Cross-Education of Muscular Endurance: A Scoping Review

Abstract

Background
It is well established that performing unilateral resistance training can increase muscle strength not only in the trained limb but also in the contralateral untrained limb, which is widely known as the cross-education of strength. However, less attention has been paid to the question of whether performing unilateral resistance training can induce cross-education of muscular endurance, despite its significant role in both athletic performance and activities of daily living.

Objectives
The objectives of this scoping review were to provide an overview of the existing literature on cross-education of muscular endurance, as well as discuss its potential underlying mechanisms and offer considerations for future research.

Methods
A scoping review was conducted on the effects of unilateral resistance training on changes in muscular endurance in the contralateral untrained limb. This scoping review was conducted in PubMed, SPORTDiscus, and Scopus.

Results
A total of 2000 articles were screened and 21 articles met the inclusion criteria. Among the 21 included studies, eight studies examined the cross-education of endurance via absolute (n = 6) or relative (n = 2) muscular endurance test, while five studies did not clearly indicate whether they examined absolute or relative muscular endurance. The remaining eight studies examined different types of muscular endurance measurements (e.g., time to task failure, total work, and fatigue index).

Conclusion
The current body of the literature does not provide sufficient evidence to draw clear conclusions on whether the cross-education of muscular endurance is present. The cross-education of muscular endurance (if it exists) may be potentially driven by neural adaptations (via bilateral access and/or cross-activation models that lead to cross-education of strength) and increased tolerance to exercise-induced discomfort. However, the limited number of available randomized controlled trials and the lack of understanding of underlying mechanisms provide a rationale for future research.

Full text

Vol.:(0123456789)
Sports Medicine (2024) 54:1771–1783
https://doi.org/10.1007/s40279-024-02042-z
REVIEW ARTICLE
Cross‑Education ofMuscular Endurance: AScoping Review
JunSeobSong1· YujiroYamada1· RyoKataoka1· WilliamB.Hammert1· AnnaKang1· JeremyP.Loenneke1
Accepted: 30 April 2024 / Published online: 17 May 2024
© The Author(s) 2024
Abstract
Background It is well established that performing unilateral resistance training can increase muscle strength not only in the
trained limb but also in the contralateral untrained limb, which is widely known as the cross-education of strength. However,
less attention has been paid to the question of whether performing unilateral resistance training can induce cross-education
of muscular endurance, despite its significant role in both athletic performance and activities of daily living.
Objectives The objectives of this scoping review were to provide an overview of the existing literature on cross-education
of muscular endurance, as well as discuss its potential underlying mechanisms and offer considerations for future research.
Methods A scoping review was conducted on the effects of unilateral resistance training on changes in muscular endurance
in the contralateral untrained limb. This scoping review was conducted in PubMed, SPORTDiscus, and Scopus.
Results A total of 2000 articles were screened and 21 articles met the inclusion criteria. Among the 21 included studies, eight
studies examined the cross-education of endurance via absolute (n = 6) or relative (n = 2) muscular endurance test, while five
studies did not clearly indicate whether they examined absolute or relative muscular endurance. The remaining eight stud-
ies examined different types of muscular endurance measurements (e.g., time to task failure, total work, and fatigue index).
Conclusion The current body of the literature does not provide sufficient evidence to draw clear conclusions on whether the
cross-education of muscular endurance is present. The cross-education of muscular endurance (if it exists) may be potentially
driven byneural adaptations (via bilateral access and/or cross-activation models that lead to cross-education of strength) and
increased tolerance to exercise-induced discomfort. However, the limited number of available randomized controlled trials
and the lack of understanding of underlying mechanisms provide a rationale for future research.
1 Introduction
Resistance training leads to improvements in strength and
muscular endurance [1–3]. When resistance training is per-
formed on one side of the body only (i.e., unilateral resist-
ance training), increased muscle strength has been observed
not only in the trained limb but also in the contralateral
untrained limb, which is widely known as the cross-educa-
tion (or cross-transfer) of strength [4, 5]. The cross-educa-
tion of strength was first reported in the scientific literature
as early as the late nineteenth century [6], and thereafter it
has been studied and reviewed extensively over the years
[4, 5, 7–9]. Although its underlying mechanisms are not
entirely understood, there is a general consensus within the
cross-education literature that the transfer of strength to the
untrained limb is mediated primarily by neural mechanisms
and likely not by mechanisms at the local muscle level (e.g.,
changes in muscle fiber type and cross-sectional area) as
these changes appear to occur within the trained limb only
[4, 7, 10, 11]. In contrast to cross-education of strength,
considerably less attention has been paid to the question
of whether performing unilateral resistance training can
increase muscular endurance in the contralateral untrained
limb (i.e., cross-education of muscular endurance).
Muscular endurance refers to the ability of muscles to
perform successive contractions at a submaximal load, and it
is considered as an important physical fitness component not
only for athletic performance in sports but also for activities
of daily living that require repetitive work [12]. Muscular
endurance can be further specified into absolute and rela-
tive muscular endurance [13]. Absolute muscular endurance
* Jeremy P. Loenneke
jploenne@olemiss.edu
1 Department ofHealth, Exercise Science, andRecreation
Management, Kevser Ermin Applied Physiology Laboratory,
The University ofMississippi, P.O. Box1848, University,
MS38677, USA
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1772 J.S.Song et al.
Key Points
Performing unilateral resistance training can increase
muscle strength not only in the trained limb but also in
the contralateral untrained limb, which is known as the
cross-education of strength. However, less attention has
been paid to the question of whether performing unilat-
eral resistance training can increase muscular endurance
in the contralateral untrained limb (i.e., cross-education
of muscular endurance).
The current body of the literature does not provide suf-
ficient evidence to draw clear conclusions whether a
cross-education of muscular endurance is present. There-
fore, further research with a nonexercise control group
(i.e., randomized controlled trials) is necessary to draw
strong conclusions.
The cross-education of muscular endurance (if it exists)
may be potentially driven by neural adaptations (via
bilateral access and/or cross-activation models that lead
to cross-education of strength) and increased tolerance to
exercise-induced discomfort.
involves performing a maximal number of repetitions with
a given absolute load regardless of changes in maximal
strength (e.g., using 60% of pretraining 1RM at pre- and
posttesting) [14]. In contrast, relative muscular endurance
involves an individual performing a maximal number of
repetitions with a load corresponding to a specific relative
intensity or percentage of the individual’s current 1RM (e.g.,
using 60% of pretraining and posttraining 1RM at pre- and
posttesting, respectively) [14]. In addition, muscular endur-
ance has been measured in several other ways when using
different types of testing (e.g., isometric, isokinetic), such as
time to task failure or total work during repeated isokinetic
contractions [15, 16]. There is evidence that resistance train-
ing can increase strength as well as induce positive mito-
chondrial and microvascular adaptations (e.g., mitochondrial
respiratory capacity, capillary to fiber ratio), which may help
explain muscular endurance adaptations in the trained limb
[17–19]. However, it remains unclear whether these mecha-
nisms can also explain the changes in muscular endurance
in the contralateral untrained limb. Therefore, the purpose
of this paper was to provide an overview of the existing
literature on cross-education of muscular endurance follow-
ing unilateral resistance training and to discuss its potential
underlying mechanisms.
2 Methods
A scoping review was conducted to evaluate the cross-
education of muscular endurance. The current study was
conducted and reported in accordance with the Preferred
Reporting for Systematic Reviews and Meta-Analyses exten-
sion for scoping reviews (PRISMA-ScR) [20].
To identify relevant articles for the current scoping
review, systematic literature searches were conducted
from inception through April 2023, using PubMed,
SPORTDiscus, and Scopus. Relevant studies were iden-
tified with the following search terms: “cross education”
OR “cross transfer” OR “contralateral effect” OR “con-
tralateral transfer” OR “interlimb transfer” OR “bilateral
transfer” AND “endurance.” An additional search was
carried out by examining the references of the included
articles. Following the removal of duplicates, articles
were screened first by title and abstract, followed by full
text screening for eligibility. The study selection process
is summarized using the PRISMA flow diagram (Fig.1).
In the present scoping review, broad inclusion criteria
were used to provide an overview of the existing litera-
ture on cross-education of muscular endurance. To be
included within the scoping review, studies were required
to fulfill the following criteria: (1) original article was
written in English language; (2) included a unilateral
resistance exercise training intervention (regardless of
strength training type and training load); (3) measured
muscular endurance (e.g., number of repetitions at an
absolute or relative load, time to task failure, total work)
in the contralateral untrained limb at pre- and posttesting;
and (4) was performed in humans with no restrictions on
age and training status. One reviewer (JSS) completed
literature searches and extraction of data. The following
information was extracted: characteristics of participants,
unilateral resistance training intervention (exercise type,
sets, repetitions, load), frequency, duration, and main out-
comes (cross-education of strength and muscular endur-
ance). Two reviewers (JSS and JPL) checked the studies
that only reported within-group changes (i.e., pre- to post-
test) for each group, and back-calculated the p-value of
between-group differences when possible.
3 Results
3.1 Search Results
The systematic search provided 2000 articles (Pub-
Med = 860, Scopus = 535, SPORTDiscus = 605), of which
377 were duplicates, leaving 1623 for screening. After
title/abstract screening, 1516 articles were excluded and
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1773
Cross-Education of Muscular Endurance
the remaining 107 articles were assessed for eligibility via
full-text screening. Ninety-six articles were omitted follow-
ing the full-text assessment, and 10 additional studies were
included by reference checking. In total, 21 studies met the
aforementioned criteria and were included in the review.
3.2 Study Characteristics
The present review included both randomized controlled
trials and nonrandomized controlled trials. Of the 21 arti-
cles included in the review (Table1), 10 studies were ran-
domized controlled trials (including a nontraining control
group) [21–30] and 11 studies were nonrandomized con-
trolled trials [31–41]. Of note, this review focused more on
randomized controlled trials, as it allows determination of
whether changes in muscular endurance in an untrained limb
(i.e., cross-education of muscular endurance) are solely due
to the training interventions.
Among the 21 included studies, nine studies employed
unilateral exercise training in the lower body (3 rand-
omized controlled trials and 6 nonrandomized controlled
trials) [21, 23, 24, 31, 33, 35–37, 39], nine studies in the
upper arm (7 randomized controlled trials and 2 nonran-
domized controlled trials) [22, 25–30, 40, 41], and three
studies used handgrip (3 nonrandomized controlled trials)
[32, 34, 38]. For the muscular endurance measurements,
absolute muscular endurance (i.e., number of repetitions
with the same given load at pre- and postintervention,
regardless of changes in maximal strength) was assessed
in six studies [21, 22, 25, 30, 39, 41], and relative mus-
cular endurance (i.e., number of repetitions with a load
corresponding to a specific relative intensity or percentage
of individual’s current 1RM) was measured in two stud-
ies [26, 27]. Of note, five studies did not provide enough
detail to determine whether absolute or relative muscular
endurance was examined for testing [23, 29, 32, 38, 40].
The remaining eight studies used several different types
of muscular endurance measurements including: time to
task failure (using absolute or relative load) [28, 31, 34,
36, 37], total work performed [24, 33], and fatigue index
(e.g., difference in work between the first three reps and
the last three reps) [35]. Among the 21 included studies,
Fig. 1 Study selection process
as per the Preferred Reporting
Items for Systematic reviews
and Meta-Analyses extension
for Scoping Reviews (PRISMA-
ScR)
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1774 J.S.Song et al.
Table 1 Studies of cross-education of muscular endurance
Study Participant Unilateral resistance training
intervention (group)
Frequency (duration) Findings (untrained limb)
Randomized controlled trials (RCTs)
Fariñas etal. [21] Young
Adults
A. Knee extension (4 sets × 8
reps, 10RM load)
B. Knee extension (32
reps × 17.4s rest between,
10RM load)
C. Control
2 sessions (× 5weeks) Endurance (AB, Reps, 10RM):
A ≈ B ≈ C
Strength (1RM): A > B ≈ C
Strength (MVIC): A ≈ B ≈ C
Fariñas etal. [22] Young
Adults
A. Biceps curl (5 sets × 6 reps,
10RM load)
B. Biceps curl (30 reps × 18.5s
rest between, 10RM load)
C. Control
2 sessions (× 5weeks) Endurance (AB, Reps, 10RM):
A ≈ B ≈ C
Strength (1RM): A > B ≈ C
Strength (MVIC): A ≈ B ≈ C
Ben Othman etal. [23] Adolescent
Males
A. Leg press (4–8 sets × 5RM)
B. Leg press (1–2 sets × 20RM)
C. Control
3 sessions (× 8weeks) Endurance (AB or RE, Reps,
60% 1RM): B > A > C
Strength (1RM): A ≈ B > C
Kannus etal. [24] 23–40years
Adults
A. Isokinetic knee extension
and flexion (5 sets × 10
maximal reps at 240°/s, 5
sets × 5 maximal reps at
60°/s, 5 sets × 25 maximal
reps at 240°/s) + isometric
knee extension (5 sets × 10s
maximal rep at a knee flexion
angle of 60°, 5 sets × 10s
maximal rep at a knee flexion
angle of 30°)
B. Control
3 sessions (× 7weeks) Group A:
Endurance (total work, isoki-
netic 240°/s): Pre < Post
Endurance (work in last 5 reps,
isokinetic 240°/s):
Pre < Post
Strength (KE, MVIC): Pre < Post
Strength (KF, MVIC): Pre ≈
Post
Strength (KE, isokinetic 60°/s):
Pre < Post
Strength (KF, isokinetic 60°/s):
Pre ≈ Post
Strength (KE, isokinetic 240°/s):
Pre < Post
Strength (KF, isokinetic 240°/s):
Pre ≈ Post
Group B:
All variables: Pre ≈ Post
Shaver [25] Young
Males
A. Elbow flexion (1 set × 30
reps/min until failure with
9.1kg)
B. Elbow flexion (1 set × 30
reps/min until failure with
9.1kg)
C. Elbow flexion (1 set × 30
reps/min until failure with
9.1kg)
D. Control
After 6weeks of training inter-
vention, each training group
received 1, 3, or 5weeks of
detraining intervention
3 sessions (× 6weeks) Group A, B, C:
Endurance (RE, Reps, 10%
MVIC): Pre < Post
Endurance (RE, Reps, 15%
MVIC): Pre < Post
Endurance (RE, Reps, 20%
MVIC): Pre < Post
Endurance (RE, Reps, 25%
MVIC): Pre < Post
Strength (MVIC): Pre < Post
Group D:
All variables: Pre ≈ Post
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1775
Cross-Education of Muscular Endurance
Table 1 (continued)
Study Participant Unilateral resistance training
intervention (group)
Frequency (duration) Findings (untrained limb)
Shaver [26] Young
Males
A. Elbow flexion (1 set × 30
reps/min until failure with
9.1kg)
B. Elbow flexion (1 set × 30
reps/min until failure with
9.1kg)
C. Elbow flexion (1 set × 30
reps/min until failure with
9.1kg)
D. Control
After 6weeks of training inter-
vention, each training group
received 1, 3, or 5weeks of
detraining intervention
3 sessions (× 6weeks) Group A, B, C:
Endurance (AB, Reps, 9.1kg):
Pre < Post
Group D:
Endurance (AB, Reps, 9.1kg):
Pre ≈ Post
Shaver [27] Young
Males
A. Elbow flexion (1 set × 10
reps with 50% of 10RM, 1
set × 10 reps with 75% of
10RM, 1 set × 10 reps with
10RM)
B. Control
3 sessions (× 6weeks) Endurance (RE, Reps 20%
MVIC): A > B
Endurance (RE, Reps, 25%
MVIC): A > B
Endurance (RE, Reps, 30%
MVIC): A > B
Endurance (RE, Reps, 35%
MVIC): A > B
Strength (MVIC): A > B
Meyers [28] Young
Males
A. Isometric elbow flexion (3
sets × 6s maximal rep at an
elbow flexion angle of 170°)
B. Isometric elbow flexion (20
sets × 6s maximal rep at an
elbow flexion angle of 170°)
C. Control
3 sessions (× 6weeks) Endurance (TTF, 100% MVIC):
A ≈ B ≈ C
Strength (MVIC 170°): A ≈ B
≈ C
Strength (MVIC 90°): A ≈ B
≈ C
Kruse and Mathews [29] Young
Males
A. Elbow flexion (1 set × 30
reps/min until failure with
3/8 of maximum strength, 2
sessions/week)
B. Elbow flexion (1 set × 30
reps/min until failure with
3/8 of maximum strength, 3
sessions/week)
C. Elbow flexion (1 set × 30
reps/min until failure with
3/8 of maximum strength, 4
sessions/week)
D. Elbow flexion (1 set × 30
reps/min until failure with
3/8 of maximum strength, 5
sessions/week)
E. Control
2–5 sessions (× 4weeks) Group A, B, C, D, E:
Endurance (AB or RE, Reps, 3/8
MVIC): Pre ≈ Post
Strength (MVIC): Pre ≈ Post
Slater-Hammel [30] Young
Males
A. Elbow flexion (1 set × 35
reps/min until failure with
6.4kg)
B. Control
3 sessions (× 3weeks) Endurance (AB, Reps, 6.4kg):
A > B
Strength: not reported
Nonrandomized and/or uncontrolled trials
Hedayatpour etal. [31] Young
Males
A. Leg press (3 sets × 15 reps,
60% 1RM)
3 sessions (× 12weeks) Endurance (RE, TTF, 50%
MVIC): Pre < Post
Strength: not reported
Yuza etal. [31] Young Females A. Handgrip exercise (1
set × 0.5s on and 0.5s off
until failure, 1/3 of maximum
handgrip strength)
5 sessions (× 4weeks) Endurance (AB or RE, Reps, 1/3
MVIC): Pre < Post
Strength (MVIC): Pre ≈ Post
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1776 J.S.Song et al.
Table 1 (continued)
Study Participant Unilateral resistance training
intervention (group)
Frequency (duration) Findings (untrained limb)
Pincivero etal. [33] Young
Adults
A. Isokinetic knee extension
and flexion (4–8 sets × 10
maximal reps, 40s rest
between sets)
B. Isokinetic knee extension
and flexion (4–8 sets × 10
maximal reps, 160s rest
between sets)
3 sessions (× 4weeks) Group A:
Endurance (total work in 30
reps, KE isokinetic 180°/s):
Pre < Post
Endurance (total work in 30
reps, KF isokinetic 180°/s):
Pre ≈ Post
Strength (KE, concentric 60°/s):
Pre ≈ Post
Strength (KF, concentric 60°/s):
Pre ≈ Post
Strength (KE, concentric
180°/s): Pre ≈ Post
Strength (KF, concentric
180°/s): Pre > Post
Group B:
Endurance (total work in 30
reps, KE isokinetic 180°/s):
Pre ≈ Post
Endurance (total work in 30
reps, KF isokinetic 180°/s):
Pre ≈ Post
Strength (KE, concentric 60°/s):
Pre ≈ Post
Strength (KF, concentric 60°/s):
Pre > Post
Strength (KE, concentric
180°/s): Pre < Post
Strength (KF, concentric
180°/s): Pre ≈ Post
Sinoway etal. [34] Young
Males
A. Handgrip exercise (1
set × 12 reps/min until fail-
ure, 30–35% MVC)
5 sessions (× 4weeks) Endurance (AB or RE, TTF,
70% of the highest sustainable
3min workload): Pre < Post
Strength (MVIC): Pre > Post
Grimby etal. [35] Old
Males
A. Isometric knee extension
(2 sets × 2 maximal reps for
4s at a knee flexion angle
of 60°, 1 set × 2 maximal
reps for 4s at a knee flexion
angle of 30°) + isokinetic
concentric knee extension
(1 set × 8 maximal reps at
30°/s, 1 set × 8 maximal
reps at 180°/s) + isokinetic
concentric/eccentric knee
extension (3 sets × 8 maximal
reps at 30°/s)
2–3 sessions (× 8–11weeks) Endurance (KE, difference in
work from the first 3 reps
to the last 3 reps during 50 reps):
Pre ≈ Post
Strength (KE, concentric 30°/s):
Pre ≈ Post
Strength (KE, eccentric 30°/s):
Pre ≈ Post
Strength (KE, concentric
120°/s): Pre ≈ Post
Strength (KE, eccentric 120°/s):
Pre ≈ Post
Parker [36] Young
Males
A. Isometric knee extension
(1 set × 10–30 brief maximal
reps at a knee flexion angle
of 90°)
B. Dynamic knee extension
(1 set × 100–300 reps with
6.4kg)
3–6 sessions (× 4months) Group A:
Endurance (AB or RE, TTF,
60% MVIC): Pre ≈ Post
Strength (MVIC): Pre < Post
Group B:
Endurance (AB or RE, TTF,
60% MVIC): Pre ≈ Post
Strength (MVIC): Pre ≈ Post
Tesch and Karlsson [37] Young
Males
A. Isometric leg press (3
sets × sustained contraction at
50% MVIC until failure)
3–4 sessions (× 6weeks) Endurance (AB or RE, TTF,
50% MVIC): Pre < Post
Strength (MVIC): Pre ≈ Post
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1777
Cross-Education of Muscular Endurance
five studies were conducted in untrained individuals [23,
24, 31, 33, 38], whereas the remaining 16 studies did not
clearly describe the training status of the participants (e.g.,
physically active, college students from physical education
program) [21, 22, 25–30, 32, 34–37, 39–41].
4 Discussion
4.1 Findings fromNonrandomized Controlled Trials
Several nonrandomized controlled trials reported changes in
muscular endurance in the untrained limb following unilat-
eral exercise training interventions. For example, 3weeks of
unilateral knee extension training increased absolute mus-
cular endurance (i.e., maximal number of repetitions using
8.2kg) in the contralateral untrained leg from pre- to posttest
[39]. Similarly, 12weeks of unilateral leg press exercise
training increased relative muscular endurance [i.e., time
to task failure during sustained isometric knee extension
at relative 50% maximum voluntary isometric contraction
(MVIC)] in the untrained leg from pre- to posttest [31]. In
addition, four studies observed an increased muscular endur-
ance (i.e., maximal number of repetitions and time to task
failure) in the untrained limb (i.e., pre- to posttest) following
4–6weeks of unilateral handgrip exercise training [32, 34,
38] and 6weeks of unilateral isometric leg press training
[37]. In those studies, however, it was unclear whether they
used an absolute or relative muscular endurance test [32, 34,
37, 38]. In one study, an increase in total work (i.e., during
30 maximum isokinetic knee extension) from pre- to post-
test was observed in the untrained leg following 4weeks
of unilateral isokinetic knee extension and flexion [33].
However, these findings were not consistent throughout the
literature. For example, no changes (i.e., pre- to posttest) in
muscular endurance (i.e., absolute and/or relative, fatigue
Table 1 (continued)
Study Participant Unilateral resistance training
intervention (group)
Frequency (duration) Findings (untrained limb)
Yasuda and Miyamura [38] Young
Males
A. Handgrip exercise (1
set × 60 reps/min until failure
with 1/3 maximum grip
strength)
B. Handgrip exercise (1
set × 60 reps/min until failure
with 1/2 maximum grip
strength)
6 sessions (× 6weeks) Group A:
Endurance (AB or RE, Reps, 1/3
MVIC): Pre ≈ Post
Strength (MVIC): Pre ≈ Post
Group B:
Endurance (AB or RE, Reps, 1/2
MVIC): Pre < Post
Strength (MVIC): Pre < Post
Hodgkins [39] Young
Females
A. Knee extension (1 set × 10
reps/min until failure with a
8.2kg boot)
3 sessions (× 3weeks) Endurance (AB, Reps, 8.2kg):
Pre < Post
Strength: not reported
Walters etal. [40] Young
Adults
A. Isometric elbow flexion (3
sets × 15s maximal rep)
B. Isometric elbow flexion
(3 sets × 15s rep at 2/3 of
maximum strength)
C. Isotonic elbow flexion (3
sets × as many repetitions as
possible within 15s, inten-
sity/load not provided)
3–5 sessions (× 2weeks) Group A:
Endurance (AB or RE, Reps, 1/3
1RM): Pre ≈ Post
Strength (MVIC): Pre < Post
Group B:
Endurance (AB or RE, Reps, 1/3
1RM): Pre ≈ Post
Strength (MVIC): Pre ≈ Post
Group C:
Endurance (AB or RE, Reps, 1/3
1RM): Pre ≈ Post
Strength (MVIC): Pre ≈ Post
Mathews etal. [41] Young
Males
A. Elbow flexion (1 set × 30
reps/min until failure)
Strength test for elbow flexion
was also performed during
each session (no detail
provided)
3 sessions (× 4weeks) Endurance (AB, Reps, 3/8
MVIC): Pre ≈ Post
Strength (EF, MVIC): Pre < Post
AB: absolute muscular endurance; AB or RE: the study did not clearly indicate whether and absolute or relative muscular endurance test was
used; KE: knee extension; MVIC: maximum voluntary isometric contraction; RE: relative muscular endurance; Reps: maximal number of repeti-
tions; TTF: time to task failure; 1RM: one-repetition maximum; > significant difference between groups (e.g., A > B indicates that group A had
greater changes in muscular endurance in the untrained limb compared to group B); ≈: no significant difference between groups (e.g., A ≈ B
indicates that the changes in muscular endurance in the untrained limb were not different between group A and B)
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1778 J.S.Song et al.
index) were observed in the contralateral untrained limb fol-
lowing unilateral knee extension training interventions [35,
36], or following unilateral elbow flexion training interven-
tions [40, 41]. Of note, however, these findings should be
interpreted with caution as it is not possible to know whether
the changes in muscular endurance are due to the exercise
training intervention or other factors outside of the training
intervention. In other words, to determine whether the cross-
education of muscular endurance is solely due to the training
interventions, a time-matched nontraining control group is
required (i.e., randomized controlled trials).
4.2 Findings fromRandomized Controlled Trials
Among ten randomized controlled trials [21–30], three
studies reported a cross-education of muscular endurance
[23, 30]. In male children (aged 10–13years), for exam-
ple, 8weeks of unilateral leg press training increased not
only strength but also muscular endurance (i.e., number
of unilateral leg press repetitions with 60% of 1RM until
failure) of the contralateral untrained leg compared with a
nontraining control group [23]. In that study, however, it
was not clear whether 60% of pre- or posttraining 1RM was
used at the posttesting (i.e., absolute or relative muscular
endurance) [23]. In healthy young males, 3weeks of uni-
lateral elbow flexion exercise training increased absolute
muscular endurance (i.e., maximal number of unilateral
elbow flexion repetitions with 6.4kg) in the contralateral
untrained arm compared with a nontraining control group
[30]. In five randomized controlled trials, only within-group
changes (i.e., pre- to posttest) in muscular endurance were
reported [24–27, 29]. For example, increases in muscular
endurance (i.e., total work performed during 25 maximal
isokinetic contractions and work performed during the last 5
repetitions) were observed in the contralateral untrained leg
from pre- to posttest in a group that performed 7weeks of
isokinetic and isometric knee extension training, whereas no
within-group changes were observed in a time-matched non-
training control group [24]. Similarly, increases in absolute
and relative [25, 26] muscular endurance from pre- to post-
test were observed in the untrained arm following 6weeks
of unilateral elbow flexion training, while no changes were
observed in a nontraining control group. In contrast, one
study found no within-group changes (pre- to posttest) in
either the training (i.e., 4weeks of unilateral elbow flex-
ion training) group or the nontraining control group [29].
Although some studies reported increases in muscular
endurance only in the training groups and not in the control
groups, this does not indicate that there was cross-educa-
tion of muscular endurance. To determine whether a cross-
education of muscular endurance is present, the changes in
muscular endurance of the training groups should be directly
compared with those of the control group. In one study,
although only within-group changes (i.e., pre- to posttest)
were reported for training and control groups, we were able
to directly compare those two groups by back-calculating
the p-value of between-group differences [27]. The calcula-
tion showed that the changes in relative muscular endur-
ance in the untrained arm following 6weeks of unilateral
elbow flexion training were significantly greater compared
with a control group, indicating a cross-education of relative
muscular endurance [27]. Three randomized controlled tri-
als did not observe cross-education of muscular endurance
[21, 22, 28]. For example, no changes in absolute muscular
endurance were observed in the contralateral untrained limb
following 5weeks of unilateral knee extension training [21]
and following 5weeks of unilateral elbow flexion training
[22] when compared with a nontraining control group. Simi-
larly, no changes in time to failure (i.e., sustaining at 100%
MVIC until force drop below 50% MVIC) were observed in
the untrained arm following 6weeks of unilateral isometric
elbow flexion training when compared with a nontraining
control group [28].
Taken together, there is very limited evidence to suggest
that performing unilateral resistance training can increase
muscular endurance in the contralateral untrained limb (i.e.,
cross-education of muscular endurance). For example, there
have been only three randomized controlled studies (out of
10 studies) that demonstrated evidence for cross-education
of muscular endurance. Among these three studies, one
showed increased absolute muscular endurance, another
showed increased relative muscular endurance, and the
third study showed increased muscular endurance (unclear
whether absolute or relative). In contrast, the remaining
seven studies either did not find or could not provide sup-
porting evidence. These discrepancies in the cross-educa-
tion of muscular endurance may be due to the differences
in training interventions (e.g., contraction type, intensity,
duration) and/or muscular endurance measurements (e.g.,
maximal number of repetitions and time to task failure using
absolute or relative load). The current body of literature does
not provide sufficient evidence to drawa clear conclusionon
whether cross-education of muscular endurance is present,
and thus requires further investigation.
4.3 Potential Underlying Mechanisms
There have been several mechanisms proposed to explain
the increase in muscular endurance in the trained limb fol-
lowing resistance training, such as increased muscle capil-
larity [17] and mitochondrial respiratory capacity/function
[42, 43]. Although these proposed mechanisms may explain
training-induced increases in muscular endurance in the
trained limb, these would be unlikely to explain the changes
in the contralateral untrained limb. The following section
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1779
Cross-Education of Muscular Endurance
will discuss potential mechanisms that might contribute to
the cross-education of muscular endurance.
4.3.1 Increases inMuscle Strength (Cross‑Education
ofStrength)
One potential adaptation that could improve absolute mus-
cular endurance in the contralateral untrained limb follow-
ing unilateral resistance training is increased strength in the
untrained limb via cross-education (i.e., cross-education
of strength). According to the size principle, motor units
are recruited in an orderly manner from the smaller motor
units (i.e., low threshold) to the larger motor units (i.e., high
threshold) as required force increases or muscle fatigues
[44]. Based on this, increases in strength following resist-
ance training may require fewer motor units to lift an abso-
lute submaximal load for the same number of repetitions,
which may delay the involvement of larger motor units and
reserve them to be recruited subsequently for sustaining the
required force as fatigue develops [14, 45]. This hypothesis
is partially supported by Ploutz etal. [45] who showed that
less muscle was recruited to lift the same submaximal load
in the untrained leg following 9weeks of unilateral knee
extension training [45], which may reserve larger motor
units to be recruited later on and consequently allow for
better performance on the absolute muscular endurance test
in the untrained limb. However, this should be interpreted
with caution since there was no time-matched control group,
which makes it difficult to know whether the changes in
muscle recruitment in the untrained limb were due to the
unilateral training or some other factor [45]. The potential
role of changes in strength on absolute muscular endurance
may be also partially supported by a secondary analysis that
examined if the changes in 1RM strength mediate changes in
absolute muscular endurance (i.e., maximal number of rep-
etitions using 42.5% pretraining 1RM) following high-load
(i.e., 70% 1RM) training compared with low-load training
interventions (i.e., 15% 1RM with or without blood flow
restriction) [18]. In that study, it was found that training-
induced increases in strength mediated the changes in mus-
cular endurance in the high-load training group relative to
the low-load training groups, suggesting that the differences
in muscular endurance between high-load and low-load
training groups may be explained by changes in strength.
However, it is of note that the mediation analysis in that
study only compared between training groups and not with
a time-matched control group, meaning that the results can
only explain the differences between training groups (i.e.,
high load versus low load). To clearly demonstrate whether
the change in strength is an underlying mechanism for
changes in muscular endurance, it may be more appropriate
to compare training groups to a nonexercise control group
in the mediation analysis. Furthermore, that analysis was on
the changes in the trained limb, and thus it remains unknown
whether increased strength from cross-education can also be
translated to improved absolute muscular endurance in the
untrained limb. One of the included studies reported con-
current increases in strength and muscular endurance in the
untrained limb [23], whereas other studies showed that the
cross-education of strength is not always accompanied by
the cross-education of absolute muscular endurance [21, 22].
Of note, simply assessing whether there were concurrent
cross-education of strength and absolute muscular endurance
may not be an appropriate approach to determine whether
cross-education of strength can be translated to cross-edu-
cation of absolute muscular endurance. A more appropriate
approach might be using a mediation analysis to examine if
the increases in strength from cross-education mediate the
changes in absolute muscular endurance in the untrained
limb [46, 47]. It is of note that some previous studies have
shown that unilateral low-load (or low-intensity) training
does not increase strength in the opposite untrained limb
(i.e., no cross-education of strength) [48, 49]. However,
this does not necessarily mean that unilateral low-load (or
low-intensity) exercise would not induce cross-education of
muscular endurance. It is plausible that cross-education of
muscular endurance can occur in the absence of strength
gain via different mechanisms.
4.3.2 Bilateral Access andCross‑Activation Model
Cross-education of relative muscular endurance likely can-
not be explained by increased strength in the contralateral
limb as relative muscular endurance is scaled to current
maximal strength. Two main theoretical models, which may
not be mutually exclusive, have been proposed to explain the
cross-education of strength and skills: “bilateral access” and
“cross-activation” models [50]. Although speculative, these
two models may also explain the cross-education of mus-
cular endurance. The “bilateral access” model involves the
development of a motor engram during unilateral resistance
training, which can be accessed not only by the trained limb,
but also by the untrained limb for the control and execu-
tion of movements [50, 51]. A widely used example is the
“callosal access” hypothesis, in which the motor engrams
developed in the trained hemisphere may be accessed by
the opposite untrained hemisphere via the corpus callosum
during motor tasks in the untrained limb [50, 51]. In this
model, it has been hypothesized that performing unilateral
resistance training may develop an effective muscle recruit-
ment pattern for maximum force production (i.e., muscle
strength), such as coordination of synergists and inhibition
of antagonists, which can be stored in neural circuits and
accessed by the untrained hemisphere [4]. Although specu-
lative, this hypothetical model may also play a role in the
cross-education of muscular endurance. In other words,
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1780 J.S.Song et al.
performing unilateral resistance training may create a motor
engram of the motor output necessary to effectively perform
repeated submaximal contractions, leading to cross-educa-
tion of muscular endurance. However, further research is
needed to determine whether or not the “bilateral access”
model plays a role in the cross-education of muscular endur-
ance in a similar way as cross-education of strength. In the
“cross-activation” model, it is proposed that performing
unilateral resistance training could induce bilateral cortical
activation, potentially leading to concurrent neural adapta-
tions in both trained and untrained hemispheres [50, 52–55].
For example, it was previously found that unilateral resist-
ance training increased corticospinal excitability in boththe
trained and untrained primary motor cortex [55]. Further-
more, decreases in interhemispheric inhibition [56], short-
interval intracortical inhibition [52, 57], and cortical silent
period [58, 59] were also observed in both the trained and
untrained side following unilateral resistance training. How-
ever, whether or not these neural adaptations can explain
the cross-education of muscular endurance is currently not
known, and further research is needed.
4.3.3 Increase inTolerance toExercise‑Induced Discomfort
Increases in tolerance to exercise-induced discomfort may
in part play a role in the cross-education of muscular endur-
ance. For example, previous studies have suggested that
the cross-education of muscular endurance may be due to
repeated exposures to uncomfortable exertions during a
training intervention, which allows individuals to accom-
modate greater exercise-induced discomfort, pain, and/
or fatigue sensation [23, 27, 30, 32]. Although it is not
directly related to exercise-induced discomfort perception,
previous cross-sectional studies have demonstrated that ath-
letes typically have higher pain tolerance when compared
with nonathlete control individuals [60, 61]. In addition,
increased pain tolerance has been observed following aero-
bic and combined (aerobic + resistance) exercise training
in healthy young adults [62]. It has been proposed that the
higher pain tolerance observed in trained individuals may be
due to enhanced pain coping strategies, developed through
repeated exposure to physical and psychological stress dur-
ing exercise [60, 63]. This is further supported by a previous
study in which 6weeks of high-intensity interval training
increased not only ischemic pain tolerance but also exercise
tolerance (i.e., time to exhaustion) when compared with vol-
ume-matched moderate-intensity continuous training [64].
In that study, it was suggested that the improvement in pain
tolerance is likely due to repeated exposure to high meta-
bolic stress and exercise-induced noxious stimuli, which
might partly explain the improvement in exercise tolerance
[64]. Based on these findings, it is possible that repeated
exposure to discomfort from unilateral resistance training
can lead to increased tolerance, resulting in increased mus-
cular endurance in the contralateral untrained limb. This
proposed mechanism is unlikely to play a role in the cross-
education of muscular endurance if the training intervention
only induces very low levels of discomfort or pain (e.g.,
low repetition with low load). However, since this proposed
mechanism is based on a study that implemented aerobic
training intervention, it needs to be further examined with
resistance training intervention.
4.4 Future Considerations
There has been extensive work on the cross-education of
strength, but far less attention has been paid to the cross-edu-
cation of muscular endurance. For example, there is a lack
of randomized controlled studies, which makes it difficult
to draw clear conclusions onthe cross-education of muscu-
lar endurance. The inclusion of a time-matched nonexercise
control group allows researchers to confidently conclude
that the increase in muscular endurance in the untrained
limb is due to the unilateral resistance training and not to
some other factor. Thus, time-matched control groups are
always recommended for future studies. In addition, it is
common to see studies reporting within-group changes (i.e.,
pre- to posttest) for each training and control group, and
when significant changes are observed only in the training
group and not in the control group, it is often concluded that
there is cross-education of muscular endurance. However,
this interpretation is problematic since the change scores
are not directly compared between groups (e.g., intervention
group versus control group). In other words, it is important
to test the group × time interaction or directly compare the
change scores between the groups if the goal is to examine
whether the changes in muscular endurance in the untrained
limb differ between the groups [65–67].
Several included studies in the present review did not
clearly indicate how they measured the cross-education
of muscular endurance. For example, a number of studies
measured the maximal number of repetitions using a cer-
tain percentage of maximum strength (e.g., 30% of 1RM);
however, they did not clearly indicate whether an absolute
or relative load/intensity was utilized at posttesting. This
lack of clarity makes it difficult to compare results across the
literature and to replicate the data in future works. Therefore,
future studies should clearly state within their methodology
whether muscular endurance was measured via an absolute
or relative muscular endurance test. In addition to absolute/
relative muscular endurance, several other types of outcome
variables have been also examined to test muscular endur-
ance (e.g., total work during a certain number of repetitions,
time to task failure). This discrepancy in methodology may
partially explain the inconsistent findings observed in the
existing literature. At present, it remains unclear which
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1781
Cross-Education of Muscular Endurance
outcome variable is the most appropriate way to test an indi-
vidual’s muscular endurance, and thus further research is
warranted. Of note, in the cross-education of strength litera-
ture, it has been suggested that the changes in strength in the
contralateral untrained limb are the greatest when it is tested
with the same movement task performed by the trained limb
(training specificity; e.g., test and train dynamically) [4].
Based on this, it may be reasonable to test the cross-educa-
tion of muscular endurance with the same movement task
used for the training intervention. However, the question of
whether the cross-education of muscular endurance follows
the principle of specificity requires further investigation.
Future studies might examine other markers of endurance
capacity (e.g., mitochondrial density, muscle capillarization)
to provide better support for the idea that the mechanism
underlying cross-education of muscular endurance may not
be local per se, but potentially via neural adaptations. A
final consideration for future studies, especially for those
attempting to address potential underlying mechanisms, may
be the use of mediation analysis. In the present review, we
suggested a number of potential underlying mechanisms
including changes in strength in the untrained limb (for
absolute muscular endurance). In one of the included stud-
ies, for example, concurrent increases in strength and mus-
cular endurance were observed in the untrained limb (i.e.,
cross-education of strength and muscular endurance) [23].
However, because there was concurrent cross-education of
strength and absolute muscular endurance, this does not
necessarily indicate that the cross-education of muscular
endurance was driven by the cross-education of strength.
One statistical approach to understanding the potential role
of strength changes in cross-education of muscular endur-
ance may be using a mediation analysis [46, 47]. Mediation
analysis can quantify the effect of the third (mediating) vari-
able (e.g., changes in strength in untrained limb) on the rela-
tionship between the independent variable (e.g., intervention
groups) and dependent variable (e.g., changes in absolute
muscular endurance in untrained limb). This approach may
help future studies with identifying the potential underlying
mechanisms that contribute to the cross-education of mus-
cular endurance (if it exists).
5 Conclusions
Performing unilateral resistance training has been shown
to increase strength not only in the trained limb but also
in the contralateral untrained limb (i.e., cross-education of
strength). However, less attention has been paid to the ques-
tion of whether performing unilateral resistance training
can also increase muscular endurance in the contralateral
untrained limb (i.e., cross-education of muscular endurance).
The current body of the literature does not provide sufficient
evidence to draw clear conclusions on whether a cross-edu-
cation of muscular endurance is present. Therefore, further
research with a nonexercise control group (i.e., randomized
controlled trials) is necessary to draw a strong conclusion.
Furthermore, some potential underlying mechanisms (i.e.,
increased strength, bilateral access model, increased tol-
erance) are discussed in the present review; however, the
proposed ideas currently lack experimental evidence and
require further research.
Acknowledgements None.
Declarations
Funding No sources of funding were used to assist in the conduct of
this review or the preparation of this article.
Conflict of Interest The authors declare that they have no conflicts of
interest relevant to the content of this article.
Ethics Approval Not applicable.
Consent to Participate Not applicable.
Consent for Publication Not applicable.
Availability of Data and Material Available upon request from the cor-
responding author.
Code Availability Not applicable.
Author Contributions JSS carried out the systematic scoping review.
YY, RK, WBH, AK, and JPL contributed to the manuscript writing.
All authors have read and approved the final version of the manuscript,
and agree with the order of presentation of the authors.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long
as you give appropriate credit to the original author(s) and the source,
provide a link to the Creative Commons licence, and indicate if changes
were made. The images or other third party material in this article are
included in the article’s Creative Commons licence, unless indicated
otherwise in a credit line to the material. If material is not included in
the article’s Creative Commons licence and your intended use is not
permitted by statutory regulation or exceeds the permitted use, you will
need to obtain permission directly from the copyright holder. To view a
copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
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