The Muscle Pump: Potential Mechanisms and Applications for Enhancing Hypertrophic Adaptations

Abstract

Cellular swelling, often referred to as "the pump," has been shown to mediate increases in muscle protein synthesis and decreased protein degradation. This paper will explore the potential hypertrophic benefits associated with the pump and discuss practical implications for resistance training program design.

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The Muscle Pump:
Potential Mechanisms
and Applications for
Enhancing Hypertrophic
Adaptations
Brad J. Schoenfeld, MSc, CSCS, CSPS, NSCA-CPT
1
and Bret Contreras, MA
2
1
Department of Health Sciences, Program of Exercise Science, City University of New York, Lehman College, New
York, New York; and
2
Department of Sport Performance, Auckland University of Technology, Auckland, New Zealand
ABSTRACT
CELLULAR SWELLING, OFTEN
REFERRED TO AS “THE PUMP,”
HAS BEEN SHOWN TO MEDIATE
INCREASES IN MUSCLE PROTEIN
SYNTHESIS AND DECREASED
PROTEIN DEGRADATION. THIS
PAPER WILL EXPLORE THE
POTENTIAL HYPERTROPHIC BEN-
EFITS ASSOCIATED WITH THE
PUMP AND DISCUSS PRACTICAL
IMPLICATIONS FOR RESISTANCE
TRAINING PROGRAM DESIGN.
INTRODUCTION
R
esistance exercise has been
shown to induce acute altera-
tions of intra- and extracellular
water balance (36), the extent of which
is dependent on the type of exercise
and intensity of training. The model
for these changes in fluid balance has
been described as such: During intense
muscular contractions, the veins taking
blood out of working muscles are com-
pressed, whereas arteries continue to
deliver blood into the working muscles,
thereby creating an increased concen-
tration of intramuscular blood plasma.
This causes plasma to seep out of the
capillaries and into the interstitial
spaces. The buildup of fluid in the
interstitial spaces brings about an extra-
cellular pressure gradient, which trig-
gers a flow of plasma back into the
muscle (i.e., reactive hyperemia) (34).
This enhanced reperfusion results in
a phenomenon commonly referred to
by sports scientists as “cellular swell-
ing” and by bodybuilders as “the
pump,” whereby muscles become en-
gorged with blood. The pump is mag-
nified by resistance exercise that relies
heavily on anaerobic glycolysis, partic-
ularly “bodybuilding-style training”
that involves moderate to higher repe-
titions with limited rest intervals (35).
Such exercise results in a substantial
accumulation of metabolic byproducts
including lactate and inorganic phos-
phate, which in turn function as osmo-
lytes and thereby draw additional fluid
into the cell (8,37).
T he pump is generally thought to be
a temporary phenomenon. Bodybuilders
“pump up” by performing high repetition
sets immediately before a competition in
an effort to make their muscles appear
full and dense while on stage (24). More-
over, there is a heightened sensation of
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1

pleasure associated with the pump,
which has been popularly described by
Arnold Schwarzenegger as a “tight feel-
ing.like somebody is blowing air into
your muscles.it feels fantastic.” (27).
Lifters therefore often will “chase the
pump” in their training regimens, struc-
turing workouts to maximize intracellu-
lar fluid accumulation. Although these
short-term effects of the pump are well
documented, recent research suggests
that the pump may, in fact, mediate
long-term adaptive responses. T his
paper will explore the potential hyper-
trophic benefits associated with the
pump and discuss practical implications
for resistance training program design.
THE ROLE OF HEAVY LOADING IN
MUSCLE HYPERTROPHY
Muscle hypertrophy represents the
dynamic balance between protein syn-
thesis and breakdown. Three primary
factors have been postulated to mediate
hypertrophic adaptations pursuant to
resistance training: mechanical tension,
metabolic stress, and muscle damage
(34). There is compelling evidence that
mechanical tension is the primary impe-
tus for this adaptive response. Goldberg
et al. (10) was the first to report that
heightened force development is the
critical factor governing increases in
muscle hypertrophy. This finding has
since been corroborated in numerous
studies (17,26,38,43,46).
Tension on muscles initiates a phen-
omenon called mechanotransduction
whereby sarcolemmal-bound mecha-
nosensors, such as integrins and focal
adhesions, convert mechanical energy
into chemical signals that mediate var-
ious intracellular anabolic and cata-
bolic pathways in a manner that
shifts muscle protein balance to favor
synthesis over degradation (48). Stud-
ies show that mechanical tension
directly stimulates mammalian target
of rapamycin (mTOR) (16), possibly
through activation of the extracellular
regulated kinase/tuberous sclerosis
complex 2 pathway (26). These actions
are believed to be carried out via syn-
thesis of the lipid second messenger
phosphatidic acid (PA) by phospholi-
pase D (16,30). Research also indicates
that PA can phosphorylate the down-
stream anabolic translational regulator
p70S6 kinase in an mTOR-indepen-
dent fashion (22), presenting yet
another path whereby mechanical
stimuli may directly drive anabolic
processes.
Given the importance of mechanical
tension in promoting anabolism, it is
logical to conclude that training with
heavy loads is an effective means for
increasing muscle growth. The use of
higher intensities places greater tension
on muscles, thus stimulating greater
mechanotransduction. As noted, how-
ever, other factors are purported to play
a role in postexercise muscle protein
accretion. In particular, there is compel-
ling evidence that exercise-induced
metabolic stress can mediate a hypertro-
phic response, and cell swelling is
believed to be an important component
to this process (35).
POTENTIAL HYPERTROPHIC
MECHANISMS OF CELL SWELLING
In simple terms, the pump represents
an increase in intracellular hydration
that causes the muscle fiber to swell.
Research shows that cell swelling acts
as a physiological regulator of cell
function (14,15), stimulating protein
accretion by both increasing protein
synthesis and decreasing protein break-
down (13,25,40). These effects have
been demonstrated in a variety of dif-
ferent cell types including hepatocytes,
osteocytes, breast cells, and muscle
fibers (21). In muscle, fast-twitch (FT)
fibers have been found to be particu-
larly sensitive to osmotic changes,
presumably related to their high con-
centration of water transport channels
called aquaporin-4 (AQP4). AQP4 is
strongly expressed in the sarcolemma
of mammalian FT glycolytic and FT
oxidative-glycolytic fibers, facilitating
the entry of plasma into the cell (8).
Numerous studies show that FT fibers
display a superior potential for growth
as compared with slow-twitch fibers
(1,2,18,39), suggesting that cell swelling
may promote hypertrophy by favor-
ably impacting net protein balance in
these fibers. Indeed, ablation of AQP4
was found to correlate with muscular
atrophy in mice (3), although it is not
clear whether this finding is related to
an inhibition of cell swelling or simply
a reduction in spontaneous physical
activity.
Although the underlying mechanisms
remain to be fully elucidated, it has
been hypothesized that cell swelling-
induced anabolism is a means of cell
survival (Figure). According to theory,
an increased pressure against the cyto-
skeleton and/or cell membrane is per-
ceived as a threat to cellular integrity,
thereby initiating an intracellular sig-
naling response that promotes rein-
forcement of its ultrastructure (20,34).
The signaling response is believed to be
facilitated by integrin-associated vol-
ume osmosensors within muscle fibers
(23). When the membrane is subjected
to swelling-induced stretch, these
sensors initiate activation of anabolic
protein-kinase transduction pathways,
potentially regulated at least in part by
growth factors that exert their influ-
ence in an autocrine/paracrine fashion
Figure. Theoretical schematic for cellular
swelling mechanisms of
action on muscle hypertrophy .
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(4,19). Research suggests that these
functions are carried out in an
mTOR-dependent (9) and/o r inde-
pendent (32) manner, a nd there is evi-
dence that mitoge n-activated protein
kinase pathways may play a role in
associated anabolic signaling (7,3 3).
Hyperhydration also may have a direct
effect on amino acid tran sport sys-
tems. Phosphatidylinositide 3-kinase
appears to be an important signaling
component in modulating glutamine
and methylaminoisobutyric acid trans-
port in muscle because of increased cel-
lular hydration (23).
It has been hypothesized that cellular
swelling may enhance hypertrophic
adaptations through increased satellite
cell activity (6). Satellite cells are muscle
stem cells that reside between the basal
lamina and sarcolemma. While resting,
these precursor cells remain quiescent.
When muscle is subjected to mechani-
cal overload, however, satellite cells
enter the cell cycle and initiate muscular
repair by first undergoing proliferation
and then differentiating into myoblast-
like cells (31). Once differentiated,
myoblasts are then able to fuse to trau-
matized myofibers and donate their
nuclei to increase the cell’s ability to
synthesize new contractile proteins
(47). Studies investigating the myogenic
properties of creatine monohydrate
(CM), an osmolyte, show a positive
impact on satellite cell accretion (29)
and differentiation (44), as well as myo-
genic regulatory factor expression (45).
Dangott et al. (6) proposed that the os-
molytic properties of CM may instigate
proliferation of satellite cells and facili-
tate their fusion to hypertrophying my-
ofibers. At this time, the satellite cell
hypothesis remains speculative, how-
ever, because it is not clear whether
myogenic effects are, in fact, mediated
by cell swelling or simply resultant to
external overload.
PRACTICAL APPLICATIONS
To date, there is a paucity of resistance
training studies directly investigating
the effects of acute cell swelling (i.e.,
the pump) on muscle hypertrophy.
However, basic research provides
a compelling reason to believe that
exercise-induced cell swelling enhances
hypertrophic gains. To achieve a pump,
local muscle activation must be high
enough to occlude venous output;
however, the contractions must be
repeated for sufficient repetitions to
allow for the pooling of blood. Further-
more, muscle tension must remain per-
sistent to prevent blood from escaping
the musculature. For these reasons,
exercise selection and manner of execu-
tion must be chosen wisely to provide
a maximal cell swelling stimulus.
Bodybuilders seeking the pump gener-
ally employ 2 different sets, repetitions,
and timing schemes. The first is the
use of several high repetition sets com-
bined with short rest periods. An
example would be 2–3 sets of ;20 rep-
etitions with 60 seconds of rest in
between sets. The second is the use
of repeated medium repetition sets
combined with short rest periods. An
example would be 5–10 sets of 8–12
repetitions with 30 seconds of rest in
between sets. Both of these strategies
are viable approaches and conceivably
can be used interchangeably to maxi-
mize the pump.
Another option for enhancing the
pump is to perform a drop set, whereby
a high intensity set is immediately fol-
lowed by a lower intensity bout with
the load decreased by ;25–50%. This
training strategy results in significant
metabolite accumulation (12), thereby
enhancing cellular hydration. Goto
et al. (11) showed that a drop set pro-
tocol resulted in a significant increase
in muscle cross sectional area, opposed
to a traditional high-intensity strength
training protocol alone. However, the
study did not control for total training
volume, leaving open the possibility
that the increased muscle protein
accretion was the result of an increased
volume rather than from the effects of
cell swelling.
Exercise selection is an important
aspect of pump training. Some exer-
cises place more constant loading on
the musculature because of their tor-
que-angle curves, whereas others load
up a particular range of motion but
diminish drastically in other ranges.
Because cellular swelling is predicated
on a prolonged venous occlusion,
those exercises that maintain constant
tension would necessarily maximize
the pump. For example, the good
morning requires the greatest muscle
force in the hip extensors at long mus-
cle lengths; the 458 hyperextension at
medium muscle lengths and the hori-
zontal back extension at short muscle
lengths (5). Although the good morn-
ing, therefore, would have the greatest
impact on inducing muscle damage
(28), the lack of tension in the upper
range of movement would diminish
cellular swelling. On the other hand,
the constant muscular tension (i.e.,
mean torque loading throughout the
repetition) associated with the 458
hyperextension heightens vascular
occlusion, thereby resulting in a greater
pump. Traditional single-joint machine
exercises such as the “pec deck,”
“reverse pec deck,” leg extension, and
seated leg curl exercises are generally
good choices for pump training
because of the constant tension they
place on the musculature.
Exercises can also be modified for
a greater pump effect. Exercises that
have diminished loading on a particular
muscle throughout the range of
motion can be altered so that perfor-
mance focuses only on the portion of
the movement that maximally stresses
the muscle is performed. For example,
bottom-half push-ups or dips are a bet-
ter strategy for achieving a pump in
the pectorals than full range push-ups
or dips. Resistance bands and chains
can also be used in concert with the
barbell to accommodate the strength
curve and place more tension that is
constant on the muscle.
Finally, when training for the pump, it
is important to perform exercises in
a continuous manner so that the target
muscles are not allowed to relax. Tani-
moto and Ishii (41) showed a significant
decrease in local muscle oxygenation—
consistent with vascular occlusion—in
the performance of low-intensity knee
extension exercise (50% 1RM) without
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a relaxation phase as compared with
high-intensity (80% 1RM) exercise per-
formed with a 1-second relaxation
between repetitions. The authors
attributed this decrease in muscle oxy-
genation level to the continuous con-
tractions of the knee extensor muscles
in exercise without relaxation. Similar
results were reported in follow-up dur-
ing multijoint lower-body exercise (42),
emphasizing the importance of main-
taining continuous tension on the
working muscles if the goal is to max-
imize cellular swelling.
CONCLUSION
In summary, progressive resistance
training in low-to-medium repetition
ranges has earned its keep in the train-
ing programs of bodybuilders and
other athletes seeking to maximize
hypertrophy, for good reason. Heavy
loads maximize muscle activation,
and progressive overload ensures that
muscles receive increased mechanical
tension over time. Therefore, increas-
ing strength on heavy multijoint
movements should be the foundation
of long-term hypertrophy training.
However, it is likely that exercise cen-
tered on achieving a “pump” through
higher repetition sets combined with
shorter rest periods also provides a
potent hypertrophic stimulus that is
synergistic to heavy compound lifting.
Therefore, individuals seeking maximal
hypertrophy should consider dedicat-
ing a component of their training ses-
sions toward “pump” training, ideally
after heavier strength work, to take
advantage of the multiple pathways
involved in muscle hypertrophy.
Future research should be undertaken
to investigate whether cell swelling,
in fact, leads to increased hypertro-
phy over that of heavy strength train-
ing alone (i.e., whethe r its inclusion
is additive or redundant). Moreover,
future research should determine the
precise mechanisms through which
“pump” training increases hypertrophy
and determine which exercises and
training methods are best suited for
eliciting a pump in the various muscles
of the body. Finally, future research
should dictate the optimal manner in
which heavier strength training and
lighter pump training can be integrated
together to maximize hypertrophic
adaptations.
Conflicts of Interest and Source of Funding:
The authors report no conflicts of interest
and no source of funding.
Brad J. Schoenfeld is a lecturer in the
exercise science program at CUNY’s
Lehman College and director of their
human performance laboratory.
Bret Contreras is currently pursuing
his PhD in Sports Science at the Auck-
land University of Technology in Auck-
land, New Zealand.
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