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Does exercise-induced muscle damage play a role in skeletal muscle hypertrophy?
Brad Schoenfeld, MSc, CSCS, NSCA-CPT
Department of Health Science
Lecturer in Exercise Science
Lehman College
Bronx, NY 10468
brad@workout911.com
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Abstract
Exercise-induced muscle damage (EIMD) occurs primarily from the performance of
unaccustomed exercise, and its severity is modulated by the type, intensity, and/or duration of
training. Although concentric and isometric actions contribute to EIMD, the greatest damage to
muscle tissue is seen with eccentric exercise, where muscles are forcibly lengthened. Damage
can be specific to just a few macromolecules of tissue or result in large tears in the sarcolemma,
basal lamina, and supportive connective tissue, as well as inducing injury to contractile elements
and the cytoskeleton. Although EIMD can have detrimental short-term effects on markers of
performance and pain, it has been hypothesized that the associated skeletal muscle inflammation
and increased protein turnover are necessary for long-term hypertrophic adaptations. A
theoretical basis for this belief has been proposed, whereby the structural changes associated
with EIMD influence gene expression, resulting in a strengthening of the tissue and thus
protection of the muscle against further injury. Other researchers, however, have questioned this
hypothesis, noting that hypertrophy can occur in the relative absence of muscle damage.
Therefore, the purpose f this paper will be twofold: 1) to extensively review the literature and
attempt to determine what, if any, role EIMD plays in promoting skeletal muscle hypertrophy,
and; 2) to make applicable recommendations for resistance training program design.
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Introduction
Damage to skeletal muscle tissue arising from physical exercise has been well-
documented in the literature (34, 42, 85). This phenomenon, commonly known as exercise-
induced muscle damage (EIMD), occurs primarily from the performance of unaccustomed
exercise, and its severity is modulated by the type, intensity, and/or duration of training (94).
Damage can be specific to just a few macromolecules of tissue or result in large tears in the
sarcolemma, basal lamina, and supportive connective tissue, as well as inducing injury to
contractile elements and the cytoskeleton (160).
EIMD is heightened from the performance of eccentric exercise, where muscles are
forcibly lengthened. Concentric and isometric exercise also contribute to EIMD, albeit to a lesser
extent than eccentric training (32, 51). Fast twitch fibers are more vulnerable to eccentrically-
induced damage than slow twitch fibers (161). Possible reasons include a reduced oxidative
capacity, higher levels of tension generated during training, and/or structural differences between
fiber phenotypes (122).
The damaging effects of eccentric exercise are believed to be related to mechanical
disruption of the actomyosin bonds as opposed to ATP-dependent detachment, which places a
higher degree of stress and strain on the involved structures compared to other muscle actions
(44). Since the weakest sarcomeres are located at different regions of each myofibril, it is
believed that the associated non-uniform lengthening causes a shearing of myofibrils. This
deforms membranes, particularly T-tubules, leading to a disruption of calcium homeostasis that
further degrades muscle by potentiating the release of calcium-activated neutral proteases (such
as calpain) involved in damaging Z-line proteins (5, 14). A dose-response relationship has been
noted, where a greater volume of exercise results in a greater magnitude of damage (112).
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Symptoms of EIMD include a reduced ability to generate muscular force, increased stiffness and
swelling, delayed onset muscle soreness (DOMS), and an increased physiological stress response
characterized by greater lactate production and an elevated heart rate response to submaximal
exercise (147).
EIMD is attenuated with subsequent bouts of the same exercise stimulus (98). This
phenomenon, dubbed the "repeated bout effect," has been attributed to strengthening of
connective tissue, increased efficiency in the recruitment of motor units, greater motor unit
synchronization, a more even distribution of the workload among fibers, and/or a greater
contribution of synergistic muscles (20, 147). Adaptations may last for up to several months,
even if no eccentric training is performed during the interim. Interestingly, the arm muscles
appear to be more predisposed to EIMD than the leg muscles, leading to the supposition that
susceptibility may be mollified in muscles that are used more during activities of daily living
(29).
Although EIMD can have detrimental short-term effects on markers of performance and
pain, it has been hypothesized that the associated skeletal muscle inflammation and increased
protein turnover are necessary for long-term hypertrophic adaptations (45, 163). A theoretical
basis for this belief has been proposed, whereby the structural changes associated with EIMD
influence gene expression, resulting in a strengthening of the tissue and thus protection of the
muscle against further injury (10). Other researchers, however, have questioned this hypothesis,
noting that hypertrophy can occur in the relative absence of muscle damage (20, 47, 87).
Therefore, the purpose of this paper will be twofold: 1) to review the literature and attempt to
determine what, if any, role EIMD plays in promoting skeletal muscle hypertrophy, and; 2) to
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make applicable recommendations as how lifters may utilize this information to optimize a
hypertrophic response.
Potential Mechanisms of Action
There is a large body of evidence indicating the EIMD is associated with factors involved
in the accretion of muscle proteins. Research shows that muscle regeneration and repair
subsequent to damaging exercise is coordinated by novel transcriptional programs involving
clusters of genes that regulate inflammatory processes, growth, stress response, and membrane
biosynthesis (93). The following is an overview of these aspects as they relate to EIMD.
Signaling Via Inflammatory Cells
The response to myodamage has been likened to the acute inflammatory response to
infection (133). Once damage is perceived by the body, neutrophils migrate to the area of trauma
and agents are then released by damaged fibers that attract macrophages to the region of injury
(97). These inflammatory cells consequently secrete other agents to damaged tissue to facilitate
repair and regeneration. Inflammatory processes can have either a beneficial or detrimental effect
on muscle function depending on the magnitude of the response, previous exposure to the
applied stimulus, and injury-specific interactions between the muscle and inflammatory cells
(151).
Neutrophils, often referred to as phagocytic cells, are the most abundant type of white
blood cells in the human body. In addition to possessing phagocytic capabilities, neutrophils also
secrete proteases that assist in degrading cellular debris produced by EIMD as well as releasing
cytolytic and cytotoxic molecules that can increase damage to injured muscle and cause damage
to healthy neighboring tissues (151). Thus, their primary role appears to be restricted to myolysis
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and other aspects involved in the removal of cellular debris rather than promoting hypertrophic
supercompensation of muscle tissue.
Although there is a lack of current evidence supporting a role for neutrophils in muscle
growth, it is conceivable that they may be responsible for signaling other inflammatory cells
necessary for muscle regeneration. One such possibility are reactive oxygen species (ROS)
(158), which can function as key cellular signaling molecules in the response to exercise (54, 77,
78, 148). Neutrophils are capable of producing a variety of ROS including superoxide, hydrogen
peroxide, hypochlorous acid, and hydroxyl radical (80). ROS have been shown to promote
growth in both smooth muscle and cardiac muscle (143), and it is theorized to have similar
hypertrophic effects on skeletal muscle (144). Transgenic mice with suppressed levels of
selenoproteins, a class of proteins that function as potent antioxidants, display increased
exercise-induced muscle growth, potentially indicating an ROS-mediated hypertrophic effect
through redox sensitive signaling pathways (69).
Hypertrophic effects associated with ROS may be carried out through heightened MAPK
signaling. In vitro analysis has shown that treatment of C2 myoblasts with an ROS variant
increases MAPK activation, with the response of the various MAPK subfamilies (ERK 1/2, JNK,
and p38-MAPK) differing over time (79). Eccentric actions have, in fact, been shown to increase
MAPK activation to a greater extent than concentric or isometric actions (93, 95), suggesting a
possible contribution from ROS activity. ROS also may modulate protein synthesis via enhanced
IGF-1 signaling. In vitro analysis by Handayaningsih et al., (62) displayed that IGF-I-induced
phosphorylation of the IGF-I receptor in C2C12 myocytes treated with ROS while this action
was markedly blunted with treatment of antioxidants, suggesting that ROS have a critical
function in the biological action of IGF-I.
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On the other hand, ROS have been found to have a negative effect on various
serine/threonine phosphatases, including calcineurin. ROS activity can interfere with calcineurin
activation by blocking its calmodulin-binding domain (26). Calcineurin is purported to play a
role in both muscle hypertrophy (41, 100) and fiber phenotype transformation (117), and thus its
inhibition may have a negative impact on muscle growth. Furthermore, some researchers have
failed to show that ROS are involved in muscle damage pursuant to resistance training (132).
Ultimately, the effects of ROS on muscle development may be dependent on the mode of
exercise (i.e. anaerobic versus aerobic), the species of ROS produced, and perhaps other factors.
Further investigation is needed to elucidate their precise role in muscle development.
As opposed to neutrophils, there is a large body of evidence that suggests a role for
macrophages in muscle regeneration and growth following EIMD (151), and some researchers
have hypothesized that they are required for compensatory hypertrophy (80). Macrophages
appear to mediate hypertrophy through the secretion of various anabolic agents, with cytokines
emerging as a particularly important player in this process. Research suggests that cytokines
synthesized within skeletal muscle (a.k.a. myokines) significantly contribute to the hypertrophic
response (111, 123, 137). Numerous myokines have been identified in the literature including
interleukin (IL)-6, IL-7, IL-8, IL-10, IL-13, IL-15, fibroblast growth factor (FGF), leukemia
inhibitory factor (LIH), and tumour necrosis factor (TNF), amongst others. Each of these agents
exerts its effects in an autocrine/paracrine fashion to bring about unique effects on skeletal
muscle adaptation, and intense exercise appears to potentiate their response.
Early evidence showed that the production of myokines was related to myodamage (21,
120). This seems logical given that cytokines are known to be involved in the response to
inflammation manifesting from EIMD. However, the results of subsequent research suggest that
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myokine production may be largely independent of damage to muscle tissue. In a study of 20
young and elderly participants, Toft et al., (153) found that 60 minutes of eccentric cycle
ergometry produced only modest increases in IL-6 relative to increases in creatine kinase (CK)
levels, indicating a only a weak correlation between muscle damage and IL-6 production. Other
studies have shown that the time course of CK and IL-6 are not well correlated (36), leading to
speculation that the release of this myokine is primarily related to contraction of muscle fibers,
perhaps as a means to mobilize substrate from fuel depots so that glucose homeostasis is
maintained during intense exercise (46).
It should be noted, however, that only IL-6 and IL-8 appear to be released from muscle in
the absence of damaging exercise (28). Conversely, systemic IL-15 levels and IL-15 mRNA in
skeletal muscle are substantially increased following eccentric resistance exercise, not concentric
exercise, and these elevations are believed to be dependent on the manifestation of tissue damage
(22, 124). This is important because IL-15 has been shown to be a potent mediator of muscle
mass, acting directly on differentiated myotubes to increase muscle protein synthesis and reduce
protein degradation (111, 123). Furthermore, studies show that eccentric exercise preferentially
upregulates FGF activity. FGFs are powerful proliferative agents that are involved in inducing
myofiber hypertrophy as well as regulating satellite cell function (164, 165). FGF has been
shown to be released directly from damaged fibers (30) and the time-course of its release
parallels an increase in CK levels associated with EIMD (31). These findings seem to suggest
that myodamage does in fact contribute to the growth process.
Satellite Cell Activity
Another possible means by which EIMD may augment muscle hypertrophy is via an
increase in satellite cell activity. Satellite cells, which reside between the basal lamina and
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sarcolemma of muscle fibers, are believed to play a critical role in the regulation of muscular
growth (65, 127). In a cluster analysis following a 16-week resistance training protocol, Petrella
et al. (121) demonstrated that subjects who experienced extreme increases in mean myofiber
cross sectional area of the vastus lateralis (>50%) possessed a much greater ability to expand the
satellite cell pool compared to those who experienced moderate or negligible increases in
hypertrophy. These results are consistent with studies showing that muscle hypertrophy is
significantly limited when satellite cells are obliterated by gamma-irradiation (127, 159).
Satellite cells remain quiescent until being aroused by a mechanical stimulus imposed on
skeletal muscle (160). Once activated, they generate precursor cells (myoblasts) that proliferate
and ultimately fuse to existing cells, providing agents needed for repair and subsequent growth
of new muscle tissue (154, 168). This may involve the co-expression of various myogenic
regulatory factors such as Myf5, MyoD, myogenin, and MRF4 (35), which bind to sequence
specific DNA elements present in the promoter of muscle genes, aiding in the hypertrophic
process (130, 139).
Satellite cells also help to retain the mitotic capability of muscles by donating their nuclei
to existing muscle fibers, thereby increasing the capacity to synthesize new contractile proteins
(12, 105). Since a muscle’s nuclear-content-to-fiber-mass ratio remains constant during
hypertrophy, the satellite cell-derived addition of new myonuclei is believed to be essential for
realizing long-term increases in muscle mass (152). This is consistent with the concept of
myonuclear domain, which proposes that the myonucleus regulates mRNA production for a
finite sarcoplasmic volume and any increases in fiber size must be accompanied by a
proportional increase in myonuclei (121).
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The interplay between muscle damage and satellite cell activity has been well established
in the literature (39, 129, 134). Damaged fibers must quickly obtain additional myonuclei to
facilitate reparation; if not, they would face cell death and thus compromise the body's functional
ability. Thus, when exercised fibers sustain damage, satellite cells proliferate and adhere to the
damaged fiber as a means to initiate repair. Given that area under the myoneural junction
contains a high population of satellite cells (67, 139), it is theorized that neurons innervating
damaged fibers might further help to stimulate satellite cell activity, enhancing the regenerative
response (160). Some researchers have proposed that a threshold of myodamage may exist
beyond which stimulated satellite cells fuse to each other to form new myofibers (11), although
this remains speculative.
The initial signal for damage-induced satellite cell activation is believed to originate from
muscle-derived nitric oxide, possibly in conjunction with hepatocyte growth factor secretion (3,
146, 151). The process is thought to be regulated, at least in part, by the cyclooxygenase (COX)-
2 pathway, which has been deemed as necessary to achieve maximal skeletal muscle hypertrophy
in response to functional overload (140). COX-2 exerts its effects by mediating the synthesis of
various prostaglandins known to stimulate satellite cell proliferation, differentiation, and fusion
(17). The inflammatory response to EIMD is believed to play a crucial role in this regenerative
response, as myogenesis has been shown to be enhanced when inflammatory cells are abundant
and impaired in their absence (17). The hypertrophic importance of exercise-induced
inflammation is further supported by research showing that non-steroidal anti-inflammatory
drugs (NSAIDs), which inhibit COX-2 production, blunt the satellite cell response (9). Most (17,
18, 92, 101), but not all (119), studies have found significant decreases in satellite cell activity
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when NSAIDs were administered in response to muscle damage, thereby potentially limiting
long-term load-induced increases in muscle hypertrophy.
It should be noted that mechanical stimuli alone can initiate satellite cell proliferation and
differentiation without subsequent damage to muscle tissue (110, 162). Therefore, it remains to
be determined whether the effects of EIMD are additive or redundant in terms of maximizing
one’s hypertrophic potential.
IGF-1 Signaling
Insulin-like growth factor (IGF-1) is an anabolic hormone that, as the name implies, has
similar structural properties to insulin. A clear cause and effect relationship has been established
between IGF-1 and skeletal muscle hypertrophy, with both mitogenic and anabolic effects seen
in muscle tissue (60). Although IGF-1 plays a general role in anabolism under normal
physiological conditions, its anabolic effects on muscle are thought to be enhanced in response to
mechanical loading (19, 61). Some consider it to be the major extracellular mediator of skeletal
muscle growth (131), and there is evidence that it may be necessary for compensatory
hypertrophy (60).
Several IGF-1 isoforms have been identified including the systemic forms IGF-1Ea and
IGF-1Eb, and the muscle-specific IGF-1Ec, which has been termed mechano growth factor
(MGF) and is believed to be the isoform principally responsible for compensatory hypertrophy
(115). These isoforms are involved in the transduction of multiple anabolic signaling pathways.
For one, IGF-1 is involved in mitogen-activated protein kinase (MAPK) signaling, particularly
via the extracellular signal-regulated kinase (ERK) cascade. Haddad and Adams (60)
demonstrated that co-infusion of a MAPK/ERK inhibitor prevented an increase in IGF-1-
mediated protein synthesis, apparently via reduced S6K1 phosphorylation. These results
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highlight the importance of the IGF-1/MAPK/ERK signaling cascade in the regulation of muscle
growth. As discussed, MAPK signaling has been shown to be highly active during eccentric
exercise. Whether this is mediated by the IGF-1 system is not entirely clear at this time.
IGF-1 also has been implicated in various Ca2+ -dependent pathways shown to stimulate
L-type calcium channel gene expression, leading to an increased intracellular Ca2+ concentration
(106). This in turn instigates the activation of calcineurin, a Ca2+-sensitive phosphatase that plays
an important regulatory role in muscle adaptation via the expression of various downstream
signaling targets including nuclear factor of activated T-cells and GATA-2 (136, 150). Levels of
the striated muscle activator of Rhos signaling (STARS) gene, a muscle specific transducer for
intracellular signaling sensitive to calcineurin, are increased 10-fold following EIMD suggesting
that STARS may be an important downstream target in the early signaling for skeletal muscle
remodeling (93). The precise effects of calcineurin in muscle development remains in question.
Although some studies suggest that calcineurin has a significant effect on muscular growth (41,
100), others indicate its primary role is in the regulation of fiber phenotype, i.e. type I, IIA, or
IIX (117). Recent research suggests that calcineurin may have a selective hypertrophic effect on
slow-twitch muscle fibers (13, 131), raising uncertainty over its role in regeneration following
EIMD.
In addition, IGF-1 is known to influence the mTOR pathway. This is important because
mTOR is widely considered a master network for controlling skeletal muscle growth (16, 55, 76,
149). IGF-1 mediates mTOR activity via phosphatidylinositol 3-kinase (PI3K)/Akt, an upstream
molecular nodal point that both facilitates anabolic signaling and inhibits catabolic signals (107,
154). The binding of IGF-1 to its receptor triggers the activation of PI3K, leading to the
phosphorylation of Akt. Akt, in turn, signals mTOR, which then exerts effects on various
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downstream targets including p70S6k to promote muscle protein synthesis through increases in
translation initiation and elongation (16). It should be noted that recent research suggests that
resistance exercise can activate mTOR independently of PI3K/Akt, presumably under the control
of phosphatidic acid and/or the ERK/TSC2 pathway (70, 71, 103, 114). With respect to EIMD,
some studies have found that phosphorylation of Akt is significantly greater in eccentric
contractions compared to isometric contractions (128) while others show it remains unaffected
regardless of contraction type (43). It therefore remains unclear what, if any, role this pathway
has in regeneration following EIMD.
Another means by which IGF-1 promotes anabolism is by increasing the protein synthetic
rate in differentiated myofibers (12, 61). Locally expressed MGF has been shown to activate
satellite cells and mediate their proliferation and differentiation (68, 166). Systemically
produced IGF-IEa, on the other hand, has been shown to upregulate fusion of satellite cells with
existing muscle fibers, facilitating the donation of myonuclei and helping to maintain optimal
DNA-to-protein ratios in muscle tissue (154, 159).
A number of studies have reported that EIMD potentiates IGF-1 production and thereby
enhances the hypertrophic response to exercise. For example, Bamman et al., (8) showed that
eccentric exercise increased IGF-1 mRNA concentrations by 62% while decreasing levels of
IGFBP-4 mRNA--a strong inhibitor of IGF-1--by 57%. Concentric exercise produced negligible
changes in these markers, indicating that structural damage to the muscle was responsible for
activation of the IGF-1 system.
McCay et al., (99) studied the in vivo response of all 3 IGF-1 isoforms to a protocol
specifically designed to bring about damage in human skeletal muscle. Eight healthy males
performed a series of 300 lengthening contractions of the knee extensors. MGF mRNA increased
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significantly 24 h after the intervention, while the expression of both IGF-1Ea and IGF-1Eb
mRNA were not elevated until 72 hours post-intervention. Because MGF expression was found
to occur earlier than other IGF-1 splice variants, researchers speculated that this isoform may
have a distinct role in the reparation process subsequent to EIMD.
Not all studies have found an association between IGF-1 production and EIMD, however.
Garma et al., (50) evaluated the acute effects of eccentric, concentric, and isometric actions on
anabolic signaling in rodents. The protocol was designed so that volume of accumulated force
was equated between exercise conditions. Nearly identical results were found in the anabolic
response of the three modes of contraction, including no differences in IGF-1 mRNA levels. The
reason for discrepancies between studies is not readily apparent and requires further
investigation.
Cell Swelling
A novel theory by which EIMD may contribute to muscle hypertrophy is via an increase
in intracellular water content. This phenomenon, known as cell swelling, serves as a
physiological regulator of cell function (64), stimulating anabolic processes both by increasing
protein synthesis and decreasing protein breakdown (57, 102, 142). It has been proposed that
increased pressure against the cytoskeleton and/or cell membrane is perceived as a threat to
cellular integrity, which in turn causes the cell to initiate a signaling response that ultimately
leads to reinforcement of its ultrastructure (86, 133).
The exact mechanisms by which cellular swelling promotes anabolism have not been
fully elucidated. There is evidence that integrin-associated volume sensors located within cells
are involved in the process (88). When the membrane is subjected to swelling-induced stretch,
these sensors activate anabolic protein-kinase transduction pathways in muscle, possibly
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mediated by autocrine effects of growth factors (30). This may also have a direct effect on amino
acid transport systems. PI3K appears to be an important signaling component in modulating
glutamine and methylaminoisobutyric acid transport in muscle due to increased cellular
hydration (88). In addition, the stimulus associated with cell swelling may trigger proliferation of
satellite cells and facilitate their fusion to hypertrophying myofibers (37).
The inflammatory response following EIMD is characterized by an accumulation of fluid
and plasma proteins within the affected tissue to an extent whereby this buildup exceeds the
capacity of lymphatic drainage resulting in swelling (59, 97, 122). Associated damage to
capillaries may augment the degree of edema (34). Howell et al., (73) found that an acute bout of
eccentric exercise for the elbow flexors in untrained subjects led to a swelling-induced increase
in arm circumference of as much as 9%, with values remaining elevated for as long as 9 days.
Using a similar protocol, Nosaka and Clarkson (113) reported up to a 4.3 cm increase in arm
circumference attributed to edema, with swelling conspicuous in all subjects by 3 days post-
exercise. Although these effects are attenuated with regimented exercise via the repeated bout
effect, significant swelling nevertheless persists even in trained subjects with increases in muscle
girth seen 48 hours post-workout (72).
To date, no study has directly evaluated the effects of EIMD-mediated cell swelling on
muscle growth. NSAID administration, which reduces the inflammatory response and thus
diminishes cell swelling, has been shown in several studies to impair the increase in muscle
protein synthesis normally associated with resistance exercise (116, 125, 155). It is possible that
these negative hypertrophic effects may be attributable to a reduction in cellular hydration.
However, a cause-effect relationship between inflammatory-derived cell swelling and protein
accretion cannot be established based on this data, and results conceivably could be related to
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confounding factors such as reduced satellite cell and/or macrophage activity. Further study is
needed to investigate this theory.
Research Examining the Relationship Between EIMD and Hypertrophy
Despite the apparently sound theoretical rationale, a direct cause-effect relationship has
yet to be established between EIMD and increased muscle growth. The following section will
discuss what is known from the literature based on studies evaluating the hypertrophic effects of
protocols designed to induce myodamage.
Direct Studies
To date, few experimental studies have directly attempted to determine if a causal
relationship exists between EIMD and muscle hypertrophy. Komulainen et al., (81) investigated
the hypothesis that severe exercise-induced damage results in greater muscle hypertrophy
compared to minor damage. Wistar rats were subjected to repeated shortening (S) or lengthening
(L) contractions of the tibialis anterior muscles. After 8 days post-exercise, the rats in the L
group displayed a 7.1-fold increase in muscle damage (measured by beta-glucuronidase activity)
compared to a 2.6-fold increase in the S group. At the conclusion of the study, increases in
muscle cross sectional area were similar between groups. These results appear to indicate that a
threshold exists beyond which damage does not have any further effect on hypertrophy. A
drawback to the study is that varying degrees of damage were not studied, making it impossible
to know whether a dose-response relationship may exist at more moderate levels of EIMD.
Moreover, these results are only applicable to early stage training adaptations, as the repeated
bout effect would preclude excessive muscle damage in well-trained individuals.
Recently, Flann et al., (47) sought to determine whether muscle damage enhanced muscle
hypertrophy in untrained human subjects. Participants were divided into two groups: 1) a naïve
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group (NA) performed eccentric exercise on an ergometer at a "somewhat hard" level (as
determined by a rating of perceived exertion scale). Training was carried out 3 times a week for
20 minutes over an 8 week period, and ; 2) a pre-trained group (PT) performed the identical
protocol to NA, except training for these subjects included a "ramp up" period lasting 3 weeks
that employed low intensity exercise to gradually acclimate their muscles to the protocol. At the
end of the study period, no statistically significant differences in muscle girth were noted
between groups.
Although these results are intriguing, several methodological limitations make it difficult
to draw relevant conclusions on the topic. First, the PT group trained for a total of 11 weeks
compared to only 8 weeks for the NA group. The authors stated that this issue was addressed by
adjusting workload so that subjects in both groups performed the same cumulative training over
the course of the study. The hypertrophic effects of packing more exercise into a shorter time
period in untrained subjects is not clear, however, and it is difficult to assess whether this may
have impacted results.
Second, both groups performed eccentric exercise throughout the protocol making it
likely that the PT group actually experienced some degree of muscle damage from the training.
CK analysis did in fact show evidence of EIMD in the PT group, although it was significantly
lower than that experienced by those in the NA group. Thus, it may be that the amount of
damage sustained by the PT group was sufficient for eliciting a maximal hypertrophic response
or perhaps that the damage sustained by the NA group was excessive and ultimately attenuated
gains by reducing their ability to train with sufficient intensity and/or delaying
supercompensatory adaptations.
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Third, the study was underpowered, thereby raising the possibility of a Type II error. A
hypertrophic advantage was actually noted for the NA group compared to the PT group (7.5%
versus 6.5%, respectively), but the results did not reach statistical significance. Whether these
results would have been significant with an adequately powered study is not clear. The
possibility also exists that had the study lasted longer, the difference may have become
statistically significant.
Finally, participants had no prior training experience, limiting generalizability to
untrained individuals. It is possible that the stimulus for hypertrophy is different between trained
and untrained individuals, and that adaptive responses related to EIMD may become more
important as one gains training experience. Further research is warranted to address these
limitations and provide more definitive evidence as to whether EIMD contributes to muscle
hypertrophy in various populations.
Indirect Studies
A recent meta-analysis indicates that eccentric exercise is superior for inducing gains
muscle mass compared to concentric exercise. Roig et al., (126) evaluated 3 studies
encompassing 73 subjects and found that eccentric exercise produced statistically significant
greater increases in muscle girth compared to concentric exercise. Moreover, the researchers
noted that 2 of the 3 studies not included in the analysis underscored the superiority of eccentric
exercise in maximizing hypertrophy. In support of these findings, it has been proposed that
optimal exercise-induced muscle growth is not attained unless eccentric muscle actions are
performed (63).
That said, several studies have failed to show any hypertrophic benefit associated with
eccentric actions (4, 104) and there is even some research indicating that concentric training may
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in fact promote greater muscle growth (96). These inconsistencies may be due at least in part to
methodological differences between studies, and the possibility remains that the hypertrophic
advantage of eccentric exercise is nullified when protocols equate for volume-load. When the
body of literature is considered, however, eccentric resistance training does appear to elicit
greater hypertrophic gains compared to other muscle actions.
There is evidence that eccentrically-induced hypertrophy may include sarcomerogenesis,
as several studies have reported a serial increase in sarcomeres associated with lengthening
actions. Lynn et al., (90) showed that rats exposed to downhill running had an increase in
sarcomere count compared to a group that ran uphill. The downhill group displayed a smaller
shift in the optimal torque angle, potentially indicative of reduced muscle damage. Similar
findings have been noted by other researchers in both rodent and human subjects (24, 89, 167).
These results are believed to be a protective response whereby the average sarcomere length is
decreased at a given muscle length so that less of the muscle's working range includes the region
of instability during future eccentric actions (122). Research seems to indicate, however, that
sarcomerogenesis is limited to the first few weeks of exercise and ceases to occur thereafter (15,
138).
Nosaka et al., (112) proposed that EIMD promotes muscle hyperplasia rather than muscle
hypertrophy. Although this is an intriguing hypothesis, studies reporting evidence of hyperplasia
in animal models have been called into question, with results attributed to a miscounting of the
intricate arrangements of elongating fibers as a greater fiber number (118). Evidence that
hyperplasia occurs in human subjects is lacking at this time (2, 91). Further research is needed to
determine if damage to tissue can, in fact, promote splitting of myofibers and thus an increase in
the number of fibers within a muscle.
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Is There a Causal Link Between EIMD, Eccentric Exercise, and Hypertrophy?
Given that eccentric exercise causes EIMD, a logical assumption is that the damage to
tissue may be responsible for its additional growth promoting effects. Although this presents an
enticing hypothesis, the underlying logic has been challenged by a number of researchers. The
following section will detail potential issues with the EIMD hypothesis and discuss proposed
alternative hypotheses that attempt to explain the hypertrophic advantages of eccentric exercise.
Challenges to the EIMD Hypothesis
Some researchers have questioned the validity of the EIMD hypothesis based on the fact
that muscles become increasingly less susceptible to damage with repeated exercise (112). As
discussed, a repeated bout effect has been consistently reported whereby EIMD is attenuated
after a single bout of exercise and becomes increasingly less prevalent thereafter. This
observation would suggest that damage does not contribute to training-induced muscle
hypertrophy. A problem with this theory, however, is that EIMD persists even in well-trained
subjects, albeit to a lesser degree than in novice trainees. Gibala et al., (52) studied the response
of 6 strength trained males to the performance of 8 sets of 8 repetitions at a load equivalent to
80% of their concentric 1-repetition maximum. Training was carried out unilaterally so that one
arm performed only concentric actions while the other arm performed only eccentric actions.
Muscle biopsy obtained 21 hours post-exercise revealed a significantly greater fiber disruption in
the eccentrically-trained arm compared to the concentrically-trained armed. These results
highlight the fact that the repeated bout effect only attenuates the extent of myodamage rather
than preventing its occurrence. Thus, the possibility that damage may in fact play a role in the
hypertrophic response cannot be ruled out based solely on the repeated bout effect.
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Another claim made to refute a potential role for EIMD in muscle hypertrophy is that
low-intensity occlusion training produces marked hypertrophy with apparently minimal damage
to muscle (1, 144). This method of exercise, called Kaatsu, employs light loads (20 to 50% of
1RM) using a pressure cuff to create a hypoxic environment. After several months of training,
hypertrophic gains have been reported to be approximately equal to those seen with traditional
"hypertrophy" loads of 70 to 80% 1RM. Given that the very light loading employed in Kaatsu
training would seemingly minimize disruption of myofibers, this would suggest the lack of a
cause-effect relationship. It should be noted, however, that muscle damage is a known
consequence of reperfusion subsequent to ischemia (48, 58). Takarada et al., (144) found that
while markers of muscle damage were indeed lower following Kaatsu training, there
nevertheless was evidence of fine microdamage within muscle tissue. Thus, the potential exists
that damage may have contributed to results. Perhaps more importantly, it is not clear whether
hypertrophy would have been more pronounced had an even greater amount of EIMD been
present in the Kaatsu group. Future protocols should seek to address these issues.
Finally, some researchers question whether a correlation exists between EIMD and
hypertrophy based on findings that aerobic exercise such as downhill running can cause
significant damage to muscle tissue without corresponding growth (20). This observation,
however, fails to account for the fact that aerobic exercise and resistance exercise elicit unique
molecular responses, activating and/or suppressing a distinctly different subset of genes and
cellular signaling pathways (66). Atherton et al., (6) found that the body has an "AMPK-PKB
switch" whereby signaling is switched to either a catabolic AMPK/PGC-1α- or anabolic PKB-
TSC2-mTOR-dominated state depending on whether the imposed demand is endurance- or
resistance-based. Specifically, aerobic training is associated with activation of AMPK and its
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various downstream targets including PGC-1α, which have been shown to mediate mitochondrial
biogenesis and progression toward a slower muscle phenotype while impairing contractile
protein synthesis (7, 141). The catabolic effects of this signaling cascade remain well into the
post-exercise period, suppressing phosphorylation of PKB or other downstream targets
associated with protein synthesis (38, 53, 157). Resistance training, on the other hand, promotes
anabolism through a variety of signaling pathways. Although AMPK is activated during the
performance of resistance exercise, these effects are reversed immediately post-workout, with
signaling rapidly switched to bring about a highly anabolic state (40). Thus, one can infer that
muscle damage by itself is not sufficient to override catabolic signaling and, if EIMD does in
fact play a role in hypertrophic remodeling, it can only do so in the presence of resistance-based
mechanical overload.
Interestingly, the damage induced by aerobic exercise manifests differently from that of
anaerobic training. CK activity has been shown to peak approximately 12-24 hours post-exercise
following downhill running while that associated with resistance training does not manifest until
after about 48 hours and can peak 4 to 6 days post-exercise (135). Moreover, peak CK levels
associated with downhill running range from 100 to 600 IU while resistance training produces
levels ranging from 2000 to 10,000 IU (33). The implications of these discrepancies have yet to
be elucidated. It also should be noted that CK levels do not necessarily reflect the degree or time
course of muscle damage (34)
Alternative Theories
If muscle damage is not responsible for the increased hypertrophy associated with
eccentric resistance exercise, the question then arises as to what mechanisms account for these
effects. A common hypothesis posits that results may be a function of a selective recruitment of
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fast-twitch muscle fibers, whereby a greater amount of tension is assumed by a reduced number
of fibers. Several EMG studies have indicated a reversal of the Henneman’s size principle of
recruitment during eccentric training, whereby the larger fast-twitch fibers are preferentially
accessed to carry out exercise performance (74, 108, 109). This seems to be consistent with
research showing that lengthening actions increase p70S6k in fast twitch fibers but not in slow
twitch fibers (145). Given that fast-twitch fibers have a significantly greater potential for growth
(82, 156), it is feasible that this could potentially account for the greater protein accretion seen in
eccentric protocols. Other studies, however, seem to refute whether a reversal of the size
principle actually does occur. An extensive review of literature by Chalmers (27) concluded that
the preponderance of evidence does not support selective recruitment of fast-twitch fibers during
eccentric contractions. These results held constant in 9 out of 10 studies deemed suitable to
address the topic and were applicable over a wide range of efforts and speeds.
Another alternative hypothesis proposes that hypertrophic benefits associated with
eccentric exercise may be due to a greater imposed mechanical stress compared to concentric or
isometric actions (112). Indeed, muscles are capable of generating greater absolute force when
contracting eccentrically versus concentrically (126). Despite this fact, however, muscle
activation during maximal eccentric actions is generally less compared to those performed
concentrically. This paradox was demonstrated by Grabiner et al., (56), who found that the
maximum EMG of the vastus lateralis during eccentric knee extension was only 84 ± 41% of that
obtained concentrically. Hence, while the potential to exert peak force is greater with eccentric
exercise, most find it extremely difficult to achieve the maximum force during eccentric actions,
ultimately resulting in an incomplete activation of the spectrum of motor neurons for a given
working muscle (44).
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Perhaps more importantly, the use of absolute maximal loads is not necessarily
paramount for optimal muscle growth. Although mechanical force appears to be the primary
stimulus for eliciting hypertrophic gains, there is evidence that a threshold may exist beyond
which other factors predominate (133). Hypertrophy-oriented routines traditionally employ
submaximal intensities, with loads in the range of 65 to 85% 1RM (25, 75, 83). Fry et al., (49)
determined that the optimum load for muscle growth was in the upper range of these values--a
figure still well below concentric maximum. Further support for this stance can be found in
studies showing that protocols using submaximal loads produce increases in anabolic signaling
and protein synthesis greater than or equal to protocols that employ high-intensity loads of
>90% 1RM (23, 75). Thus, the theory that the hypertrophic superiority of eccentric actions is
solely a function of higher force output remains speculative at this time.
Practical Applications
There is a sound theoretical rationale supporting a potential role for EIMD in the
hypertrophic response. While it appears that muscle growth can occur in the relative absence of
muscle damage, potential mechanisms exist whereby EIMD may enhance the accretion of
muscle proteins including the release of inflammatory agents, activation of satellite cells, and
upregulation of the IGF-1 system, or at least set in motion the signaling pathways that lead to
hypertrophy. Although research suggests that eccentric exercise has greater hypertophic effects
compared to other types of actions, however, a cause-effect relationship directly linking these
gains to EIMD has yet to be established. Moreover, if such a relationship does in fact exist, it is
not clear what extent of damage is optimal for inducing maximum muscle growth.
Evidence does seem to show that a threshold exists beyond which damage does not
further augment muscle remodeling and may in fact interfere with the process. Given that a high
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degree of EIMD causes a reduction in the force-producing ability of the affected muscle,
excessive damage can impair an individual's ability to train, which necessarily would have a
detrimental effect on muscle growth. Moreover, while training in the early recovery phase of
EIMD does not seem to exacerbate muscle damage, it may interfere with the recovery process
(84, 112). Thus, current research indicates that a protocol that elicits a moderate amount of
damage would be most appropriate for maximizing the hypertrophic response.
Future research should seek to clarify whether a causal relationship exists between EIMD
and muscle hypertrophy and, if so, evaluate optimal levels of damage to maximize the response.
Furthermore, the vast majority of studies have been carried out on untrained subjects.
Considering that a ceiling effect slows the rate of muscle growth as one gains training
experience, it is possible that myodamage may become an increasingly important mediator of
hypertrophy in highly trained individuals. Elucidating these issues will help to increase our
understanding of the mechanisms of muscle development and allow for the optimization
hypertrophy-oriented training programs.
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