. The
present meta-analysis included 48 control units for
the cross education limb reporting an average
strength gain of 2.2% (median: 2.1%, range:
6%11%). Therefore, the inclusion of a control
group is important to account for the over-estima-
tion of cross education due to extraneous factors
such as task familiarization.
It has been shown that task familiarity and famil-
iarization contractions can increase force approxi-
mately 311% within a single session [110113].
Carroll et al. [4] estimated that the effect of famil-
iarization on the overestimation of cross education
was approximately 4%. Therefore, it is surprising
that there was no significant difference in the
strength gain between groups that were familiarized
and those that were not. It was hypothesized that a
lack of familiarization would overestimate the mag-
nitude of the cross education and training strength
gain. The likely reason for the absence of difference
in the strength gain is the lack of reporting in the
majority of studies as to what was considered to be
familiarization. Since most studies neglected to
detail the method of familiarization, any study
which noted that its participants were familiarized,
be it a demonstration, a single test contraction, or
an entire session, was included in the
The large number of units included in the pre-
sent meta-analysis allowed for the comparison of
cross education between upper and lower limbs and
between sexes in 135 units of able-bodied partici-
pants. Manca et al. [7], separated 31 studies into
upper and lower limb training finding a larger mag-
nitude of cross education in the lower limb (16.4%)
compared to the upper limb (9.4%). However, the
present meta-analysis found no significant difference
between cross education in the lower (18%) and
upper (17%) limbs. Similarly, there was no signifi-
cant difference (p¼0.60) in the magnitude of cross
education between males (16%) and females (17%),
However, comparison between sexes in the trained
limb revealed slightly larger (p¼0.06) training adap-
tations in females (33%) compared to males (26%).
This resulted in a slightly larger (p¼0.17) cross-
body transfer of strength in males (65% transfer)
compared to females (54% transfer).
To date, many studies have assumed an equality
between sexes in the magnitude of cross education,
often citing the review by Zhou [8], which does not
compare sexes. In the literature, only two studies
[43,100] included sex comparisons following unilat-
eral training. Both studies also found significant dif-
ferences between sexes in the magnitude of the
training adaptation, but no difference in the magni-
tude of cross education. This indicates that there is
a difference in the amount of transfer (or ratio
between trained and untrained limbs) between the
sexes, however previous literature is conflicting.
Hubal et al. [43] found a significantly higher
strength cross-body transfer ratio in females (21%)
compared to males (16%). Alternatively, Tracy et al.
Table 4. The number of units that fall within each category: sex of the unit, the usage of familiarization, the limb involved,
and the presence of a control group from the able-bodied participants.
Number of units Number of participants % gain (untrained) % gain (trained) Cross-body transfer
Overall: able-bodied 135 2362 29 18 68%
Limb trained
Lower 68 791 18 28 68%
Upper 67 1571 17 30 69%
Sex of unit
Males only 65 997 16 26 65%
Females only 27 747 17 33 54%
Both sexes 34 506
Unknown sex 9 112
Yes 67 1387 18 29
No 68 975 17 29
Control group (Y/N) 79/56
Note: the presence of a control group (Y) does not indicate the reporting of control results. significant difference in % gain or cross-body transfer
between unit categories (lower vs upper; males vs females; yes vs no familiarization), p<0.05.
[100] found a significantly lower strength transfer
ratio in females (32% transfer) compared to males
(36% transfer).
A review of 141 unilateral training units resulted in
a cross education strength gain of 18% in young
adults, 15% in older adults, and 29% in a patient
population, which is higher than previous estimates
[4,6,7] of 8% to 12%. The cross education effect
was accompanied by a significant moderate to large
effect size in each population. The average cross-
body transfer ranged from 48% to 77% slightly
higher that previous estimates of 3560% [4,6]. The
present analysis identified: the presence of cross
education in young and older able-bodied partici-
pants as well as patient populations; the efficacy of
EMS training over voluntary modalities; and the
equivalence in cross education between upper and
lower limbs as well as in males and females. The
1529% magnitude of cross education is promising
for the use of unilateral training in rehabilitation.
Disclosure Statement
No potential conflict of interest was reported by
the authors.
Notes on contributors
Lara A. Green recently completed her Ph.D. in health
biosciences at Brock University examining the phenom-
enon of cross education. David A. Gabriel completed his
Ph.D. in biomechanics at McGill University in 1995. He
worked as a post-doctoral fellow in orthopedic biomech-
anics at the Mayo Clinic until 1997. He is currently a
professor at Brock University.
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... It is well known that muscle strength gain conferred by a unilateral limb resistance training is transferred to a nontrained homologous muscle of the contralateral limb, which is referred to as the cross-education effect (6)(7)(8). A meta-analysis study by Munn et al. (8) showed that the magnitude of increase in muscle strength of the contralateral limb was 35% (95% confidence interval (CI), 20.9%-49.3%) of that of the ipsilaterally trained limb. Green and Gabrial (7) revealed that the cross-education effect was similar between upper-and lower-limb muscles, between sexes, and between young and old individuals, and the ratio between the nontrained and trained muscle strength gain ranged between 48% and 77% among 96 studies. ...
... A meta-analysis study by Munn et al. (8) showed that the magnitude of increase in muscle strength of the contralateral limb was 35% (95% confidence interval (CI), 20.9%-49.3%) of that of the ipsilaterally trained limb. Green and Gabrial (7) revealed that the cross-education effect was similar between upper-and lower-limb muscles, between sexes, and between young and old individuals, and the ratio between the nontrained and trained muscle strength gain ranged between 48% and 77% among 96 studies. Importantly, the magnitude of the cross-education effect seems to be greater after eccentric (ET) than concentric resistance training (CT). ...
... It has been reported that the magnitude of increase in muscle strength of the contralateral limb was 35% (95% CI: 20.9%-49.3%) (8) or 48%-77% of that of the ipsilaterally trained limb (7). Thus, it is likely that the greater cross-education effect by ET than CT was due to the greater training effects induced by ET than CT. ...
Introduction: The present study tested the hypothesis that eccentric training (ET) of non-immobilized arm would attenuate negative effects of immobilization and provide greater protective effects against muscle damage induced by eccentric exercise after immobilization, when compared with concentric training (CT). Methods: Sedentary young men were placed to ET, CT or control group (n = 12/group), and their non-dominant arms were immobilized for 3 weeks. During the immobilization period, ET and CT groups performed 5 sets of 6 dumbbell curl eccentric-only and concentric-only contractions, respectively at 20-80% of maximal voluntary isometric contraction (MVCiso) strength over six sessions. MVCiso torque, root-mean square (RMS) of electromyographic activity during MVCiso, and bicep brachii muscle cross-sectional area (CSA) were measured before and after immobilization for both arms. All participants performed 30 eccentric contractions of the elbow flexors (30EC) by the immobilized arm after the cast was removed. Several indirect muscle damage markers were measured before, immediately after, and for 5 days following 30EC. Results: ET increased MVCiso (17 ± 7%), RMS (24 ± 8%), and CSA (9 ± 2%) greater (P < 0.05) than CT (6 ± 4%, 9 ± 4%, 3 ± 2%) for the trained arm. The control group showed decreases in MVCiso (-17 ± 2%), RMS (-26 ± 6%), and CSA (-12 ± 3%) for the immobilized arm, but these changes were attenuated greater (P < 0.05) by ET (3 ± 3%, -0.1 ± 2%, 0.1 ± 0.3%) than CT (-4 ± 2%, -4 ± 2%, -1.3 ± 0.4%). Changes in all muscle damage markers after 30EC were smaller (P < 0.05) for the ET and CT than control, and ET than CT group (e.g., peak plasma creatine kinase activity, ET: 860 ± 688, CT: 2,390 ± 1104, control: 7,819 ± 4,011 IU/L). Conclusions: These results showed that ET of the non-immobilized arm was effective for eliminating the negative effects of immobilization and attenuating eccentric exercise-induced muscle damage after immobilization.
... A growing body of evidence has demonstrated the potential importance of contralesional pathways toward functional recovery, providing a potential solution for harnessing plasticity within the contralesional hemisphere to facilitate functional recovery in severe stroke (Bradnam et al., 2013;Buetefisch, 2015). Unilateral motor training, which has been extensively studied in healthy individuals and some patient populations, has demonstrated bilateral training effects (Green & Gabriel, 2018). This phenomenon, termed as 'cross-education', is defined as the transfer of strength or motor skill gains to the contralateral untrained limb following unilateral motor training (Ruddy & Carson, 2013). ...
... There is adequate evidence to support that strength or skill training of one limb can benefit the untrained muscle/limb (Cirer-Sastre et al., 2017;Farthing, 2009;Manca et al., 2017). A recent meta-analysis that included 95 cross-education studies performed in neurologically intact individuals demonstrated approximately 18% increase in strength of the untrained muscle after unilateral strength training (Green & Gabriel, 2018). Studies have also demonstrated cross-education of motor skills following skill-based training in healthy individuals (Carroll et al., 2008;Hinder et al., 2013;Lee et al., 2010;Leung et al., 2018;Leung et al., 2015). ...
... Five out of seven studies reported significant bilateral strength gains following unilateral strength training in stroke survivors. This result is consistent with a recent meta-analysis in healthy individuals which reported 28% strength gain in the trained muscle and 18% strength gain in the untrained muscle (Green & Gabriel, 2018). The strength gains in stroke were greater with an average increase of 34% in the trained/non-paretic muscle and 93% in the untrained/paretic muscle (percentage changes calculated from the group mean data of the five studies that demonstrated bilateral strength gains). ...
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Current stroke rehabilitation interventions focus on intensive task specific training of the paretic limb, which may not be feasible for individuals with higher levels of impairment or in the early phase of stroke. Cross-education, a mechanism that improves strength or skill of the untrained limb following unilateral motor training, has high clinical relevance for stroke rehabilitation. Despite its potential benefits, our knowledge on the application and efficacy of cross-education in stroke is limited. We performed a scoping review to synthesize the current evidence regarding neurophysiological and motor effects of cross-education training in stroke. Low to strong evidence from five studies demonstrated strength gains ranging from 31-200% in the untrained paretic limb following non-paretic muscle training. Neurophysiological mechanisms underlying cross-education were unclear as the three studies that used transcranial magnetic stimulation to probe functional connectivity demonstrated mixed results in low sample size. Our review suggests that cross-education is a promising clinical approach in stroke, however high quality studies focusing on neurophysiological mechanisms are required to establish the efficacy and underlying mechanisms of cross-education in stroke. Recommendations regarding future directions and clinical utility are provided.
... Conversely, the "bilateral-access hypothesis" suggests that neural adaptations in the trained hemisphere following unilateral strength training can be accessed by the untrained hemisphere. 6,7 Both types of adaptations would eventually lead to enhanced neural signaling during voluntary contractions in both limbs, resulting in greater motor unit recruitment, firing frequency, and improved synchronicity. 8,9 This results in improved strength and power output from the ipsilateral trained limb and its untrained counterpart. ...
... Previous research in healthy and functionally disabled populations reported that contralateral strength increased by 17% and 29%, respectively. 7 Ankle injuries, requiring surgery and immobilization, are common in both sporting and general populations, with Achilles tendon rupture incidence being reported as 29.3 per 100,000 person years. 10 The plantar flexors are particularly susceptible to immobilization-induced muscle atrophy due to a higher proportion of slow type I fibers to fast type II fibers. ...
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Context: Cross-education (CE) refers to neuromuscular gains in the untrained limb upon contralateral limb training. To date, only laboratory-based exercise programs have demonstrated CE. Home-based exercise prescription eliciting CE could have greater clinical applicability. Objective: To determine the effect of an 8-week, home-based unilateral strength training intervention on isokinetic muscle strength, muscular excitation, and power in trained and untrained plantar flexors. Design: Randomized controlled trial. Methods: Thirty-four healthy participants were randomized to intervention (n = 20) or control (n = 14). The intervention group completed 3 sets of 12 repetitions of progressively loaded unilateral calf raises 3 days per week. Concentric and eccentric peak torque were measured using isokinetic dynamometry at 30°/s and 120°/s. Maximal electromyogram amplitude was simultaneously measured. Power was measured using a jump mat. All variables were measured at preintervention, midintervention, and postintervention. Results: Strength significantly increased bilaterally pre-post at both velocities concentrically and eccentrically in intervention group participants. Maximal electromyogram amplitude significantly increased pre-post bilaterally at both velocities in the medial gastrocnemii of the intervention group. Power significantly increased bilaterally pre-post in the intervention group, with a dose-response effect demonstrated in the untrained plantar flexors. The CE effects of strength, power, and electromyogram activation were 23.4%, 14.6%, and 25.3%, respectively. All control group values were unchanged pre-post. Conclusion: This study shows that a simple at-home unilateral plantar flexor exercise protocol induces significant increases in contralateral strength, muscular excitation, and power. These results suggest the applicability of CE in home rehabilitation programs aiming to restore or maintain neuromuscular function in inactive individuals or immobilized ankles.
... However, the robot-assisted technique is always limited to fixed movement trajectories and it is difficult to train fine movement of the fingers. Another approach to facilitate post-stroke training of the paretic hand is cross-limb transfer via visual feedback, i.e. the initial practice of unilateral motor tasks with one hand improves subsequent performance with the unpracticed/paretic hand [13,14]. For example, practicing a motor task with one limb had been shown to improve performance with the limb opposite in a wide range of motor tasks including mirror tracing, pursuit tracking, sequential finger tapping, the serial reaction time task, and reaching while exposed to force perturbations [14][15][16]. ...
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We have previously shown that healthy subjects can transfer coordination skills to the unpracticed hand by performing a unimanual task with the other hand and visualizing a bimanual action using a game-like interactive system. However, whether this system could be used to transfer coordination skills to the paretic hand after stroke and its underlying neural mechanism remain unknown. Here, using a game-like interactive system for visualization during physical practice in an immersive virtual reality environment, we examined coordination skill improvement in the unpracticed/paretic hand after training in 10 healthy subjects and 13 chronic and sub-acute stroke patients. The bimanual movement task was defined as simultaneously drawing non-symmetric three-sided squares (e.g., U and C), while the training strategy was performing a unimanual task with the right/nonparetic hand and visualizing a bimanual action. We found large decreases in the intra-hand temporal and spatial measures for movement in the unpracticed/paretic hand after training. Furthermore, a substantial reduction in the inter-hand temporal and spatial interference was observed after training. Additionally, we examined the related cortical network evolution using EEG in both the healthy subjects and stroke patients. Our studies show that the cortical network became more efficient after training in the healthy subjects and stroke patients. These results demonstrate that our proposed method could contribute to the transference of coordination skill to the paretic/unpracticed hand by promoting the efficiency of cortical networks.
... Despite a lot of studies describing CE, there is an inconsistency between their findings [1,3,7]. However, some evidence seems to be well established. ...
The purpose of this study was to determine the effect of training of one side of the body on the muscle torques and power output on the trained and untrained side. Seventeen female and twenty-two male students were subject to a four-week knee joint power training regimen on a specially designed stand. The subjects were divided into two groups: a training group (female - N = 11 and male - N = 16) and a control group (female - N = 6 and male - N = 6). Effectiveness of power training on the stand described previously was estimated based on bilateral knee torque and power under static and isokinetic conditions. The experiment lasted for 39 days and was preceded by preliminary studies (pre-training). Control measurements in training groups were made after four weeks of training (post-training) and after the next two weeks (de-training). Power training caused an insignificant increase in force and power in both groups for the untrained leg and a significant increase in RMS EMG. Therefore, the study confirmed the hypothesis that resistance training performed in dynamic conditions can affect the contralateral limb and may also trigger delayed adaptations to training conditions during the detraining phase. Sex differences in adaptation to power training are not clear; however, the differences in gains in contralateral effects between men and women were not confirmed.
... Recently, several studies on NMES showed that electric stimulation for one limb, has improved the contralateral muscle strength, which occurred by activating the contralateral motor pathway, contralateral hemisphere, and the ipsilateral sensory or motor cortical areas (Arkov et al 2010;Kadri et al 2017;Minetto et al 2018;Cattagnia et al., 2018). These finding show that the unilateral motor and sensory activity affect structures bilaterally producing cross education or neural adaptation (Green and Gabriel 2018), which are relatively dependent on direct or indirect corticospinal projections (Hendy and Lamon 2017). ...
Background In the field of rehabilitation, the acute application of neuromuscular electrical stimulation (NMES) causes not only peripheral muscle contraction but also involve the central nervous system by the transient increase in spinal motor neuron and cortical activity. Therefore it has been used in several fields of rehabilitation. Previous studies used surface electromyography to assess this effect. But we conducted our study to assess the effect of NMES on contralateral quadriceps muscle in normal individuals using another method needle electromyography. Methods A self-control study, carried out on 20 normal males, who were subjected to (i) NMES Training Program for 60 min for the right quadriceps muscle. (ii) Assessment of EMG activity for rectus femoris muscle (RF) on the contralateral side. An assessment was done for minimal volition and maximal volition or interference pattern analysis, this assessment was done twice: before the start of NMES and during the session. Results EMG of voluntary activity (Minimal volition) and Maximum voluntary activity analysis for RF muscles showed increased duration (in millisecond), amplitude (in millivolt) (P < 0.01), increased activity in turn per second, amplitude/turn (M) (uV) compared to the result before NMES application. Conclusion Our study provides a new evident date that the acute NMES application to the contralateral quadriceps muscles, leads to significant facilitation of the maximal voluntary power in the ipsilateral muscles through activation of efferent neural control. This facilitating effect of motor neurons in the contralateral muscles is likely due to the complex combination interaction between spinal and supraspinal control. Trial registration Trial registration: PACTR202010887172053.
Context: More studies are needed to compare the effect of voluntary contraction, electrical stimulation, and electrical stimulation superimposed onto voluntary contraction in improving trained and untrained homolog muscle strength and lower-extremity endurance. Design: Seventy-six healthy young adults (age = 20.41 [3.07] y, 61 females and 15 males) were included in the study. Subjects were randomly divided into 3 groups as voluntary isometric contraction (IC) group, Russian current (RC) group, and superimposed Russian current (SRC) group. Methods: All training regimens were performed under physiotherapist supervision for a total of 18 sessions (3 times per week for 6 wk). In each session, 10 ICs were achieved with voluntary isometric exercise only, RC only, or RC superimposed onto ICs. Main outcome measures were trained and untrained quadriceps strength (maximal voluntary isometric contraction [MVIC]) and lower-extremity endurance (sit-to-stand test). Results: After 6 weeks of training, all outcome measures improved in all groups (P < .05), except the untrained quadriceps MVIC score of RC group (P = .562). The trained quadriceps MVIC score (P < .001, η2 = .478), untrained quadriceps MVIC score (P = .011, η2 = .115), and sit-to-stand test score (P < .001, η2 = .357) differed significantly among the 3 groups; post hoc analysis revealed that the trained quadriceps MVIC score was higher in SRC and RC groups than in the IC group, untrained quadriceps MVIC score was higher in SRC group than in the RC group, and sit-to-stand test score was higher in SRC group than in the RC group and IC group. Conclusions: RC and RC superimposed onto IC are superior to IC in improving quadriceps muscle strength, and RC superimposed onto IC is superior to RC and IC in improving lower-extremity endurance. RC superimposed onto IC and voluntary IC created cross-education effect on untrained quadriceps.
Ballistic motor training induces plasticity changes and imparts a cross-transfer effect. However, whether there are age-related differences in these changes remain unclear. Thus, the purpose of this study was to perform a meta-analysis to determine the corticospinal responses and cross-transfer of motor performance following ballistic motor training in young and older adults. Meta-analysis was performed using a random-effects model. A best evidence synthesis was performed for variables that had insufficient data for meta-analysis. There was strong evidence to suggest that young participants exhibited greater cross-transfer of ballistic motor performance than their older counterparts. This meta-analysis showed no significant age-related differences in motor-evoked potentials (MEPs), short-interval intracortical inhibition (SICI) and surface electromyography (sEMG) for both hands following ballistic motor training.
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This study aimed to identify the ipsilateral corticospinal responses of the contralateral limb following different types of unilateral motor-training. Three groups performing unilateral slow-paced strength training (SPST), non-paced strength training (NPST) or visuomotor skill training (VT) were compared to a control group. It was hypothesised that 4 weeks of unilateral SPST and VT, but not NPST, would increase ipsilateral corticospinal excitability (CSE) and reduce short-interval cortical inhibition (SICI), resulting in greater performance gains of the untrained limb. Tracking error of the untrained limb reduced by 29 and 41% following 2 and 4 weeks of VT. Strength of the untrained limb increased by 8 and 16% following 2 and 4 weeks of SPST and by 6 and 13% following NPST. There was no difference in cross-education of strength or tracking error. For the trained limb, SPST and NPST increased strength (28 and 26%), and VT improved by 47 and 58%. SPST and VT increased ipsilateral CSE by 89 and 71% at 2 weeks. Ipsilateral CSE increased 105 and 81% at 4 weeks following SPST and VT. The NPST group and control group showed no changes at 2 and 4 weeks. SPST and VT reduced ipsilateral SICI by 45 and 47% at 2 weeks; at 4 weeks, SPST and VT reduced SICI by 48 and 38%. The ipsilateral corticospinal responses are determined by the type of motor-training. There were no differences in motor performance between SPST, NPST and VT. The data suggests that the corticospinal responses to cross-education are different and determined by the type of motor-training.
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Purpose: Cross-education (CE) of strength is a well-known phenomenon whereby exercise of one limb can induce strength gains in the contralateral untrained limb. The only available meta-analyses on CE, which date back to a decade ago, estimated a modest 7.8% increase in contralateral strength following unilateral training. However, in recent years new evidences have outlined larger contralateral gains, which deserve to be systematically evaluated. Therefore, the aim of this meta-analysis was to appraise current data on CE and determine its overall magnitude of effect. Methods: Five databases were searched from inception to December 2016. All randomized controlled trials focusing on unilateral resistance training were carefully checked by two reviewers who also assessed the eligibility of the identified trials and extracted data independently. The risk of bias was assessed using the Cochrane Risk-of-Bias tool. Results: Thirty-one studies entered the meta-analysis. Data from 785 subjects were pooled and subgroup analyses by body region (upper/lower limb) and type of training (isometric/concentric/eccentric/isotonic-dynamic) were performed. The pooled estimate of CE was a significant 11.9% contralateral increase (95% CI 9.1-14.8; p < 0.00001; upper limb: + 9.4%, p < 0.00001; lower limb: + 16.4%, p < 0.00001). Significant CE effects were induced by isometric (8.2%; p = 0.0003), concentric (11.3%; p < 0.00001), eccentric (17.7%; p = 0.003) and isotonic-dynamic training (15.9%; p < 0.00001), although a high risk of bias was detected across the studies. Conclusions: Unilateral resistance training induces significant contraction type-dependent gains in the contralateral untrained limb. Methodological issues in the included studies are outlined to provide guidance for a reliable quantification of CE in future studies.
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Cross-education of strength occurs when strength-training one limb increases the strength of the untrained limb and is restricted to the untrained homologous muscle. Cortical circuits located ipsilateral to the trained limb might be involved. We used transcranial magnetic stimulation (TMS) to determine the corticomotor responses from the untrained homologous (biceps brachii) and non-homologous (flexor carpi radialis) muscle following strength-training of the right elbow flexors. Motor evoked potentials were recorded from the untrained left biceps brachii and flexor carpi radialis during a submaximal contraction from 20 individuals (10 women, 10 men, aged 18-35 years; training group; n = 10 and control group; n = 10) before and after 3-weeks of strength-training the right biceps brachii at 80% of 1-repetition maximum (1-RM). Recruitment-curves for corticomotor excitability and inhibition of the untrained homologous and non-homologous muscle were constructed and assessed by examining the area under the recruitment curve (AURC). Strength-training increased strength of the trained elbow flexors (29%), resulting in a 18% increase in contralateral strength of the untrained elbow flexors (P <0.0001). The trained wrist flexors increased by 19%, resulting in a 12% increase in strength of the untrained wrist flexors (P = 0.005). TMS showed increased corticomotor excitability and decreased corticomotor inhibition for the untrained homologous muscle (P < 0.05); however, there were no changes in the untrained non-homologous muscle (P > 0.05). These findings show that the cross-education of muscular strength is spatially distributed; however, the neural adaptations are confined to the motor pathway ipsilateral to the untrained homologous agonist.
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Purpose: To compare the effects of unilateral strength training by stimulated and voluntary contractions on muscle strength and monopedal postural control of the contralateral limb. Methods: 36 non-active healthy male subjects were recruited and split randomly into three groups. Two groups of 12 subjects took part in a strength-training program (3 sessions a week over 8 weeks) comprising 43 contractions of the quadriceps femoris of the ipsilateral limb (at 20% of the MVC). One group carried out voluntary contractions exclusively (VOL group), while the other group benefited exclusively from electro-induced contractions (NMES group). The other 12 subjects formed the control (CON) group. Assessments of MVC and monopedal postural control in static and dynamic postural tasks were performed with the ipsilateral (ISPI) and contralateral (CONTRA) limbs before (PRE) and after (POST) completion of the training program. Results: After the training program, the MVC of the IPSI and CONTRA limbs increased similarly for both experimental groups (VOL and NMES). There were no significant improvements of monopedal postural control for the IPSI or CONTRA limbs in either the VOL or NMES experimental group. No change was observed for the CON group over the protocol period. Conclusion: The purposed training program with NMES vs VOL contractions induced strength gains but did not permit any improvement of contralateral monopedal postural control in healthy young subjects. This has potential for therapeutic application and allows clinicians to focus their training programs on dynamic and poly-articular exercises to improve the postural control in young subjects.
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Purpose: To test the effects of 4 weeks of unilateral low-load resistance training (LLRT), with and without blood flow restriction (BFR), on maximal voluntary contraction (MVC), muscle thickness, volitional wave (V wave), and Hoffmann reflex (H reflex) of the soleus muscle. Methods: Twenty-two males were randomly distributed into three groups: a control group (CTR; n = 8); a low-load blood flow restriction resistance training group (BFR-LLRT; n = 7), who were an inflatable cuff to occlude blood flow; and a low-load resistance training group without blood flow restriction (LLRT; n = 7). The training consisted of four sets of unilateral isometric LLRT (25% of MVC) three times a week over 4 weeks. Results: MVC increased 33% (P < 0.001) and 22% (P < 0.01) in the trained leg of both BFR-LLRT and LLRT groups, respectively. The soleus thickness increased 9.5% (P < 0.001) and 6.5% (P < 0.01) in the trained leg of both BFR-LLRT and LLRT groups, respectively. However, neither MVC nor thickness changed in either of the legs tested in the CTR group (MVC -1 and -5%, and muscle thickness 1.9 and 1.2%, for the control and trained leg, respectively). Moreover, V wave and H reflex did not change significantly in all the groups studied (Vwave/M wave ratio -7.9 and -2.6%, and H max/M max ratio -3.8 and -4%, for the control and trained leg, respectively). Conclusions: Collectively, the present data suggest that in spite of the changes occurring in soleus strength and thickness, 4 weeks of low-load resistance training, with or without BFR, does not cause any change in neural drive or motoneuronal excitability.
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There is solid evidence on the cross-training phenomenon, but the training load required to achieve it has yet to be established. The aim of this meta-analysis was to deduce which unilateral strength training load (duration, frequency, intensity, rest and type) would enable the biggest strength increases to be obtained in the inactive contralateral limb. The examined studies were limited to those written in the English language within the Web of Science, PubMed and SPORTDiscus databases. Ten of the 43 eligible studies were included, covering a total of 409 participants. The studies included in the meta-analysis showed a low risk of bias and had an estimated pooled effect size of 0.56 (95% CI from 0.34 to 0.78). Greater effect sizes were observed in lengthy protocols involving fast eccentric exercises using designs of 3 sets of 10 repetitions and a 2-minute rest time. Effect size did not relate to absolute volume, relative intensity, absolute duration and speed of execution. In conclusion, to optimize contralateral strength improvements, cross-training sessions should involve fast eccentric sets with moderate volumes and rest intervals.
The contralateral effects of unilateral strength training, known as cross-education of strength, date back well over a century. In the last decade, a limited number of studies have emerged demonstrating the preservation or "sparing" effects of cross-education during immobilization. Recently published evidence reveals that the sparing effects of cross-education show muscle site specificity and involve preservation of muscle cross-sectional area. The new research also demonstrates utility of training with eccentric contractions as a potent stimulus to preserve immobilized limb strength across multiple modes of contraction. The cumulative data in nonclinical settings suggest that cross-education can completely abolish expected declines in strength and muscle size in the range of ∼13% and ∼4%, respectively, after 3-4 weeks of immobilization of a healthy arm. The evidence hints towards the possibility that unique mechanisms may be involved in preservation effects of cross-education, as compared with those that lead to functional improvements under normal conditions. Cross-education effects after strength training appear to be larger in clinical settings, but there is still only 1 randomized clinical trial demonstrating the potential utility of cross-education in addition to standard treatment. More work is necessary in both controlled and clinical settings to understand the potential interaction of neural and muscle adaptations involved in the observed sparing effects, but there is growing evidence to advocate for the clinical utility of cross-education.
Efferent neural drive during strong muscle contractions is attenuated with age, even after lifelong strength training. However, it is unknown if this deterioration may impede contralateral neural plasticity, and limit the clinical value of unilateral strength training. We assessed muscle force-generating capacity, evoked potentials recordings (V-wave and H-reflex normalized to M-wave; V/M-ratio and H/M-ratio) and voluntary activation (VA) in the plantar flexors of the contralateral limb following unilateral maximal strength training (MST) with the dominant limb for three weeks (9 sessions). Twenty-three 73±4(SD) year old males were randomized to a MST group (N=11), exercising with an intensity of ~90% of maximal strength, or a control group (CG, N=12). MST improved contralateral maximal strength (107.6±27.0 to 119.1±34.8Nm;10%) and rate of force development (197.3±54.1 to 232.8±77.7Nms ⁻¹;18%) (both p<0.05). These strength gains were associated with (r=0.465 to 0.608) an enhanced soleus V/M-ratio (0.12±0.09 to 0.21±0.17) and VA (79.5±5.1 to 83.3±5.2%) (all p<0.05). H/M-ratio (10% maximal strength) remained unaltered after MST, and no changes were apparent in the CG. In conclusion, cross-limb effects in older adults are regulated by efferent neural drive enhancement, and advocate the clinical relevance of MST to improve neuromuscular function in individuals with conditions that results in unilateral strength reductions.
Purpose This study aimed to assess the efficacy of applying anodal transcranial direct-current stimulation (a-tDCS) to the ipsilateral motor cortex (iM1) during unilateral strength training to enhance the neurophysiological and functional effects of cross-education. Methods Twenty-four healthy volunteers were randomly allocated to perform either of the following: strength training during a-tDCS (ST + a-tDCS), strength training during sham tDCS (ST + sham), or a-tDCS during rest (a-tDCS) across 2 wk. Strength training of the right biceps brachii involved four sets of six repetitions at 80% of one-repetition maximum three times per week. Anodal tDCS was applied to the iM1 at 1.5 mA for 15 min during each strength training session. Outcome measures included one-repetition maximum strength of the untrained biceps brachii, corticomotoneuronal excitability, cross-activation, and short-interval intracortical inhibition (SICI) of the iM1 determined by transcranial magnetic stimulation. Results Immediately after the final training session, there was an increase in strength for both the ST + a-tDCS (12.5%, P < 0.001) and the ST + sham group (9.4%, P = 0.007), which was accompanied by significant increases in corticomotoneuronal excitability and decreases in SICI for both groups. After a 48-h retention period, strength increase was maintained in the ST + a-tDCS (13.0%, P = 0.001) group, which was significantly greater than the ST + sham group (7.6%, P = 0.039). Similarly, increases in corticomotoneuronal excitability and decreases in SICI were maintained in the ST + a-tDCS group but not in the ST + sham group. No main effects were reported for the a-tDCS group (all P > 0.05). Conclusions The addition of a-tDCS to the iM1 during unilateral strength training prolongs the benefits of cross-education, which may have significant implications to enhancement of rehabilitation outcomes after a single-limb injury or impairment.