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European Journal of Sport Science
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Muscular adaptations in low- versus high-load
resistance training: A meta-analysis
Brad J. Schoenfelda, Jacob M. Wilsonb, Ryan P. Loweryb & James W. Kriegerc
a Department of Health Sciences, CUNY Lehman College, Bronx, NY, USA
b Department of Health Sciences and Human Performance, University of Tampa, Tampa, FL,
USA
c Weightology LLC, Redmond, WA, USA
Published online: 20 Dec 2014.
To cite this article: Brad J. Schoenfeld, Jacob M. Wilson, Ryan P. Lowery & James W. Krieger (2014): Muscular
adaptations in low- versus high-load resistance training: A meta-analysis, European Journal of Sport Science, DOI:
10.1080/17461391.2014.989922
To link to this article: http://dx.doi.org/10.1080/17461391.2014.989922
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REVIEW ARTICLE
Muscular adaptations in low- versus high-load resistance training:
A meta-analysis
BRAD J. SCHOENFELD
1
, JACOB M. WILSON
2
, RYAN P. LOWERY
2
, & JAMES W. KRIEGER
3
1
Department of Health Sciences, CUNY Lehman College, Bronx, NY, USA,
2
Department of Health Sciences and Human
Performance, University of Tampa, Tampa, FL, USA,
3
Weightology LLC, Redmond, WA, USA
Abstract
There has been much debate as to optimal loading strategies for maximising the adaptive response to resistance exercise.
The purpose of this paper therefore was to conduct a meta-analysis of randomised controlled trials to compare the effects of
low-load (≤60% 1 repetition maximum [RM]) versus high-load (≥65% 1 RM) training in enhancing post-exercise muscular
adaptations. The strength analysis comprised 251 subjects and 32 effect sizes (ESs), nested within 20 treatment groups and
9 studies. The hypertrophy analysis comprised 191 subjects and 34 ESs, nested with 17 treatment groups and 8 studies.
There was a trend for strength outcomes to be greater with high loads compared to low loads (difference = 1.07 ± 0.60;
CI: −0.18, 2.32; p= 0.09). The mean ES for low loads was 1.23 ± 0.43 (CI: 0.32, 2.13). The mean ES for high loads was
2.30 ± 0.43 (CI: 1.41, 3.19). There was a trend for hypertrophy outcomes to be greater with high loads compared to low
loads (difference = 0.43 ± 0.24; CI: −0.05, 0.92; p= 0.076). The mean ES for low loads was 0.39 ± 0.17 (CI: 0.05, 0.73).
The mean ES for high loads was 0.82 ± 0.17 (CI: 0.49, 1.16). In conclusion, training with loads ≤50% 1 RM was found to
promote substantial increases in muscle strength and hypertrophy in untrained individuals, but a trend was noted for
superiority of heavy loading with respect to these outcome measures with null findings likely attributed to a relatively small
number of studies on the topic.
Keywords: Muscle recruitment, low-load exercise, light weights
It has been well established that regimented resist-
ance exercise can promote marked increases in
muscle strength and hypertrophy, with improve-
ments in these outcome measures seen irrespective
of age and gender (Ivey et al., 2000; Kosek,
Kim, Petrella, Cross, & Bamman, 2006). Exercise-
induced muscular adaptations are at least in part
attributed to a phenomenon called mechanotrans-
duction, whereby sarcolemmal-bound mechanosen-
sors, such as integrins and focal adhesions, convert
mechanical energy into chemical signals that medi-
ate myocellular anabolic and catabolic pathways
(Zou et al., 2011). When subjected to mechanical
overload, the signalling cascade upregulates anabolic
processes in a manner that results in a net increase in
muscle protein synthesis, thereby leading to an
enlargement of fibres (Glass, 2005). The extent
of hypertrophy has been shown to vary by fibre
type, with fast-twitch (FT) fibres displaying an
approximately 50% greater capacity for growth in
comparison to their slow-twitch (ST) counterparts
(Adams & Bamman, 2012; Kosek et al., 2006). It
should be noted, however, that a high degree of
inter-individual variability exists in this regard, with
some individuals displaying substantially greater ST
hypertrophy than others (Kosek et al., 2006).
Proper manipulation of programme variables is
considered essential to optimise post-exercise mus-
cular adaptations (Kraemer & Ratamess, 2004). One
such variable is the amount of load lifted, generally
quantified as a percentage of 1 repetition maximum
(RM). Studies show that alterations to the training
load have a significant effect on acute post-exercise
metabolic, hormonal and neural responses –factors
that have been postulated to mediate enhancements
in muscle strength and hypertrophy (Kraemer &
Ratamess, 2004). Accordingly, there has been
much debate as to optimal loading strategies for
maximising the adaptive response to resistance exer-
cise. Some have hypothesised that a load of at least
Correspondence: B. J. Schoenfeld, Department of Health Sciences, CUNY Lehman College, 250 Bedford Park Blvd West, APEX Building,
Bronx, NY 10468, USA. E-mail: brad@workout911.com
European Journal of Sport Science, 2014
http://dx.doi.org/10.1080/17461391.2014.989922
© 2014 European College of Sport Science
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65% 1 RM is necessary to elicit favourable increases
in hypertrophy, with even higher loads needed to
maximise strength gains (Kraemer & Ratamess, 2004;
Kraemer et al., 2002; McDonagh & Davies, 1984).
This belief is predicated on the premise that heavier
loading is required to achieve full recruitment of the
higher threshold motor units and that optimal
improvements in strength and hypertrophy can only
be accomplished through complete motor unit activa-
tion (Kraemer & Ratamess, 2004).
The assertion that heavy weights are necessary for
optimising the post-exercise muscular response has
recently been challenged, however, with some
researchers claiming that very low loads can promote
adaptations similar to high-load training (Burd,
Mitchell, Churchward-Venne, & Phillips, 2012).
It has been surmised that as long as intensity of
effort is maximal, even experienced lifters can realise
significant increases in muscle hypertrophy from
training with low loads (Burd, Moore, Mitchell, &
Phillips, 2012). Proponents claim that complete
high-threshold motor unit recruitment can be
achieved with low-load training provided repetitions
are carried out to momentary muscular failure. But
although research clearly shows FT fibres are indeed
recruited during low-load training, there is evidence
that recruitment does not equal what is achieved
from the use of heavier loads (Akima & Saito, 2013;
Cook, Murphy, & Labarbera, 2013). Recent work
from our lab supports these findings, with both mean
and peak electromyographic values showing mark-
edly and significantly higher activation during per-
formance of the leg press at 75% 1 RM versus 30% 1
RM (Schoenfeld, Contreras, Willardson, Fontana, &
Tiryaki-Sonmez, 2014). Despite this evidence, train-
ing to failure at 30% 1 RM has been found to
produce a similar acute muscle protein synthetic
response to a 90% 1 RM protocol 4-h post-exercise,
with myofibrillar muscle protein synthesis remaining
elevated only in the 30% to failure condition at the
24-h mark (Burd et al., 2010). Moreover, phosphor-
ylation of p70S6K was significantly increased at 4 h
only in the 30% to failure condition, and this
elevation was correlated with the degree of stimula-
tion of myofibrillar muscle protein synthesis (MPS).
These findings suggest that low-load exercise when
performed to muscular failure results in greater
acute adaptive responses compared to training with
heavy loads. The study did not report muscle protein
breakdown, and results were limited by an inability
to localise MPS based on fibre type. Results were
also confounded by a greater total volume of weight
lifted in the low-load condition. Moreover and
importantly, the evaluation of MPS following an
acute bout of resistance exercise does not always
occur in parallel with chronic upregulation of caus-
ative myogenic signals (Coffey, Shield, et al., 2006a)
and may not reflect experienced hypertrophic
responses subsequent to regimented resistance train-
ing carried out over a period of weeks or months
(Mitchell, Churchward-Venne, Parise, et al., 2014;
Timmons, 2011). This is a complex area of research,
however, and interested readers are referred to the
recent review by Mitchell, Churchward-Venne,
Cameron-Smith, and Phillips (2014) for an in-depth
discussion of the topic.
A number of longitudinal studies have been carried
out that compare muscular adaptations in low- versus
high-load training programs. Results of these studies
have been conflicting. One issue with the current body
of research on the topic is that studies have employed
small sample sizes. Thus, it is possible that null
findings may be attributable to a Type II error as a
result of the studies being underpowered. In addi-
tion, various methodological issues between proto-
cols confound results, making it difficult to draw
definitive conclusions. Thus, by increasing statistical
power and controlling for confounding variables, a
meta-analysis may help to provide clarity on the
topic. The purpose of this paper therefore was to
conduct a meta-analysis to compare muscular adapta-
tions between low- and high-load resistance training
programmes.
Methodology
Inclusion criteria
Only randomised controlled trials or randomised
crossover trials involving both low- and high-load
training were considered for inclusion. High-load
training was defined here as lifting weights ≥65% 1
RM; low-load training was defined as lifting loads
≤60% 1 RM. Resistance training protocols had to
span at least 6 weeks and directly measure dynamic
muscle strength and/or hypertrophy as a primary
outcome variable. In addition, the training proto-
cols had to be carried out to momentary muscular
failure –the inability to complete another concentric
repetition while maintaining proper form.
Search strategy
To carry out this review, English-language literature
searches of the PubMed, EBSCO and Google Scho-
lar databases were conducted from January 1980 to
December 2013. Combinations of the following key-
words were used as search terms: “skeletal muscle”;
“hypertrophy”;“growth”;“cross-sectional area”;
“intensity”;“strength”;“loading”;“low load”;“light
load”;“resistance training”and “resistance exercise”.
Consistent with methods outlined by Greenhalgh and
Peacock (2005), the reference lists of articles
retrieved in the search were then screened for any
2B. J. Schoenfeld et al.
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additional articles that had relevance to the topic.
Abstracts from conferences, reviews and unpublished
dissertations/theses were excluded from analysis.
A total of 846 studies were evaluated based on
search criteria. To reduce the potential for selection
bias, each of these studies was independently per-
used by two of the investigators (B. J. S. and R. P.
L.), and a mutual decision was made as to whether
or not they met basic inclusion criteria. Any inter-
reviewer disagreements were settled by consensus
and/or consultation with the third investigator. A to-
tal of 13 studies were identified that investigated
low- versus high-load training in accordance with the
criteria outlined (see Figure 1). Three studies (Leger
et al., 2006; Weiss, Coney, & Clark, 1999,2000)
had to be omitted from analysis due to lack of
adequate data thereby leaving 10 studies for analysis.
Figure 1 shows a flowchart of the literature search.
Table I summarises the studies included for analysis.
Coding of studies
Studies were read and individually coded by two
of the investigators (B. J. S. and R. P. L.) for
the following variables: descriptive information of
subjects by group including gender, body mass
index, training status (trained subjects were defined
as those with at least one year resistance training
experience), age and stratified subject age (classified
as either young [18–29 years], middle-aged [30–49
years] or elderly [50+ years]); whether the study was
a parallel or within-subject design; the number of
subjects in each group; duration of the study;
exercise volume (single set, multi-set or both);
whether volume was equated between groups; rest
interval between sets (short rest of less than 2
minutes vs. long rest of 2 minutes or more); type of
hypertrophy measurement (magnetic resonance
imaging, computerised tomography, ultrasound,
biopsy, etc.) and region/muscle of body measured
(upper, lower or both) and strength measure(s)
employed for testing (free weights or isokinetic/
isometric dynamometry). Coding was cross-checked
between coders, and any discrepancies were resolved
by mutual consensus. To assess potential coder drift,
30% of the studies were randomly selected for
recoding as described by Cooper, Hedges, and
Valentine (2009). Per case agreement was deter-
mined by dividing the number of variables coded the
Records idenfied through
database searching
(n = 843)
Screening
Included Eligibility Idenficaon
Addional records idenfied
through other sources
(n = 3)
Total records screened
(n = 846)
Full-text arcles assessed
for eligibility
(n = 87)
Full-text arcles excluded,
with reasons
(n = 74)
Studies included in
meta-analysis (n = 13)
(n = 9)
Studies included in
meta-analysis
(n = 10)
Addional arcles
excluded, with reasons
(n = 3)
Figure 1. Flow diagram of the literature search.
Muscular adaptations in low- versus high-load resistance training 3
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Table I. Overview of studies meeting inclusion criteria
Study Subjects Design
Volume
equated?
Hypertrophy
measurement Findings
Anderson and
Kearney
(1982)
Forty-three
untrained
young men
Random assignment to either three
sets of high-intensity (6–8 RM),
two sets of medium intensity
(30–40 RM) or one set of low
intensity (100–150 RM). Exercise
consisted of the bench press
performed 3 days a week for
9 weeks. Tempo was consistent
between conditions
Yes N/A Significantly greater increases in
strength for the high- vs. medium-
and low-intensity groups
Campos
et al. (2002)
Thirty-two
untrained young
men (five served
as non-exercising
controls)
Random assignment to either high-
intensity (3–5 RM), intermediate-
intensity (9–11 RM) or low-
intensity (20–28 RM) exercises.
Exercise consisted of 2–4 sets of
squat, leg press and leg extension,
performed 3 days a week for
8 weeks. Tempo was consistent
between conditions
Yes Muscle
biopsy
Significant increases in CSA for
high-intensity exercise; no
significant increase in CSA for
low-intensity exercise.
Significantly greater increases in
muscle strength for high vs. low
intensity
Mitchell
et al. (2012)
Eighteen
untrained
young men
Randomly assignment to perform
two of three unilateral leg
extension protocols: three sets at
30% RM, three at 80% RM and
one set at 80% RM. Tempo was
consistent between conditions.
Training was carried out 3 days per
week for 10 weeks
No MRI, muscle
biopsy
No differences in CSA between
low- and high-intensity exercise.
Significantly greater strength
gains in high vs. low load
Ogasawara,
Loenneke,
Thiebaud,
and
Abe(2013)
Nine untrained
young men
Non-randomised crossover design
to perform four sets of bench press
exercise at 75% 1 RM. Training
was carried out 3 days a week for
6 weeks. Tempo was consistent
between conditions. After a
12-month washout period, the
same protocol was performed at
30% 1 RM
No MRI No differences in CSA between
low- and high-intensity exercise.
Significantly greater increases in
strength favouring high vs.
low load
Popov
et al. (2006)
Eighteen
untrained
young men
Random assignment to either high
intensity (80% of MVC) or low
intensity (50% MVC) without
relaxation. Exercise consisted of
leg press exercise performed 3 days
a week for 8 weeks. Tempo was
consistent between conditions
No MRI No differences in CSA or strength
between groups
Schuenke
et al. (2012)
Thirty-four
untrained young
women
Randomised assignment to either
moderate intensity (80–85% RM)
at a tempo of 1–2 seconds, a low
intensity (~40–60% RM) at a
tempo of 1–2 seconds or slow
speed (~40–60% RM) at a tempo
of 10 seconds concentric and
4 seconds eccentric. Exercise
consisted of three sets of squat, leg
press and leg extension, performed
2–3 days a week for 6 weeks
No Muscle
biopsy
Significant increases in CSA for
high-intensity exercise; no
significant increase in CSA for
low-intensity exercise
Stone and
Coulter
(1994)
Fifty untrained
young women
Three sets of 6–8 RM, two sets of
15–20 RM and one set of 30–40
RM. A combination of free weight
and machine exercises were
performed for the upper and lower
body. Tempo was consistent
between conditions. Training was
carried out 3 days a week for
9 weeks
Yes N/A No differences in strength
between groups
4B. J. Schoenfeld et al.
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same by the total number of variables. Acceptance
required a mean agreement of 0.90.
Calculation of effect size
For each 1-RM strength or hypertrophy outcome, an
effect size (ES) was calculated as the pretest–posttest
change, divided by the pretest standard deviation
(SD; Morris & DeShon, 2002). The sampling
variance for each ES was estimated according to
Morris and DeShon (2002). Calculation of the
sampling variance required an estimate of the popu-
lation ES and the pretest–posttest correlation for
each individual ES. The population ES was esti-
mated by calculating the mean ES across all studies
and treatment groups (Morris & DeShon, 2002).
The pretest–posttest correlation was calculated using
the following formula (Morris & DeShon, 2002):
r¼s2
1þs2
2s2
D
2s1s2
ðÞ
where s
1
and s
2
are the SD for the pre- and posttest
means, respectively, and s
D
is the SD of the difference
scores. s
D
was estimated using the following formula
(Technical guide: Data analysis and interpretation
[online]):
sD¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
s2
1
n
þs2
2
n
s
Statistical analyses
Meta-analyses were performed using hierarchical
linear mixed models, modelling the variation
between studies as a random effect, the variation
between treatment and control groups as a random
effect nested within studies and the low- versus high-
load comparison as a fixed effect (Hox & de Leeuw,
2003). The within-group variances were assumed
known. Observations were weighted by the inverse of
the sampling variance (Morris & DeShon, 2002).
Model parameters were estimated by the method of
restricted maximum likelihood (Thompson & Sharp,
1999). Denominator degree of freedom for statistical
tests and CIs was calculated according to Berkey,
Hoaglin, Mosteller, and Colditz (1995). Separate
Table I. (Continued)
Study Subjects Design
Volume
equated?
Hypertrophy
measurement Findings
Tanimoto and
Ishii (2006)
Twenty-four
untrained
young men
Random assignment to either 50%
RM with a 6 second tempo and no
relaxing phase between repetitions,
80% RM with a 2 second tempo
and 1 second relaxation between
repetitions or 50% RM with a
2 second tempo and 1 second
relaxation between repetitions.
Exercise consisted of three sets of
knee extensions, performed 3 days
a week for 12 weeks.
No MRI No differences in CSA or strength
between low- and high-intensity
exercise
Tanimoto
et al. (2008)
Thirty-six
untrained young
men (12 served as
non-exercising
controls)
Random assignment to either
~55% RM with a 6-second tempo
and no relaxing phase between
repetitions or 80–90% RM with a
2-second tempo and 1-second
relaxation between repetitions.
Exercise consisted of three sets of
squat, chest press, lat pulldown,
abdominal bend and back
extension, performed 2 days a
week for 13 weeks
No B-mode
ultrasound
No differences in CSA or strength
between low- and high-intensity
exercise
Van Roie,
Delecluse,
Coudyzer,
Boonen,and
Bautmans
(2013)
Fifty-six untrained
elderly adults
Random assignment to perform leg
press and leg extension training at
either high load (2 × 10–15
repetitions at 80% 1 RM), low load
(1 × 80–100 repetitions at 20%
1 RM) or low load + (1 × 60
repetitions at 20% 1 RM, followed
by 1 × 10–20 repetitions at 40%
1 RM) for 12 weeks. Tempo was
consistent between conditions
No CT No differences in muscle volume
between groups. Greater increases
in strength for high- and low+- vs.
low-load conditions
RM, repetition maximum; CSA, cross-sectional area; CT, computerised tomography; MRI, magnetic resonance imaging; MVC, maximal
voluntary contraction.
Muscular adaptations in low- versus high-load resistance training 5
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analyses were performed for strength and hyper-
trophy. Due to sample size limitations, no subgroup
analyses were performed. All analyses were per-
formed using SAS Enterprise Guide Version 4.2
(Cary, NC, USA). Effects were considered signific-
ant at p≤0.05, and trends were declared at 0.05 < p
≤0.10. Data are reported as means (±SEs) and
95% CIs.
Results
Study characteristics
The strength analysis comprised 251 subjects and
32 ESs, nested within 20 treatment groups and 9
studies. The weighted mean strength ES across all
studies and groups was 1.75 ± 0.34 (CI: 0.96, 2.55).
The hypertrophy analysis comprised 191 subjects
and 34 ESs, nested with 17 treatment groups and 8
studies. The weighted mean hypertrophy ES across
all studies and groups was 0.61 ± 0.12 (CI:
0.33, 0.89).
Strength model
The mean muscle strength ES difference between
high- and low-load groups for each individual study,
along with the overall weighted mean difference
across all studies, is shown in Figure 2. No signific-
ant differences between groups were seen, but there
was a trend for strength outcomes to be greater with
high loads compared to low loads (difference = 1.07
± 0.60; CI: −0.18, 2.32; p= 0.09). The mean ES
for low loads was 1.23 ± 0.43 (CI: 0.32, 2.13). The
mean ES for high loads was 2.30 ± 0.43 (CI:
1.41, 3.19).
Hypertrophy model
The mean muscle hypertrophy ES difference
between high- and low-load groups for each indi-
vidual study, along with the overall weighted mean
difference across all studies, is shown in Figure 3.
No significant differences between groups were seen,
but there was a trend for hypertrophy outcomes to be
greater with high loads compared to low loads
(difference = 0.43 ± 0.24; CI: −0.05, 0.92; p=
0.076). The mean ES for low loads was 0.39 ± 0.17
(CI: 0.05, 0.73). The mean ES for high loads was
0.82 ± 0.17 (CI: 0.49, 1.16).
Discussion
This is the first meta-analysis to investigate muscu-
lar adaptations in low- versus high-load training.
The study produced several important and novel
Figure 2. Forest plot of the impact of load on strength by study.
6B. J. Schoenfeld et al.
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findings. With respect to muscle hypertrophy, we
found no significant differences in ES between high-
versus low-load training protocols. To an extent,
these results run contrary to established guidelines
for hypertrophy training, which state that loads
greater than ~65% 1 RM are needed to maximise
the hypertrophic response (Kraemer & Ratamess,
2004; Kraemer et al., 2002; McDonagh & Davies,
1984). The current study provides compelling evid-
ence that substantial muscle growth can in fact be
achieved training with loads ≤60% 1 RM, with gains
approaching those using higher percentages of 1
RM. It should be noted, however, that there was a
trend for greater growth when using heavy loads as
compared to light loads (p= 0.076). This is reflected
in the mean ES data, where high-load training
showed a strong effect for hypertrophy (0.82 ±
0.17) while low-load training showed only a moder-
ate effect (0.39 ± 0.17). These data suggest that
those seeking to maximise hypertrophy might benefit
from the use of heavier loads.
With respect to increases in muscular strength, no
significant differences in ES were seen between high-
and low-load training protocols. Consistent with the
concept of a “strength-endurance continuum,”it is
generally believed that adaptations associated with
light-load protocols are specific to enhancing mus-
cular endurance and that any improvements in the
ability to exert maximal force are minimal (Campos
et al., 2002). On the surface, results of the present
study would seem to refute these assertions as
pooled analysis of results showed that significant
increases in strength are possible when low-load
training is carried out to muscular failure. However,
closer scrutiny of data indicates that these findings
must be interpreted with caution. Although both
high- and low-load training showed a strong effect
for increases in strength, the magnitude of the
difference in means for ES was large between the
two protocols (2.30 ± 0.43 versus 1.23 ± 0.43,
respectively), and the p-value for the difference
showed a trend for significance in favour of high-
load training (p= 0.09). In addition, the 95% CI
differential favoured high-load training (CI: −0.18–
2.32). Moreover, as seen in Figure 2, all nine studies
investigating the topic favoured high-load training,
and six of these studies showed a moderate to strong
difference in magnitude of effect. Taken in com-
bination, it can be inferred that the relative paucity of
studies on the topic limited statistical power and thus
obscured our ability to detect significant differences.
While it is evident that low-load training can promote
robust increases in muscular strength, the body of
research would seem to suggest that the use of
heavier loads might be required for maximum effect.
Figure 3. Forest plot of the impact of load on hypertrophy by study.
Muscular adaptations in low- versus high-load resistance training 7
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It is not clear from our analysis whether hyper-
trophy in low- versus high-load training manifested
in a fibre-type specific manner. As previously men-
tioned, there is evidence that muscle fibre recruit-
ment is suboptimal when training with low loads
(Akima & Saito, 2013; Cook et al., 2013). Recent
work from our lab found that heavy-load training
produced significantly greater mean and peak mus-
cle activation of thigh musculature compared to low-
load training (by 35% and 22%, respectively) during
performance of the leg press (Schoenfeld et al.,
2014). On the other hand, the duration of the light-
load set was three- to four-fold higher compared to
the heavy-load set, indicating that fibres activated
during light-load training received considerably
greater time-under-load (TUL) versus that achieved
during heavy loading. Given that Type I fibres have a
higher threshold for fatigability, the greater TUL
during low-load training would conceivably max-
imise their stimulation and thus promote a greater
hypertrophic response. This hypothesis is consistent
with the findings of Mitchell et al. (2012), who
compared knee extension training at 80% of 1-RM
versus 30% of 1-RM over 10 weeks. Although
similar increases between groups were reported in
whole muscle hypertrophy of the quadriceps as
assessed by magnetic resonance imaging, tissue
analysis from muscle biopsy revealed an increased
Type I fibre area in the low-load condition (~23%
versus ~16% in low versus high load, respectively),
whereas the high-load condition favoured greater
Type II fibre area (~15% versus ~12% in high versus
low load, respectively). Other studies that have
investigated the topic have failed to show a fibre-
type-specific response across the strength-endurance
continuum (Campos et al., 2002; Schuenke et al.,
2012). The reason for discrepancies between studies
is not clear. Further research is needed to provide
clarity on whether differences do in fact exist
between loading strategies with respect to fibre-type
hypertrophy and, if so, quantify the magnitude of
these differences.
An important consideration to take into account
when generalising results of this meta-analysis is that
all of the included studies employed untrained or
recreationally trained subjects. There is evidence
that regular performance of resistance training can
modulate the hypertrophic response to loading
(Schoenfeld, 2013). As an individual gains lifting
experience, a “ceiling effect”makes it progressively
more difficult to increase muscle mass, perhaps
mediated by an altered anabolic intracellular signal-
ling cascade (Coffey, Zhong, et al., 2006; Ogasa-
wara, Kobayashi, et al., 2013). In support of this
point, untrained subjects have been shown to
increase muscle mass from cardiovascular exercise –
a modality that is not sufficient to induce hypertrophy
in a well-trained population (Konopka & Harber,
2014). Thus, more demanding resistance training
protocols may be needed to elicit a hypertrophic
response in those who regularly lift weights, perhaps
including the use of heavier loads. Similarly, while
loads ≤60% 1 RM can promote significant strength
increases, some researchers have put forth the notion
that greater loading is required as an individual attains
a more advanced training status to realise further
improvements (Kraemer & Ratamess, 2004). This
hypothesis is based on the fact that early-phase
strength-related adaptations are characterised by
enhancements in motor learning and coordination
and that heavier loads are therefore necessary to
maximise strength-related outcomes once an indi-
vidual acquires these basic motor skills. A ceiling
effect might have implications with respect to strength
gains as well. Future research should seek to invest-
igate the response to different loading strategies in
those with at least one year of consistent resistance
training experience.
A limitation of current research on high versus low
loading is the relatively brief duration of the training
protocols. The longest study spanned 13 weeks and
several were as short as 6 weeks. Although these time
frames are certainly long enough to realise significant
increases in muscular strength and hypertrophy, it
remains to be determined whether results would
diverge over longer training periods. This conun-
drum should be addressed in future research. It
should also be noted that the studies included for
analysis had inherent differences in manipulation of
variables, such as exercise selection, rest intervals
and frequency. Thus, pooling of data may not
necessarily reflect these discrepancies and their
potential impact on results. Although no statistically
significant differences were noted between condi-
tions, the variability in the response to training and
the observed trend for a positive effect of heavy-load
training warrants further research with larger sample
sizes.
Practical applications
This meta-analysis provides compelling evidence
that training with loads ≤60% 1 RM can promote
substantial increases in muscle strength and hyper-
trophy in untrained individuals. However, a strong
trend was noted for superiority of heavy loading with
respect to these outcome measures, with null find-
ings likely attributed to the relatively small number
of studies meeting inclusion criteria. It may well be
that a combination of heavy and light loading is best
for maximising muscular adaptations associated with
resistance training, conceivably by promoting
optimal hypertrophy of both Type I and Type II
muscle fibres. Our findings also have important
8B. J. Schoenfeld et al.
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implications for the elderly and those suffering from
musculoskeletal conditions, such as osteoarthritis,
who clearly can enhance muscular adaptations by
training with lighter loads that are more easily
tolerated. Given the dearth of data on experienced
lifters, future research should focus on elucidating
the response of well-trained populations to low-
versus high-load protocols.
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