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REVIEW ARTICLE
Is There a Minimum Intensity Threshold for Resistance
Training-Induced Hypertrophic Adaptations?
Brad J. Schoenfeld
Published online: 19 August 2013
Ó Springer International Publishing Switzerland 2013
Abstract In humans, regimented resistance training has
been shown to promote substantial increases in skeletal
muscle mass. With respect to traditional resistance training
methods, the prevailing opinion is that an intensity of
greater than *60 % of 1 repetition maximum (RM) is
necessary to elicit significant increases in muscular size. It
has been surmised that this is the minimum threshold
required to activate the complete spectrum of fiber types,
particularly those associated with the largest motor units.
There is emerging evidence, however, that low-intensity
resistance training performed with blood flow restriction
(BFR) can promote marked increases in muscle hypertro-
phy, in many cases equal to that of traditional high-inten-
sity exercise. The anabolic effects of such occlusion-based
training have been attributed to increased levels of meta-
bolic stress that mediate hypertrophy at least in part by
enhancing recruitment of high-threshold motor units.
Recently, several researchers have put forth the theory that
low-intensity exercise (B50 % 1RM) performed without
BFR can promote increases in muscle size equal, or per-
haps even superior, to that at higher intensities, provided
training is carried out to volitional muscular failure. Pro-
ponents of the theory postulate that fatiguing contractions
at light loads is simply a milder form of BFR and thus
ultimately results in maximal muscle fiber recruitment.
Current research indicates that low-load exercise can
indeed promote increases in muscle growth in untrained
subjects, and that these gains may be functionally, meta-
bolically, and/or aesthetically meaningful. However,
whether hypertrophic adaptations can equal that achieved
with higher intensity resistance exercise (B60 % 1RM)
remains to be determined. Furthermore, it is not clear as to
what, if any, hypertrophic effects are seen with low-
intensity exercise in well-trained subjects as experimental
studies on the topic in this population are lacking. Practical
implications of these findings are discussed.
1 Introduction
Muscle tissue displays a high level of plasticity, allowing it
to readily adapt to both acute and chronic imposed
demands [1]. Studies have clearly demonstrated that when
subjected to functional overload, muscle tissue responds by
increasing its cross-sectional area (CSA). Animal models
using passive stretch, synergist ablation (surgical removal
of one muscle to cause increased overload of the syner-
gists), and neuromuscular electrical stimulation produce
hypertrophic increases of as much as 100 % [2]. In
humans, regimented resistance training has been shown to
promote marked increases in skeletal muscle mass [3, 4].
Although hypertrophy occurs in all fiber types, fast-twitch
(FT) fibers display an approximately 50 % greater capacity
for growth compared with their slow-twitch (ST) counter-
parts [2, 5]. That said, there is a high degree of inter-
individual variability with respect to the extent of hyper-
trophic adaptation across the full spectrum of fiber types
[5].
Three primary factors have been proposed to mediate
hypertrophic adaptations pursuant to resistance training:
mechanical tension, metabolic stress, and muscle damage
[3]. A number of researchers have surmised that tension is
the primary driving force in this process [6, 7]. However,
assuming that a given level of mechanical tension is
B. J. Schoenfeld (&)
Department of Health Sciences, Program of Exercise Science,
APEX Building, Room # 219, Lehman College, CUNY,
250 Bedford Park Blvd West, Bronx, NY 10468, USA
e-mail: brad@workout911.com
Sports Med (2013) 43:1279–1288
DOI 10.1007/s40279-013-0088-z
Author's personal copy
achieved, both metabolic stress and tissue damage may
become increasingly important factors in optimizing a
hypertrophic response [8, 9]. Studies to date are incon-
clusive as to whether one particular parameter predomi-
nates with respect to activating the cellular and molecular
mechanisms responsible for regulating muscle growth [2].
With respect to traditional resistance training methods,
the prevailing opinion is that a concentric intensity of
greater than *60 % of 1 repetition maximum (RM) is
necessary to elicit significant increases in muscle size [10–
12]. It has been surmised that this is the minimum
threshold required to activate the complete spectrum of
fiber types, particularly those associated with the largest
motor units (MUs) [13]. There is emerging evidence,
however, that low-intensity resistance training performed
with blood flow restriction (BRF) can promote significant
increases in muscle hypertrophy, in many cases equal to
that of traditional high-intensity exercise [14]. Restriction
of blood flow is achieved by wrapping an elastic implement
(such as knee or elbow wraps) at the proximal portion of a
limb so that circulation is occluded to working muscles
during performance of resistance exercise. The anabolic
effects of such occlusion-based training have been attrib-
uted to increased levels of metabolic stress, i.e., a buildup
of metabolites pursuant to glycolytic energy production. It
is theorized that metabolic stress mediates hypertrophy at
least in part by enhancing recruitment of high-threshold
MUs [15], but other mechanisms are also believed to play a
role in the process including cell swelling, elevated hor-
monal levels, and increased production of reactive oxygen
species [16, 17].
Recently, several researchers have put forth the theory
that low-intensity exercise (B50 % 1RM) performed
without BFR can promote increases in muscle size equal,
or perhaps even superior, to that at higher intensities,
provided training is carried out to volitional muscular
failure [4, 18]. Proponents of the theory postulate that
fatiguing contractions at light loads is simply a milder form
of BFR and thus ultimately results in maximal muscle fiber
recruitment [19]. It has been surmised that as long as
progressive overload is employed, even the most serious
lifters can realize significant increases in muscle hyper-
trophy from such low-intensity training [19]. The purpose
of this review therefore is to evaluate the literature on the
topic in an attempt to determine the minimum intensity
required for optimal hypertrophic adaptations. Evidence-
based recommendations are then made to help guide pro-
gram design when devising hypertrophy-oriented routines.
To carry out this review, English-language literature
searches of the PubMed, EBSCO, and Google Scholar
databases were conducted for all time periods up to
December 2012. Combinations of the following keywords
were used as search terms: ‘skeletal muscle’;
‘hypertrophy’; ‘muscle growth’; ‘cross sectional area’;
‘intensity’; ‘loading’; ‘low load’; ‘repetition range’;
‘resistance training’; and ‘resistance exercise’. The refer-
ence lists of articles retrieved in the search were then
screened for any additional articles that had relevance to
the topic.
2 Theoretical Basis for Lower Intensity Hypertrophic
Adaptations
Maximal muscle hypertrophy is predicated on recruiting as
many MUs as possible in the target muscles and achieving
high firing rates in these MUs for a sufficient length of time
[11]. The mechanisms by which mechanical forces lead to
muscular adaptations are still not fully understood. Current
theory proposes that the process is regulated by a phe-
nomenon called mechanotransduction whereby sarcolem-
mal-bound mechanosensors, such as integrins and focal
adhesions, convert mechanical energy into chemical sig-
nals that mediate intracellular anabolic and catabolic
pathways, ultimately leading to a shift in muscle protein
balance that favors synthesis over degradation [20]. A
summation of anabolic signals of an adequate magnitude is
required to generate sustained responses that lead to muscle
protein accretion [21].
Many signaling pathways have been identified as play-
ing a part in the regulation of muscle mass, with certain
pathways acting in a permissive role and others providing
direct mediation of cellular processes that influence mes-
senger RNA translation and hypertrophy [22]. Signaling
pathways that have been identified include phosphatidyl-
inositol 3-kinase-protein kinase B-mammalian target of
rapamycin, mitogen-activated protein kinase (MAPK), and
various calcium (Ca
2?
)-dependent pathways, amongst
others. Although these pathways may overlap at key reg-
ulatory steps, evidence suggests that they are interactive
rather than redundant [23]. For example, although Akt and
MAPK/extracellular signal-related kinase (ERK) both have
been shown to stimulate mammalian target of rapamycin to
a similar extent, the combined effects of both lead to an
even greater stimulation compared with either pathway
alone [24]. A complete discussion of these signaling
pathways and their functions is beyond the scope of this
article. For further information, interested readers are
referred to recent reviews by Bassel-Duby and Olson, [25],
Miyazaki and Esser [26], and Glass [27].
Claims for a hypertrophic effect of low-intensity resis-
tance exercise are based on the premise that recruitment of
the full spectrum of MUs is achieved at virtually any
intensity, provided training is carried out to the point of
concentric muscular failure [18]. It remains questionable,
however, whether this belief holds true in practice. There is
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evidence that fatiguing contractions result in a corre-
sponding increase in electromyography (EMG) activity,
presumably resulting from an increased contribution of
high-threshold MUs recruited to maintain force output
[28], but it is not clear what level of intensity is required to
initiate activation of these high-threshold MUs. Further-
more, beyond a certain intensity level, the resistive exercise
would become more reliant on aerobic metabolism and
thus could be continued for extended periods of time in the
upper levels of steady state. This shift in energy system
contribution would conceivably result in a competitive
interaction between anabolic and catabolic signaling
pathways that leads to adaptations associated more with
endurance than strength [29].
Studies corroborating the supposition that low-intensity
training to failure equates to a milder form of BFR are
lacking. Wernbom et al. [4] demonstrated that peak EMG
activity was similar between three sets of low-intensity
(30 % 1RM) unilateral knee extensions performed with and
without BFR to muscular failure. Mean values were not
reported thereby prohibiting analysis of the effects on
muscle recruitment over the course of the entire range of
sets. Further research is necessary to better determine the
relationship between muscle recruitment and low-intensity
exercise with and without BRF.
There is evidence that muscle recruitment is indeed
greater in high-intensity exercise compared with low-
intensity blood flow-restricted exercise. Employing a
model that examined inorganic phosphate splitting via
31
P-
magnetic resonance spectroscopy, Suga et al. [30] dis-
played that FT fiber recruitment occurred in only 31 % of
subjects who performed BFR training at 20 % 1RM com-
pared with 70 % of those who trained at 65 % 1RM. This
finding is consistent with other research showing that
exercise performed at high intensities produces substan-
tially greater EMG activity compared with BFR exercise at
20 % 1RM, indicating an attenuated recruitment at the
lower training intensity [31, 32]. Follow-up work by Suga
et al. [33] showed that splitting of P
i
peaks at 30 % 1RM
approached those of higher intensity exercise, but never-
theless did not reach levels indicative of equal muscle fiber
recruitment. Only when blood flow-restricted exercise was
carried out at an intensity of 40 % 1RM did P
i
peaks equate
to, and actually exceed, those associated with traditional
high-intensity training. Lending further support to these
findings, Cook et al. [34] recently demonstrated that EMG
amplitude of the vastus lateralis, vastus medialis, and rec-
tus femoris during knee extension exercise to failure was
significantly greater at a high intensity (70 % 1RM) than at
a low intensity (20 % 1RM) both with and without BFR.
The aforementioned studies are limited to the use of the
knee extension; further research is needed using a variety
of single- and multi-joint movements with varying
percentages of 1RM performed to failure to provide a
better understanding of the subject.
3 Acute Responses to Varying Resistance Exercise
Intensities
Several animal studies have evaluated the effects of differ-
ent intensities on acute signaling responses. Using an in situ
model, Martineau et al. [35] subjected rat plantaris muscles
to peak concentric, eccentric, and isometric actions via
electrical stimulation. Results showed tension-dependent
phosphorylation of c-Jun N-terminal kinase (JNK) and
ERK, with higher mechanical tension resulting in progres-
sively greater phosphorylation. This suggests that peak
tension is a better predictor of MAPK phosphorylation than
either time-under-tension or rate of tension development.
Interestingly, follow-up work by the same laboratory found
a linear relationship between time under tension and sig-
naling of JNK, whereas rate of tension change showed no
effect, highlighting the importance of time under tension in
anabolic signaling [36]. Taken together, these findings point
to the importance of overall training volume for maximizing
the acute molecular responses related to skeletal muscle
hypertrophy irrespective of training intensity.
In an attempt to qualify the acute effects of resistance
training intensity in humans, Kumar et al. [37] investigated
the acute exercise responses at 20–90 % 1 RM in healthy
young and old men. The protocol was designed so that
volume of training was approximately equal between
training intensities. Thus, at 20 % intensity, participants
performed three sets of 27 repetitions; at 40 % intensity,
three sets of 14 repetitions were performed; at 60 %
intensity, training consisted of three sets of nine repetitions;
at 75 % intensity, three sets of eight repetitions were per-
formed; and at 90 % intensity, six sets of three repetitions
were performed. Increases in myofibrillar muscle protein
synthesis (MPS) were minimal after exercise at 20 % and
40 % 1RM but values significantly and markedly increased
at 60 % 1RM, plateauing thereafter. Similarly, phosphor-
ylation of p70S6K was maximized at intensities of
60–90 % 1RM, peaking just prior to the maximal rise in
MPS. These results held true in both younger and older
subjects, suggesting that the stimulatory effect on MPS
reaches a maximum at *60–75 % 1RM of isotonic exer-
cise. The authors did not state whether low-load training
was carried out to muscular failure, but based on the study
design this does not appear to be the case. This is an
important limitation as research indicates the hypertrophic
response to low-load training is predicated on lifting to the
point of voluntary muscular failure [38–40].
Burd et al. [39] sought to determine whether resistance
exercise intensity had a differential effect on MPS and
Intensity Threshold for Hypertrophic Adaptations 1281
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anabolic signaling. A quasi within-subject design was used
where 15 young, recreationally active men performed four
sets of unilateral knee extensions at 30 % and 90 % 1RM to
volitional muscular failure. A third condition involved
performing the exercise at 30 % 1RM with external work
(repetitions 9 load) matched to the 90 % condition. At 4-h
post-exercise, measures of MPS were elevated at all con-
ditions studied, but levels in the 30 % work-matched
condition were approximately half that of the other two
conditions. Interestingly, myofibrillar MPS remained ele-
vated at 24-h post-exercise only in the 30 % to failure
condition. Phosphorylation of p70S6K was significantly
increased at 4 h only in the 30 % to failure condition, and
this elevation was correlated with the degree of stimulation
of myofibrillar MPS. These findings suggest that low-
intensity exercise performed to volitional fatigue induces
greater acute muscular responses compared with high-
intensity exercise. The fact that volume was substantially
greater in the 30 % condition versus the 90 % condition
confounds the ability to isolate the impact of intensity on
the variables studied.
Although these studies provide relevant clues as to the
anabolic effects of various intensities of exercise, their
findings are not necessarily predictive of long-term chan-
ges in lean body mass. Evaluation of measures of MPS
following an acute bout of resistance exercise do not
always occur in parallel with chronic upregulation of
causative myogenic signals [41] and may not reflect
hypertrophic responses experienced pursuant to regimented
resistance training carried out over a period of weeks or
months [42]. Moreover, the acute responses of subjects
with minimal training experience, in particular, must be
viewed with caution as results may be primarily a function
of the unfamiliarity of exercise and thus not applicable to
the response of well-trained individuals [2, 43]. Given
these inherent limitations, any attempt to extrapolate find-
ings from such data to hypertrophic adaptations is specu-
lative, at best.
4 Chronic Adaptations to Varying Resistance Exercise
Intensities
A number of studies have attempted to directly evaluate
long-term hypertrophic adaptations along the strength-
endurance intensity continuum. Findings between these
studies are inconsistent and discrepant. Table 1 summa-
rizes the relevant research to date.
Campos et al. [44] was the first to investigate the topic
in a well-controlled experimental fashion. Thirty-two
untrained men ( mean ± SD: age 22.5 ± 5.8 years) were
randomly assigned to one of three lower body training
protocols: a low-repetition group (n = 9) that performed
3–5 RM for four sets of each exercise with 3-min rest
intervals between sets; an intermediate repetition group
(n = 11) that performed 9–11 RM for three sets with 2-min
rest intervals; or a high-repetition group (n = 7) that per-
formed 20–28 RM for two sets with 1-min rest intervals. A
control group (n = 5) performed no resistance exercise.
The exercise regimen consisted of the leg press, squat, and
knee extension with total volume load approximately equal
between groups. Training was carried out 2 days a week
for the first 4 weeks and 3 days a week for the final
4 weeks. Resistance was progressively increased through-
out the training period to maintain repetition ranges and all
sets were performed to momentary concentric muscular
failure. Muscle biopsy was used to assess changes in fiber
CSA of the vastus lateralis. After 8 weeks, both the high
and intermediate repetition groups displayed significant
increases of 12.5 %, 19.5 %, and 26 % in CSA for type I,
IIA, and IIX fibers, respectively. Increases in muscle fiber
CSA for the high-repetition group did not reach statistical
significance for any of the fiber types, indicating that lower
intensity exercise is substandard for promoting increases in
hypertrophy.
Employing the same basic training program as Campos
et al. [44], Leger et al. [45] divided 25 healthy men into
either a low- or high-repetition group; an intermediate
group was not included as part of the study design. Subjects
were older than in the Campos et al. [44] study (age
36 ± 4.9 years) and had not participated in a resistance
training program for at least 1 year. Muscle volume was
assessed by computed tomography (CT). After 8 weeks, an
approximately 10 % increase in quadriceps CSA was noted
in both groups with no significant differences found
between training protocols. Follow-up work by this labo-
ratory [46], in a similar population demographic, also
reported 10 % increases in quadriceps hypertrophy with no
significant differences between groups using the same
training protocol. The researchers attributed the discrep-
ancy between their results and that of Campos et al. [44]to
the detrained status of the somewhat older subjects, theo-
rizing that any type of resistance training in this population
would promote a sufficient overload stimulus to elicit
increases in muscle growth.
Tanimoto and Ishii [40] evaluated the muscular response
of low-intensity exercise performed with slow movement
and tonic force generation to a traditional higher intensity
routine in 24 untrained men. Subjects were randomly
assigned to perform repetitions of the leg extension at
either 50 % RM with a 6-s cadence (3 s for both concentric
and eccentric actions) and no relaxing phase between
repetitions (LST; n = 8) or 80 % RM at a tempo of 1 s for
both concentric and eccentric actions with 1-s relaxation
between repetitions (HN; n = 8). Both of these groups
performed approximately eight repetitions per set until
1282 B. J. Schoenfeld
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failure and the intensity was progressively adjusted based
on performance in the previous session. A third group (LN;
n = 8) performed low-intensity exercise (50 % RM) using
the normal tempo employed in the high-intensity protocol
and thus did not work to volitional failure. Training was
carried out 3 days a week for 12 weeks. At the end of the
study period, muscle CSA as determined by magnetic
resonance imaging (MRI) increased significantly in both
the LST and HN groups (5.4 ± 3.7 % vs. 4.3 ± 2.1 %,
respectively), with no significant differences noted between
the groups. The LN group did not significantly increase
muscle mass. These results again emphasize the
Table 1 Summary of long-term studies evaluating the effects of training intensity on muscle hypertrophy
Study Subjects Design Volume
equated
Train
to
failure
Measurement Findings
Campos
et al. [44]
32 untrained young
men (5 served as
non-exercising
controls)
Random assignment to either low intensity (3–5
RM), intermediate intensity (9–11 RM) for 3
sets with 2-min rest intervals, or high-intensity
(20–28 RM) exercise. Exercise consisted of 2–4
sets of squat, leg press, and leg extension,
performed 3 days a week for 8 weeks
Yes Yes Muscle
biopsy
Significant increases in CSA for
high-intensity exercise; no
significant increase in CSA for
low-intensity exercise
Leger et al.
[45]
24 untrained
middle-aged men
Random assignment to either low intensity (3–5
RM) or a high intensity (20–28 RM) exercise.
Exercise consisted of 2–4 sets of squat, leg
press, and leg extension, performed 3 days a
week for 8 weeks
Yes Yes CT No differences in CSA between
low- and high-intensity exercise
Lamon
et al. [46]
25 untrained
middle-aged men
Random assignment to either low-intensity (3–5
RM) or high-intensity (20–28 RM) exercise.
Exercise consisted of 2–4 sets of squat, leg
press, and leg extension, performed 3 days a
week for 8 weeks
Yes Yes CT No differences in CSA between
low- and high-intensity exercise
Tanimoto
and Ishii
[40]
24 untrained young
men
Random assignment to either 50 % RM with a 6-s
tempo and no relaxing phase between
repetitions, 80 % RM with a 2-s tempo and 1-s
relaxation between repetitions, or 50 % RM
with a 2-s tempo and 1-s relaxation between
repetitions. Exercise consisted of 3 sets of knee
extensions, performed 3 days a week for
12 weeks
No Yes MRI No differences in CSA between
low- and high-intensity exercise
Tanimoto
et al. [47]
36 untrained young
men (12 served
as non-exercising
controls)
Random assignment to either 55–60 % RM with a
6-s tempo and no relaxing phase between
repetitions or 80–90 % RM with a 2-s tempo
and 1-s relaxation between repetitions. Exercise
consisted of 3 sets of squat, chest press, lat
pulldown, abdominal bend, and back extension,
performed 2 days a week for 13 weeks
No Yes B-mode
ultrasound
No differences in CSA between
low- and high-intensity exercise
Holm et al.
[48]
11 untrained young
men
Random, counterbalanced performance of 10 sets
of unilateral leg extensions, training one leg at
70 % 1RM and the contralateral leg at 15.5 %
1RM, performed 3 days a week for 12 weeks
Yes No MRI Significantly greater increases in
CSA in high-intensity versus
low-intensity exercise
Mitchell
et al. [50]
18 untrained young
men
Randomized assignment to perform 2 of 3
unilateral leg extension protocols: 3 sets at 30 %
RM; 3 at 80 % RM; or 1 set at 80 % RM.
Training was carried out 3 days per week for
10 weeks
No Yes MRI, muscle
biopsy
No differences in CSA between
low- and high-intensity exercise
Schuenke
et al. [51]
34 untrained young
women
Randomized assignment to either moderate
intensity (80–85 % RM) at a tempo of 1–2 s, a
low intensity (*
40–60 % RM) at a tempo of
1–2 s, or slow speed (*40–60 % RM) at a
tempo of 10 s concentric and 4 s eccentric.
Exercise consisted of 3 sets of squat, leg press,
and leg extension, performed 2–3 days a week
for 6 weeks
No Yes Muscle
biopsy
Significant increases in CSA for
high-intensity exercise; no
significant increase in CSA for
low-intensity exercise
Ogasawara
et al. [52]
9 untrained young
men
Non-randomized crossover design to perform 4
sets of bench press exercise at 75 % 1RM.
Training was carried out 3 days a week for
6 weeks. After a 12-month washout period, the
same protocol was performed at 30 % 1RM
No Yes MRI No differences in CSA between
low- and high-intensity exercise
RM repetition maximum, CSA cross-sectional area, CT computed tomography, MRI magnetic resonance imaging
Intensity Threshold for Hypertrophic Adaptations 1283
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importance of training to muscular failure for eliciting a
hypertrophic response during low-load training.
This study design was subsequently replicated by the
same laboratory [47] using a total-body resistance training
program consisting of three sets of the squat, chest press,
lat pull-down, abdominal bend, and back extension.
Intensity was slightly higher for both groups (55–60 % in
the LST group and 80–90 % in the HN group), as neces-
sitated by the multi-joint nature of the exercises. Again,
significant increases in muscle size were detected in both
the LST and HN groups (mean ± SD: 6.8 ± 3.4 % vs.
9.1 ± 4.2 %, respectively), with no significant differences
noted between groups. While these findings are intriguing,
they are confounded by the altered repetition cadence,
thereby making it impossible to draw relevant conclusions
as to traditional intensity recommendations. Moreover,
although results did not reach statistical significance in the
total-body protocol, high-intensity exercise produced an
approximately 34 % greater absolute increase in hyper-
trophy. Thus, it seems likely that the small sample size
resulted in a type II error.
Holm et al. [48] studied the effects of light-load resis-
tance exercise in 11 sedentary young men. A within-subject
design was employed whereby subjects performed ten sets
of unilateral leg extensions, training one leg at 70 % 1RM
and the contralateral leg at 15.5 % 1RM in a randomized,
counterbalanced fashion. Training was carried out 3 days a
week for a total of 12 weeks. Muscle CSA of the quadri-
ceps as determined by MRI was greater by threefold in the
high-intensity leg compared with the leg that performed
low-intensity exercise. It should be noted that the low-
intensity exercise involved performing one repetition every
5 s for 3 min, calling into question the extent of fatigue
experienced during exercise performance, and thus
obscuring the ability to extrapolate conclusions to low-load
training to failure. Interestingly, a subsequent study using
the same protocol in healthy, young men showed a sig-
nificant 18 % increase in satellite cell number associated
with the low-load protocol after 12 weeks of training,
indicating that low-intensity exercise has a favorable effect
on early-stage myogenesis [49].
In a follow-up to their previously mentioned, acute
training study [39], Stuart Phillips’ laboratory employed a
quasi within-subject design to test the hypothesis that these
results would translate into long-term gains in muscle
hypertrophy [50]. Eighteen untrained men (mean ± SD:
age 21 ± 1 years) were randomly assigned to perform two
of three different resistance training protocols involving
unilateral knee extension exercise for each leg to
momentary concentric muscular failure as follows: three
sets of low-intensity exercise at 30 % RM; three sets of
high-intensity exercise at 80 % RM; and one set of high-
intensity exercise at 80 % RM. Training was carried out 3
days per week for 10 weeks. Muscular adaptations of the
vastus lateralis was assessed by MRI and muscle biopsy. At
the end of the study period, both the low- and multi-set
high-intensity groups realized significant increases in
muscle volume (mean ± SD: 6.8 ± 1.8 % vs.
7.2 ± 1.9 %, respectively), with no differences found
between the groups. The single-set high-intensity group
also showed significant increases in hypertrophy, although
the gains were less than half that of the other two groups
(mean ± SD: 3.2 ± 0.8 %). Interestingly, fiber analysis by
muscle biopsy showed that the low-intensity group dis-
played greater hypertrophy of type I fibers while the high-
intensity group displayed greater hypertrophy of type II
fibers, suggesting a fiber type-specific adaptive response
along the strength-endurance continuum. The study was
limited by the sole use of the leg extension exercise, which
is not representative of training routines normally
employed in a hypertrophy-oriented program.
A recent study by Schuenke et al. [51] investigated both
the effects of intensity as well as tempo on muscle
hypertrophy. Thirty-four untrained women were randomly
divided into one of three groups: A traditional strength
(TS) group that performed sets of 6–10 RM at a cadence of
1–2 s on the concentric and eccentric portion of the repe-
tition; a traditional muscular endurance (TE) group that
performed 20–30 repetitions at the same speed as TS; and a
slow-speed (SS) group that performed 6–10 repetitions at a
tempo of 10 s on concentric action and 4 s on the eccentric
action. Both TE and SS trained at an intensity of
*40–60 % 1RM while TS trained at *80–85 % 1RM.
The longer duration of cadence in the SS routine is asso-
ciated with a reduced momentum and a greater consistency
in average force (vs. peak force) over a complete repetition
compared with training with TE. Training consisted of
three sets of the leg press, squat, and knee extension to
momentary muscular failure with *2-min rest intervals
afforded between sets. Training was carried out 2 days a
week for the first week and 3 days a week for the
remaining 5 weeks. Muscle biopsy was used to assess CSA
of the vastus lateralis. After 6 weeks, significant increases
were noted in TS for types I, IIA, and IIX fiber areas
(mean ± SD: 26.6 ± 22.7 %, 32.9 ± 20.4 %, and
41.1 ± 32.7 % respectively), whereas no significant dif-
ferences were seen in TE. Interestingly, SS displayed sig-
nificant increases in both types IIA IIX CSA, although
these changes were less than half that of that experienced
by TS. It remains conceivable that hypertrophy might
manifest more gradually in lower intensity exercise and, if
so, would therefore not have been evident in this study
given its short duration.
Most recently, Ogasawara et al. [52] found similar
increases in CSA of the pectoralis major and triceps brachii
in subjects performing free-weight bench press exercise at
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75 % 1RM versus 30 % 1RM to concentric muscle failure.
The study employed a within-subject design whereby nine
previously untrained subjects performed the higher inten-
sity exercise for the initial 6 weeks of the study and then,
after a 12-month washout period of detraining, performed
6 weeks of the low-load exercise in a non-randomized
fashion. Although intriguing, these findings must be
viewed with caution as ‘muscle memory’ via neural
mechanisms and/or satellite cell accretion may have
influenced results [5, 53, 54].
The mixed and conflicting results between these studies
are hard to justify and likely a function of the varied study
designs and methods of assessment. One issue of note is the
use of different techniques for measuring muscular adap-
tations including biopsy, MRI, ultrasound, and/or CT. Each
of these techniques has various inherent strengths and
weaknesses, causing difficulties when attempting to rec-
oncile research findings [2].
The use of different exercise protocols serves to further
confound results. Some of the studies involved only a few
sets of single-joint exercise while others employed multi-
set routines consisting of combinations of single- and
multi-joint exercises more representative of traditional
hypertrophy training practices. In addition, some studies
equated volume between training conditions while other
studies did not. These confounding issues hinder the ability
to draw relevant comparisons between studies.
Another major limitation of the current body of litera-
ture is a lack of statistical power because of small sample
sizes. Studies to date have generally involved fewer than
*30 exercising subjects with B12 subjects per group
studied. This substantially raises the possibility of a type II
error, whereby significant differences cannot be determined
when in fact they do exist. Greater statistical power could
be achieved by pooled analysis of data; however, the dis-
parate methods employed in existing studies to date make
such a meta-analysis problematic.
Finally, and importantly, all studies to date have been
carried out in untrained or minimally trained subjects. It is
well established that highly trained individuals respond
differently than those who lack training experience [55]. A
‘ceiling effect’ makes it progressively more difficult for
trained individuals to increase muscular gains, thereby
necessitating more demanding resistance training protocols
to elicit a hypertrophic response. Moreover, there is
emerging evidence that consistent resistance exercise can
alter anabolic intracellular signaling in rodents [56] and
humans [57], indicating an attenuated hypertrophic
response. As such, current findings cannot necessarily be
generalized to a well trained population. Future research
should therefore focus on the hypertrophic effects of
training intensity in those with at least 1 year or more of
regular, consistent resistance training experience.
5 Conclusions
Although it is evident that a minimum intensity threshold
exists to promote increases in muscle mass, the precise
level of intensity needed to achieve hypertrophic adapta-
tions has yet to be elucidated. Based on current research, it
does appear that low-load exercise can indeed promote
increases in muscle growth in untrained subjects, and that
these gains may be functionally, metabolically, and/or
aesthetically meaningful. However, whether hypertrophic
adaptations can equal that achieved with higher intensity
resistance exercise (B60 % 1RM) remains dubious. Fur-
thermore, it is not clear as to what, if any, hypertrophic
effects are seen with low-intensity exercise in well-trained
subjects, as experimental studies on the topic in this pop-
ulation are lacking.
The preponderance of evidence indicates that blood
flow-restricted resistance exercise at intensity levels
B20 % 1RM does not result in recruitment of the full
spectrum of MUs, making it highly unlikely that non-
occluded resistance exercise at similar intensities would
achieve comparable muscle activation to high-intensity
exercise. Recruitment of FT fibers with blood flow-
restricted resistance exercise at 30 % has been shown to
approach, but not equal, that of high-intensity exercise
[33]. Given these findings, it would appear that intensities
above 30 % are needed for complete muscle fiber recruit-
ment. It therefore stands to reason that if traditional resis-
tance exercise B30 % 1RM does in fact promote muscular
gains equal to that of high-intensity exercise, as has been
found in a limited number of studies [45, 46, 50], the dif-
ferences in protein accretion seemingly would have to be
made up by a greater degree of hypertrophy in type I and
perhaps type IIA fibers. It is conceivable that other factors
attributed to metabolic stress (e.g., cell swelling, autocrine/
paracrine factors, and systemic hormonal elevations) may
allow for such enhanced adaptations. This appears to be the
case with BFR, as marked hypertrophy is routinely seen at
intensities B30 % 1RM [
14], presumably mediated by a
heightened metabolic build-up [16, 58]. Whether similar
effects are realized in low-load training without BFR
remains to be elucidated. Moreover, some exercises may
lend themselves well to promoting BFR and thus height-
ening metabolic stress at lower loads (those with consistent
torques) while others may not (those with torque curves
that considerably drop off during the lift). Further research
is needed to investigate these issues.
Another consideration that needs to be taken into
account is the necessity to train to fatigue during low-
intensity training. It has been hypothesized that persistently
training to volitional muscular failure increases the
potential for overtraining and psychological burnout [59].
Indeed, Izquierdo, et al. [60] found that training to failure
Intensity Threshold for Hypertrophic Adaptations 1285
Author's personal copy
caused reductions in resting insulin-like growth factor-1
levels and a blunting of resting testosterone levels over a
16-week protocol, suggesting that subjects may have been
over-trained by the end of the study. The negative effects
of overtraining generally take time to manifest and thus
likely would not have been evident in the current studies on
training intensity given their relatively short duration
(B12 weeks). In practice, however, this would necessitate
the implementation of more frequent unloading periods
over the course of a periodic training program compared
with higher intensity exercise. It is not clear how such
alterations might affect long-term hypertrophic gains.
Research seems to suggest that a moderate repetition
range (6–12 RM) using a controlled lifting cadence may be
optimal for maximizing gains in muscle hypertrophy [7,
61, 62], although evidence is far from conclusive on the
subject. This so-called ‘hypertrophy range’ may conceiv-
ably provide an optimal combination of mechanical ten-
sion, metabolic stress, and muscle damage, thereby
generating a sustained anabolic response that maximizes
muscle protein accretion [3]. Regardless of the existence of
an ideal hypertrophy range, however, a strong case can be
made for incorporating the use of a variety of training
intensities into a hypertrophy-oriented program. Low-rep-
etition resistance training (1–5 RM) enhances neuromus-
cular adaptations necessary for the development of
maximal strength [61]. These adaptations allow the use of
heavier loads, and thereby greater mechanical tension, at a
given moderate intensity. Conversely, higher repetition
training (15? RM) can help to attenuate the exercise-
induced rise in blood lactate [63], delaying the onset of
fatigue and thus leading to a greater inroading of fibers
during hypertrophy-type training. This varied approach
would seem to be of particular importance for those with
considerable training experience, as a greater degree of
overload is necessary for continued adaptation in these
individuals.
It is also conceivable that people may respond differently
to exercise intensity based on individual muscle morphol-
ogy. There is clear evidence that men and women exhibit
large variations in their response to the same resistance
training protocol, with some subjects displaying little to no
muscular gains and others showing profound increases in
muscle mass [64, 65]. These variances may be due, at least
in part, to differences in muscle fiber-type distribution.
Studies show a large genetic variability between individuals
in the percentage of FT versus ST fibers for a given muscle
[66], and these disparities can have implications in the
response to exercise [67]. This raises the possibility that
adaptations to low- and high-training intensities may be
specific to the fiber-type profile of the target muscle. For
example, there is evidence that the predominantly ST soleus
muscle is much less responsive to traditional resistance
exercise compared with primarily FT muscles such as the
vastus lateralis and the biceps brachii [68]. Would the
soleus respond better to a high-repetition protocol given its
high percentage of ST fibers? Although this concept is
intriguing in theory, a fiber-type exercise prescription based
on training intensity has not been confirmed through
research and thus remains speculative. Moreover, given the
inter-individual variability of fiber-type composition, it
would be difficult if not impossible to non-invasively
determine fiber-type ratios of each muscle, thus making
application impractical for the vast majority of people.
In conclusion, there is evidence that low-load training
can increase muscle mass in untrained subjects. Therefore,
low-load training to failure appears to be an effective
strategy to increase muscle mass during early-stage train-
ing. This may have particular relevance in populations such
as the elderly and individuals who may not be able to
perform resistance exercise at higher intensities. It remains
questionable, however, as to whether the extent of hyper-
trophy in low-load training is comparable to what can be
achieved through heavy resistance exercise. Based on
recruitment data, it would appear that intensities above
30 % RM are needed to optimize type II fiber activation in
the absence of active BFR. Moreover, research evaluating
the effects of such training in promoting hypertrophic
benefits in experienced lifters is lacking at this time. Future
research should seek to clarify the extent of hypertrophic
effects along the intensity continuum using realistic train-
ing programs, as well as elucidating these effects in indi-
viduals with considerable training experience.
Acknowledgments This review was not funded by any outside
organization. Brad Schoenfeld is the sole author of this work. There
are no conflicts of interest present.
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