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Brief Review
Sex Differences in Resistance Training: A
Systematic Review and Meta-Analysis
Brandon M. Roberts,
1
Greg Nuckols,
2
and James W. Krieger
3
1
University of Alabama at Birmingham, Birmingham, Alabama;
2
Stronger by Science LLC, Raleigh, North Carolina; and
3
Weightology
LLC, Issaquah, Washington
Abstract
Roberts,BM,Nuckols,G,andKrieger,JW.Sexdifferencesinresistance training: A systematic review and meta-analysis. J
Strength Cond Res XX(X): 000–000, 2020—The purpose of this study was to determine whether there are different responses to
resistance training for strength or hypertrophy in young to middle-aged males and females using the same resistance training
protocol. The protocol was pre-registered with PROSPERO (CRD42018094276). Meta-analyses were performed using robust
variance random effects modeling for multilevel data structures, with adjustments for small samples using package robumeta in
R. Statistical significance was set at P,0.05. The analysis of hypertrophy comprised 12 outcomes from 10 studies with no
significant difference between males and females (effect size [ES] 50.07 60.06; P50.31; I
2
50). The analysis of upper-body
strength comprised 19 outcomes from 17 studies with a significant effect favoring females (ES 5-0.60 60.16; P50.002; I
2
5
72.1). The analysis of lower-body strength comprised 23 outcomes from 23 studies with no significant difference between sexes
(ES 520.21 60.16; P50.20; I
2
574.7). We found that males and females adapted to resistance training with similar effect
sizes for hypertrophy and lower-body strength, but females had a larger effect for relative upper-body strength. Given the
moderate effect size favoring females in the upper-body strength analysis, it is possible that untrained females display a higher
capacity to increase upper-body strength than males. Further research is required to clarify why this difference occurs only in the
upper body and whether the differences are due to neural, muscular, motor learning, or are an artifact of the short duration of
studies included.
Key Words: resistance exercise, gender differences, strength, hypertrophy
Introduction
It is well-established that both males and females can increase
muscle size and strength in response to resistance training (RT)
(29). Furthermore, several studies have shown RT has multiple
benefits for overall health (39,41,58). Although there have not
been any studies that use dose-response models to determine
whether males and females respond differently to chronic RT,
several studies have compared the adaptations of males and
females using the same training protocol. However, whether
there are sex-specific adaptations to the same training is still
unclear.
In most studies, males increase absolute strength more than
females (10,12,68). Yet, some find that the relative increase in
muscle strength and hypertrophy are similar between sexes
(1,21,28,30,32,36,40,67,70,78,85). However others find
females have a greater relative strength increase
(7,9,29,34,36,38,48,55,56,63,79). In one of the largest
studies to date, Hubal et al. (29) found females have higher
relative strength increases than males.
A key consideration in comparing the responses in males and
females is that pre-training levels of muscle size and strength are
generally greater in males, independent of training status
(3,35,67). Another well-known set of differences between males
and females are hormonal, which may influence muscle hyper-
trophy and strength adaptations. There also may be some
differences in types of occupation that could cause basal strength
differences. However, there is currently no review bringing to-
gether the major differences between sexes at the neuromuscular,
muscular, and hormonal level in the context of RT.
Considering the importance of muscle strength and size to
overall health and exercise performance, it is important to un-
derstand sex differences in response to RT if they exist. Therefore,
the purpose of this study is to determine whether there are dif-
ferent responses to RT for strength or hypertrophy in young to
middle-aged males and females.
Methods
Experimental Approach to the Problem
Inclusion Criteria. Research publications were considered eligible
for this systematic review if they (a) were experimental in design,
(b) were published in a peer-reviewed, English-language journal,
(c) were conducted in human populations, (d) included at least 1
method of estimating changes in muscle mass and/or dynamic,
isometric, or isokinetic strength, and (e) had subjects who were
between 18 and 50 years old (Table 1).
Exclusion Criteria. Studies were considered ineligible for this
review if (a) the training protocol lasted for ,5 weeks, (b) the
study involved subjects with medical conditions, pregnancy, or
injuries impairing training capacity, (c) subjects were taking
supplements or hormone replacement therapy. Case studies were
not included. Studies that were not written in English, conference
abstracts, thesis, or posters were also excluded from this review.
Address correspondence to Dr. Brandon M. Roberts, robertsb21@gmail.com.
Journal of Strength and Conditioning Research 00(00)/1–13
ª2020 National Strength and Conditioning Association
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Table 1
Overview of studies meeting inclusion criteria.*
Hypertrophy and strength adaptations to resistance training in males and females
Study nSession per week Training status Study duration (wks) Strength measurement Within-sex ES for strength Measurement Within-sex ES for hypertrophy
Abe et al. (1) Male: 17
Female: 15
3 Untrained .12 mo 12 Chest press
Knee extension
Male: 1.33
Female: 0.96
Ultrasound Male: 1.31
Female: 1.03
Ahtiainen et al. (2) Male: 61
Female: 27
2 Untrained 24 Leg press Male: 1.06
Female: 0.96
MRI
DXA
Ultrasound
NR
Alway et al. (4) Male: 5
Female 5
2 Trained body builders 24 NR NR Computed tomography,
fiber cross-sectional area
CT
Male: 0.42
Female: 0.25
fCSA
Male: 0.08
Female: 0.41
Carlsson et al. (7) Male: 7
Female: 7
2NR
Athletes
6 Bar-dips, chin-ups Male: 0.20
Female: 0.64
NR NR
Colliander et al. (8) Male: 11
Female: 11
3 Untrained
Physically active
12 Leg extension Male: 2.3
Female: 1.61
NR NR
Cureton et al. (9) Male: 8
Female: 7
3 Untrained .6 mo 16 Elbow flexion
Leg extension
Male: 0.43
Female: 1.14
Computed tomography Arm
Male: 1.2
Female: 1.03
Thigh
Male: 0.43
Female: 0.25
Daniels et al. (10) Male: 11
Female: 7
5NR
Physically active
104 Pull-down
Leg extension
Upper body
Male: 1.41
Female: 1
Lower body
Male: 1.3
Female: 1.54
NR NR
Dias et al. (12) Male: 23
Female: 15
3 Untrained .6mo
Physically active
12 Bench press
Squat
Upper body
Male: 0.52
Female: 0.98
Lower body
Male: 0.36
Female: 0.75
NR NR
Donges et al. (13) Male: 16
Female: 19
3 Untrained .12 mo
Sedentary
10 Chest press
Leg press
NR NR NR
Dorgo et al. (14) Male: 14
Female: 14
2 Untrained
Physically active
12 Leg extension Male: 1.28
Female: 2.45
NR NR
Fernandez-Gonzalo et al. (15) Male: 16
Female: 16
2–3 Untrained .6mo
Physically active
6 Leg press Male: 1.92
Female: 0.88
NR NR
Garthe et al. (17) Male: 11
Female: 13
4NR
Athletes
12 Squat NR NR NR
Gentil et al. (18) Male: 44
Female: 47
2 No systematic RT .3 mo 10 Elbow flexion Male: 0.56
Female: 0.6
NR NR
Guadalupe-Grau et al. (19) Male: 24
Female: 23
2NR
Physically active
9 Leg press Male: 1.15
Female: 5.63
NR NR
Sex Differences in Training (2020) 00:00
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Table 1
Overview of studies meeting inclusion criteria.* (Continued)
Hypertrophy and strength adaptations to resistance training in males and females
Study nSession per week Training status Study duration (wks) Strength measurement Within-sex ES for strength Measurement Within-sex ES for hypertrophy
H¨akkinen et al. (24) Male: 9
Female: 9
2–3 Untrained
Physically active
12 Leg extension Male: 2.45
Female: 0.62
NR NR
H¨akkinen et al. (22) Male: 12
Female: 12
2 Untrained
Physically active
12 Leg extension Male: 0.98
Female: 1.2
NR NR
Hakkinen et al. (21)
Hakkinen et al. (23)
Hakkinen et al. (25)
Male: 42
Female: 39
2 Untrained
Physically active
24 Leg extension Male: 0.31
Female: 0.38
Muscle fiber size Male: 0.66
Female: 0.59
Hostler et al. (28) Male: 5
Female: 5
3 Untrained .6 mo 16 Bench press Male: 1.18
Female: 3.49
NR NR
Male: 5
Female: 4
2 Male: 1.9
Female: 1.74
Hurlbut et al. (32)
Lemmer et al. (42)
Lemmer et al. (43)
Roth et al. (67)
Male: 10
Female: 9
3 Untrained .6mo
Sedentary
24 Chest press
Leg press
Upper body
Male: 0.94
Female: 2
Lower body
Male: 1.38
Female: 2.23
MRI Male: 0.43
Female: 0.09
Hubal et al. (29)
Liu et al. (45)
Peterson et al. (56)
Male: 43
Female: 40
2 Untrained .12 mo 12 Bicep flexion Male: 0.04
Female: 0.06
MRI Male: 0.67
Female: 0.65
Hunter (30) Male: 11
Female: 10
3 NR 7 Bench press Male: 0.08
Female: 0.06
NR NR
Male: 14
Female: 11
4 Male: 0.04
Female: 0.15
Ivey et al. (33)
Ivey et al.(34)
Male: 11
Female: 11
3 Untrained .6mo
Sedentary
9 Leg extension Male: 1.27
Female: 1.88
MRI Male: 0.49
Female: 0.31
Jozsi et al. (36) Male: 6
Female: 9
2 Untrained .12 mo 12 Chest press
Leg press
Upper body
Male: 0.61
Female: 0.62
Lower body
Male: 2.25
Female: 2.41
NR NR
Kell et al. (38) Male: 20
Female: 20
3 Trained
Physically active
12 Bench press
Back squat
Upper body
Male: 1.31
Female: 2.37
Lower body
Male: 1.48
Female: 2.22
NR NR
Kosek et al. (40) Male: 13
Female: 11
3 Untrained .12 mo 16 Leg press Male: 1.8
Female: 2.78
Muscle fiber size Male: 1.84
Female: 1.23
Martin-Ginis et al. (48) Male: 25
Female: 15
5 Untrained .6mo
Sedentary
12 Chest press
Leg press
Upper body
Male: 1.33
Female: 3.31
Lower body
Male: 2.56
Female: 2.62
NR NR
Sex Differences in Training (2020) 00:00 |www.nsca.com
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Table 1
Overview of studies meeting inclusion criteria.* (Continued)
Hypertrophy and strength adaptations to resistance training in males and females
Study nSession per week Training status Study duration (wks) Strength measurement Within-sex ES for strength Measurement Within-sex ES for hypertrophy
O’Hagan et al. (55) Male: 6
Female 6
3 NR 20 Elbow flexion Male: 2
Female: 4.18
CT Male: 0.09
Female: 0.43
Reichman et al. (64) Male: 62
Female: 58
3 Trained ,3 h per week 10 wk Chest press
Leg press
NR NR NR
Ribiero et al. (63)
Ribiero et al. (62)
Ribiero et al. (60)
Female: 31
Male: 28
3 Untrained .6 mo 16 wk Bench press Male: 0.69
Female: 1.22
NR NR
Rutherford et al. (68) Male: 11
Female: 9
3 Untrained 12 wk Leg extension Male: 0.81
Female: 0.22
NR NR
Spurway et al. (77) Male: 10
Female: 10
3 Untrained
Physically active
6 wk Leg extension NR NR NR
Salvador et al. (71) Male: 33
Female: 23
3 Untrained .6 mo 8 wk Bench press
Back squat
Upper body
Male: 0.41
Female: 0.81
Lower body
Male: 0.55
Female: 0.67
NR NR
Schmidt et al. (72) Male: 43
Female: 53
3 Untrained
Physically active
8 wk Push-ups NR NR NR
Staron et al. (79) Male: 13
Female: 8
2 Untrained
Physically active
9 wk NR NR Muscle fiber size NR
Stock et al. (80) Male: 17
Female: 17
2 Untrained .6 mo 10 wk Leg extension Male: 0.44
Female: 0.63
NR NR
Washburn et al. (85) Male: 17
Female: 13
3 NR 24 wk Chest press
Leg press
NR NR NR
Weiss et al. (86) Male: 12
Female: 14
3 Untrained .3 mo 8 wk Plantar flexion Male: 0.91
Female: 0.52
Ultrasound Male: 0.24
Female: 0.17
Williamson et al. (90)
Raue et al. (58)
Male: 6
Female: 6
3 Untrained .12 mo 12 wk Leg extension Male: 7.91
Female: 11.8
NR NR
Wilmore et al. (91) Male: 26
Female: 46
2 NR 10 wk Bench press
Leg press
Male: 2.21
Female: 3.5
NR NR
*ES 5effect size; MRI 5magnetic resonance imaging; NR 5Not recorded; DXA 5dual-energy X-ray absorptiometry; CT 5computed tomography; RT 5resistance training; fCSA 5fiber cross-sectional area.
Sex Differences in Training (2020) 00:00
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Search Strategy. Our protocol was pre-registered with PROS-
PERO (CRD42018094276). The systematic review was per-
formed in accordance with the guidelines provided by the
Preferred Reporting Items for Systematic Reviews and Meta-
Analyses (PRISMA). A literature review was conducted up until
April 2018 using Medline and SportDiscus. Combinations of the
following terms were used to produce search results: gender or sex
AND strength training or RT or powerlifting AND strength or
hypertrophy or 1 repetition maximum. Search terms were added
using the NOT term to reduce the number of irrelevant studies
according to exclusion criteria (concurrent, children, disease,
supplement). Citations from studies were also scanned for addi-
tional studies (Figure 1).
Subjects
A total of 1,162 studies were identified using the aforementioned
search terms, and 24 were additionally identified through other
sources. Eighty studies were identified as being eligible for the
review. After full-text review, 30 were removed for not meeting
the inclusion criteria. Ultimately, 50 studies were deemed to have
satisfied the inclusion criteria. Of those studies, 10 were analyzed
for hypertrophy measures, 17 for upper-body strength, and 23 for
lower-body strength.
Procedures
Coding of Studies. Studies were independently searched and
coded by 2 of the authors (G.N. and B.M.R.) for the following
variables: descriptive information (age, sex, training status), the
number of subjects per group, training mode, duration of study,
training frequency, repetition range, mode of muscle measure-
ment (magnetic resonance imaging, fiber cross sectional area
[fCSA], ultrasound, and computed tomography). Results were
cross-checked between coders, and any discrepancies were re-
solved by mutual consensus.
Calculation of Effect Size. For each hypertrophy and strength
outcome, a within-group effect size (ES) for each male and female
group was calculated as the pretest-posttest change, divided by
the pretest standard deviation (SD) (54). A study level ES was then
calculated as the difference between the male group ES and female
group ES. A small sample bias adjustment was applied to each ES
(54). The sampling variance around each ES was calculated using
the sample size in each study (6).
Statistical Analyses
Meta-analyses were performed using robust variance random
effects modeling for multilevel data structures, with adjustments
for small samples using package robumeta in R (27,82). The study
was used as the clustering variable to account for correlated group
effects within studies. Observations were weighted by the inverse
of the sampling variance. Separate analyses were conducted for
hypertrophy, upper-body strength, and lower-body strength. A
fail-safe N was performed to calculate the number of null studies
needed to achieve a pvalue of 0.05 or greater using the Rosenthal
approach.
Figure 1. PRISMA diagram.
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All analyses were performed in R version 3.5 (The R Foun-
dation for Statistical Computing, Vienna, Austria). Effects were
considered significant at P#0.05. Data are reported as means 6
SEM and 95% confidence intervals (CIs) unless otherwise
specified.
Methodological Quality
The quality of studies is important for analysis of systematic
reviews and meta-analysis. However, due to the nature of
studies on sex differences, it is difficult to blind subjects,
therapists, assessors, conceal allocation, or randomly
allocate, which are integral parts of quality assessment.
Because this eliminates half of the questions in most scales, we
felt it was unworthy to perform these types of quality
assessments.
Results
Hypertrophy
The analysis of hypertrophy comprised 12 outcomes from 10
studies. There was no significant difference between males and
females (ES 50.07 60.06; 95% CI: 20.09 to 0.23; P50.31;
Figure 2). Heterogeneity was low (I
2
50) (Figure 3).
Figure 2. Forest plot of studies comparing changes in hypertrophy in males and females. The data
shown are mean 695% CI; the size of the plotted squares reflects the statistical weight of each study.
CI 5confidence interval.
Sex Differences in Training (2020) 00:00
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Upper-Body Strength
The analysis of upper-body strength comprised 19 outcomes from
17 studies. There was a significant effect favoring females (ES 5
20.60 60.16; 95% CI: 20.93 to 20.26; P50.002; Figure 4).
Heterogeneity was high (I
2
572.1). Adding training status
(trained or untrained), single or multijoint strength measurements
(e.g., leg extension or leg press), training duration (weeks), or
sessions per week as covariates did not substantially reduce het-
erogeneity (I
2
569.7) (Figure 5).
Lower-Body Strength
The analysis of lower-body strength comprised 23 outcomes from
23 studies. There was no significant difference between sexes (ES
520.21 60.16; 95% CI: 20.54 to 0.12; P50.20; Figure 6).
Heterogeneity was high (I
2
574.7). Adding training status
(trained or untrained), single or multijoint strength measurements
(e.g., leg extension or leg press), training duration (weeks), or
sessions per week as covariates did not substantially reduce het-
erogeneity (I
2
577.4) (Figure 7).
Sensitivity Analysis
Because of the limited sample size, we completed a sensitivity
analysis on all 3 outcomes where 1 study at a time was removed to
determine whether that a particular study had any significant
impact on the outcomes. However, we did not identify any in-
fluential studies.
Publication Bias
A rank correlation test for funnel plot asymmetry was performed
for the upper-body strength results because there was a significant
finding (Figure 8). It was not significant (P50.41). We also used
a fail-safe N to calculate the number of null studies needed to
achieve a pvalue of 0.05 or greater using the Rosenthal approach.
The fail-safe N was 294. Thus, there was no evidence of publi-
cation bias for the upper-body strength outcomes.
Discussion
This review meta-analyzed studies that compared strength or
direct measures of hypertrophy in males and females who used the
same RT program. A majority of the studies were completed in
untrained individuals. The main finding was that effect sizes in
hypertrophy and lower-body strength were similar between
sexes. However, there was a significant effect in favor of females
for upper-body strength (ES 520.60; 95% CI: 20.93 to 20.26;
P50.002).
Muscular strength increases in response to RT are a com-
bination of neurological and muscular adaptations. Initial,
rapid improvements in strength seem to result primarily from
neurological adaptation, whereas subsequent gains are pri-
marilytheresultofmuscularadaptations (53). In one of the
first studies comparing untrained males and females, Wilmore
etal.,foundthatstrengthwassimilarwhennormalizedto
body weight after 10 weeks of intensive RT (90). Interestingly,
relative upper-body strength increased 29% in females com-
pared with 17% in males, whereas relative increases in lower-
body strength were similar (90). These data were the first data
to indicate there may be differences in strength changes be-
tween sexes. However, a limitation was that both groups ex-
perienced considerable decreases in body fat percentage over
the course of the study, indicating they were likely not in an
optimal nutritional environment for gaining or maintaining
muscle mass or strength (90). More recent data have indicated
that both sexes respond to upper-body strength in a similar
manner (18). Yet, in the largest study to date with ;342
females and ;243 males, there was a significant difference in
relative upper strength changes in favor of females (29).
Herein, we cover a number of potential variables that could
help explain the differences in strength we and others have
found.
Neuromuscular adaptations are one factor that could explain
the larger increase for females in upper-body strength. However,
one study compared the number of motor units in the biceps
brachii and vastus medialis but found no differences between
sexes (50). The same research group also found no difference in
Figure 3. Percent change in muscle hypertrophy in males and females.
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Figure 4. Forest plot of studies comparing changes in upper-body strength in males and females. The
data shown are mean 695% CI; the size of the plotted squares reflects the statistical weight of each
study. CI 5confidence interval.
Sex Differences in Training (2020) 00:00
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motor unit activation for elbow flexion or knee extension (50).
Others have found that males are no better able to activate motor
units than females (5,91). Yet, neuromuscular fatigue from RT is
generally greater in males than females, and acute recovery may
be slower in males (20). Because the included studies are short in
nature, this could have an effect on strength adaptations if sub-
jects are not fully recovered during testing. The average untrained
female may also have a lower initial level of fitness compared with
a male (74). This could cause a ceiling effect for motor skills that
may explain differences in upper-body strength because the
studies were conducted in mostly untrained subjects. Ultimately,
there are very few known differences at the neuromuscular level
between sexes that could explain our findings, but more research
is warranted.
It is well established that sex differences exist in skeletal muscle
mass and distribution (35). Females often have less total and lean
body mass, a higher body fat percentage, and a smaller muscle
fiber cross-sectional area (65,78). One explanation of why dif-
ferences could occur in strength or hypertrophy is muscle phe-
notype. Females have a greater proportion of type I fibers (65,75)
in the vastus lateralis and the biceps brachii (3,50,69). There are
currently no studies that compare the number of muscle fibers
between sexes, but a classic study has shown the muscle fiber
number decreases with age in males, although our data did not
include those over 50 (44). Furthermore, there seem to be similar
responses in muscle protein synthesis between sexes (47,76,86),
and muscle damage due to RT is also similar between, yet the
inflammatory response may be attenuated in females compared
with males (80). However, there are some data to indicate that
although there are similar indirect markers of muscle damage
after RT, males could have longer-lasting muscle soreness than
females (11). On a single fiber level, force per CSA and contractile
velocity of type I and type II fibers are similar when comparing
sexes (83). Taken together, there are relatively few differences in
skeletal muscle between sexes, which helps explain our finding
that hypertrophy is similar.
It was once postulated that females achieved small increases in
muscle size after RT because of low androgen levels whereby
a lesser amount of work-induced muscle hypertrophy would
prevent them from gaining strength to the same extent as males
(85). Although it is true that absolute hypertrophy and gains in
strength are larger in males after RT, it seems that relative
increases in both muscularity and lower-body strength are similar
between the sexes, and relative gains in upper-body strength may
be larger in females. Indeed, it is well established that females have
lower levels of testosterone, free-testosterone, and insulin-like
growth factor-binding protein 1 compared with males (65).
Heavy training decreases gonadotropin-releasing hormone pul-
satility in females (66). Males exhibit lower serum cortisol due to
chronic RT, whereas females do not (78). Females do not expe-
rience elevations in postexercise testosterone compared with
males (16,86). This differential change in testosterone has led to
speculation that females may have an attenuated potential for
resistance exercise-induced hypertrophy, which we did not find in
our analysis (72). Another difference is that males have more
upper-body muscle, which has more androgen receptors (37).
Gentil et al. (18) suggest that this could affect strength gains over
time. Another potential confounder is the menstrual cycle. Some
evidence suggests that females who complete training during the
follicular phase can have larger strength gains and more muscle
growth (59, 81, 88) while females may take longer to recover
during the luteal phase (46), and most of the studies included did
not adjust for menstrual cycle. However, other evidence suggests
that the changes in protein kinetics across the menstrual cycle may
not play a large role in muscle accrual (51). Although some studies
suggest that hormonal differences play a role in changes, larger
and well-controlled studies are needed to understand why that
occurs or how it affects strength or hypertrophy adaptations.
In a recent review, Hunter presents evidence that sex differ-
ences in muscle fatigue of repeated dynamic contractions are
specific to the task requirements (31). Females may have less
skeletal muscle fatigue compared with males during single-limb
isometric contractions. It has also been suggested that there are
independent responses to fatiguing contractions (31). Likewise,
shortening velocity is a potential factor to tease out the contri-
bution of voluntary activation and contractile mechanisms (73).
There is also evidence that female tendons have a smaller capacity
for adaptation to training (52,87), which may be exacerbated by
oral contraceptive use (26) and could potentially affect strength
adaptations.
Figure 5. Percent change in muscle upper-body strength in males and females.
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Figure 6. Forest plot of studies comparing changes in lower-body strength in males and females.
The data shown are mean 695% CI; the size of the plotted squares reflects the statistical weight
of each study. CI 5confidence interval; the size of the plotted squares reflects the statistical
weight of each study.
Sex Differences in Training (2020) 00:00
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Although a strength of this study is that it is the first meta-
analysis completed on sex differences, there are several limi-
tations. First, most subjects included in the analysis are untrained
individuals. It is possible that a longer training duration or other
factors could change the results. In addition, the untrained sub-
jects could have different levels of basal activity between studies as
it is often not well described in exercise science research. The
studies included also vary with regard to mode, duration, and
intensity of exercise utilized. However, our analysis of upper-
body strength found no evidence publication bias and no single
studies of major influence. It has been argued (1) that many of the
earlier studies conducted on sex comparisons for both strength
and muscular hypertrophic changes were hampered by low sta-
tistical power resulting from small sample sizes (all #20 subjects).
Another potential limitation is missing studies due to unused
search terms or databases. There is also a possibility that male
subjects could be more familiar with upper-body movements
(e.g., bench press) that could have resulted in females having
greater neuromuscular adaptations. Finally, heterogeneity was
high for the outcomes of studies assessing both upper and lower-
body strength, yet incorporating training status, testing modality,
duration, or sessions did not substantially decrease heterogeneity.
Although there was a mean effect in favor of females for upper-
body strength gains and no significant difference between the
sexes for lower-body strength gains, more research is needed to
understand the sources of this heterogeneity.
We found that males and females adapted to RT with similar
effect sizes for hypertrophy and lower-body strength, but females
had a larger effect size for relative upper-body strength. Current
research indicates there are few differences at the skeletal muscle
level between sexes. However, hormonal fluctuations, daily
physical activity, and exercise recovery may play a role in our
findings. In sum, well-designed studies with a primary goal of
comparing male and females are relatively few, and our un-
derstanding of sex differences in the physiology of RT is in-
complete, which makes studies on sex-differences warranted.
Figure 7. Percent change in lower-body strength in males and females.
Figure 8. Funnel plot, using data from studies with upper-body strength outcomes.
Sex Differences in Training (2020) 00:00 |www.nsca.com
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Practical Applications
Given the moderate effect size favoring females in the upper-
body strength analysis, it is possible that untrained females
display a higher capacity to increase upper-body strength than
males. Further research is required to clarify why this differ-
ence occurs only in the upper body and whether the differences
are due to neural, muscular, or motor learning adaptations. In
practice, it is important to know that both males and females
can considerably increase muscle strength and size with RT.
Because there are is a paucity of studies comparing multiple
RT programs between sexes, it is currently difficult to know if
exercise prescription should be different between sexes.
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