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New fundamental resistance exercise determinants of molecular and cellular muscle adaptations


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Abstract Physical activity relies on muscular force. In adult skeletal muscle, force results from the contraction of postmitotic, multinucleated myofibres of different contractile and metabolic properties. Myofibres can adapt to (patho-)physiological conditions of altered functional demand by radial growth, longitudinal growth, and regulation of fibre type functional gene modules. The adaptation's specificity depends on the distinct molecular and cellular events triggered by unique combinations of conditional cues. In order to derive effective and tailored exercise prescriptions, it must be determined (1) which mechano-biological condition leads to what molecular/cellular response, and (2) how this molecular/cellular response relates to the structural, contractile, and metabolic adaptation. It follows that a thorough mechano-biological description of the loading condition is imperative. Unfortunately, the definition of (resistance) exercise conditions in the past and present literature is insufficient. It is classically limited to load magnitude, number of repetitions and sets, rest in-between sets, number of interventions/week, and training period. In this review, we show why the current description is insufficient, and identify new determinants of quantitative and/or qualitative effects on skeletal muscle with respect to resistance exercise in healthy, adult humans. These new mandatory determinants comprise the fractional and temporal distribution of the contraction modes per repetition, duration of one repetition, rest in-between repetitions, time under tension, muscular failure, range of motion, recovery time, and anatomical definition. We strongly recommend to standardise the design and description of all future resistance exercise investigations by using the herein proposed set of 13 mechano-biological determinants (classical and new ones).
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Eur J Appl Physiol (2006) 97: 643–663
DOI 10.1007/s00421-006-0238-1
Marco Toigo · Urs Boutellier
New fundamental resistance exercise determinants of molecular
and cellular muscle adaptations
Accepted: 8 May 2006 / Published online: 15 July 2006
© Springer-Verlag 2006
Abstract Physical activity relies on muscular force. In
adult skeletal muscle, force results from the contraction of
postmitotic, multinucleated myoWbres of diVerent con-
tractile and metabolic properties. MyoWbres can adapt to
(patho-)physiological conditions of altered functional
demand by radial growth, longitudinal growth, and regu-
lation of Wbre type functional gene modules. The adapta-
tion’s speciWcity depends on the distinct molecular and
cellular events triggered by unique combinations of condi-
tional cues. In order to derive eVective and tailored exer-
cise prescriptions, it must be determined (1) which
mechano-biological condition leads to what molecular/
cellular response, and (2) how this molecular/cellular
response relates to the structural, contractile, and meta-
bolic adaptation. It follows that a thorough mechano-bio-
logical description of the loading condition is imperative.
Unfortunately, the deWnition of (resistance) exercise con-
ditions in the past and present literature is insuYcient. It is
classically limited to load magnitude, number of repeti-
tions and sets, rest in-between sets, number of interven-
tions/week, and training period. In this review, we show
why the current description is insuYcient, and identify
new determinants of quantitative and/or qualitative eVects
on skeletal muscle with respect to resistance exercise in
healthy, adult humans. These new mandatory determi-
nants comprise the fractional and temporal distribution of
the contraction modes per repetition, duration of one rep-
etition, rest in-between repetitions, time under tension,
muscular failure, range of motion, recovery time, and ana-
tomical deWnition. We strongly recommend to standardise
the design and description of all future resistance exercise
investigations by using the herein proposed set of 13
mechano-biological determinants (classical and new ones).
Keywords Exercise · Skeletal muscle · Cellular
mechanotransduction · Skeletal muscle satellite cells ·
From stimulus to adaptational eVect
Physiological conditions such as resistance exercise per-
turb the skeletal muscle’s tensional integrity (Ingber
2003a, b). These perturbations are mechano-chemically
transduced into a molecular and cellular response within
and between myoWbres and satellite cells (i.e. muscle stem
cells) (Tidball 2005). The mechano-chemical transduction
is based on the genetic background, age, gender, and sev-
eral other factors. Finally, this molecular and cellular
response leads to speciWc structural adaptations that
result in task-speciWc functional enhancement (i.e. the
adaptational eVect) (Fig. 1). However, a causal connection
only exists between the molecular/cellular response (i.e.
signal transduction) and the adaptation (Fig. 1). “More
strength”, i.e. the adaptational eVect, is not necessarily the
result of more muscle mass, since several distinct adapta-
tions can lead to the same eVect (at least in the short
term). Conversely, training in the “6–12-repetition-maxi-
mum zone with multiple sets for 2–3 days week
”, the so-
called “hypertrophy training” (Kraemer and Ratamess
2004), doesn’t necessarily mean that muscle hypertrophy
(i.e. an increase in muscle mass due to the increase in the
size, as opposed to the number, of preexisting skeletal
muscle Wbres) will result. This is due to the fact that the
ability to exercise is distinct from the ability to adapt. It
has recently been shown for a large cohort of men and
women that signiWcant variability in muscle size and
strength gain exists after unilateral resistance exercise of
the elbow Xexors (Hubal et al. 2005). While some subjects
showed little to no gain, other showed profound changes.
Also, sex diVerences were apparent. Men had a slight
advantage in relative size gains compared to women,
whereas women outpaced men considerably in relative
gains in strength. Age diVerences exhibit a profound
M. Toigo (&) · U. Boutellier
Institute of Human Movement Sciences, and Institute
of Physiology, ETH Zurich, and University of Zurich,
Y23 K 12, Winterthurerstrasse 190, 8057 Zurich, Switzerland
Tel.: +41-44-6355062
Fax: +41-44-6356863
impact on the molecular response to resistance exercise,
too (Hameed et al. 2003). Following a single bout of high
load knee extensor resistance exercise, mechanogrowth
factor (MGF) response was attenuated in older subjects.
This is indicative of age-related desensitivity to mechani-
cal loading. Also, various human polymorphisms at
genetic loci have a quantitative eVect on muscle pheno-
type (Thompson et al. 2004). Such polymorphisms, also
known as muscle quantitative trait loci, drive the genetic
variation that underlies muscle size and strength.
Therefore, several prerequisites for the identiWcation
of eVective and speciWc exercise perturbations with
desired functional enhancement are to be met. First,
causal relationships between changes at the molecular
and cellular level and the resulting adaptation need to be
identiWed. These relationships need to be established on
the basis of several factors such as genetics, age, sex, etc.
Further, it must be ascertained whether the adaptation
leads to functional enhancement. Finally, the various
causal connections between signal transduction and
adaptation can be interrogated by multiple exercise
perturbations in order to identify the most eVective and
speciWc exercise perturbations. However, for the identi-
Wcation of eVective and speciWc exercise perturbations it
is imperative to unequivocally deWne and describe the
exercise stimulus in mechano-biological relevant terms.
These mechano-biological terms should be directly asso-
ciated with the molecular response. Unfortunately, in
current investigations into the molecular response to
exercise, perturbation design and deWnition remain
largely undeWned or ignored. This review aims at identi-
fying mechano-biological determinants of exercise
conditions that have quantitative and/or qualitative
eVects on skeletal muscle phenotype. These mechano-
biological determinants are proposed for standardised
description of resistance exercise stimuli in both aca-
demic and recreational settings.
Fundamental mechanical stimuli decoded by skeletal
Basically, muscles can adopt three strategies of quantita-
tive or qualitative eVect on muscular phenotype to adjust
for altered functional demands (Goldspink 1985): (1)
positive or negative longitudinal growth; (2) positive or
negative radial growth and (3) contractile [myosin heavy
chain (MyHC)] and metabolic tuning. These adaptational
strategies can be adopted concurrently or separately,
depending on the speciWcity of the (patho-)physiological
condition. Generally, exercise-induced physiological con-
ditions can be viewed as the perturbations of the muscle
cells’ tensional integrity. Perturbation of tensional integ-
rity occurs by increasing or decreasing myocellular active
and/or passive tension, as well as energy production or
absorption. Additionally, these tensional and energetic
alterations can be sustained for diVerent duration. Thus,
every exercise condition is coded by a speciWc combina-
tion of changes in constant or intermittent active and/or
passive tension of diVerent duration. These temporal
changes in active and/or passive tension, together with
the inferred structural insults, are then decoded at the
molecular level and transduced into an appropriate
Fig. 1 SimpliWed model for the
transduction of exercise-related
skeletal muscle perturbations
into structural adaptations with
associated adaptational eVect.
An exercise stimulus with deW-
ned mechano-biological charac-
teristics (described in this paper)
(a) is mechano-chemically trans-
duced (c) into a quantitative
and/or qualitative adaptation
(d) of the muscle phenotype,
based on the respective response
matrix (b). The adaptation (d) is
associated with the correspond-
ing adaptational eVect (e). A
causal connection exists be-
tween (c) and (d) (red shading)
m = f(G, Y, S, A, M, T, H, N, I,...)
Genotype (G
Age (Y
Gender (S
Muscular architecture (A
Muscular subsystem (M
Muscular anthropometry (T
Hormonal status (H
Nutritional status (N
Immunological status (I
, mechano-biological exercise
stimulus determinants
e = f(x
s = f(e, m, ...)
a = f(s)
z = f(a)
exercise stimulus (e)
signal transduction (s)
adaptation (a)
adaptational effect (z)
response matrix (m)
adaptational response. In the following sections, we
provide a detailed description on how the three funda-
mental adaptations are regulated at the molecular and
cellular levels. From this, we derive the relevant mec-
hano-biological determinants for resistance exercise-
induced muscular conditioning.
Molecular and cellular determinants of longitudinal
muscle growth
Mechanical measurements of rabbit muscle strips before
and after skinning indicate that total passive tension
increases with increasing sarcomere length (second-order
polynomial) (Prado et al. 2005), i.e. with increased strain.
Total passive tension is believed to develop due to the
lengthening of extramyoWbrillar elements (especially the
collagen content in the extracellular matrix) as well as to
the lengthening of titin. Titin is a giant (» 3–3.7 MDa)
sarcomeric protein that contains a series of spring ele-
ments within its I-band region, which contribute to the
elastic properties of myoWbrils (Prado et al. 2005). Exter-
nal or internal forces applied to the myoWbril lengthen or
shorten the myoWbril to above or below the slack length,
respectively. The lengthening or shortening of the myoW-
bril creates a titin force, which is directed to restore rest-
ing length (Miller et al. 2003). MyoWbrils can be
lengthened actively or passively. This means that myoW-
brils can lengthen while contracting (“lengthening con-
traction”, “eccentric contraction”, “active stretch”) or
without concurrent contraction. In contrast, myoWbrils
can only actively shorten, i.e. while contracting (“short-
ening contraction”, “concentric contraction”). Conse-
quently, passive tension in myoWbrils can develop with or
without concurrent lengthening contraction or with
shortening contraction.
Molecular sensing of myoWbrillar strain
In line with titin’s structural and elastic properties,
mounting evidence indicates that myoWbrillar strain
mediates signalling pathways that involve titin’s Z-line
region (Miller et al. 2003). Signalling pathways comprise
(1) the titin-muscle LIM protein (MLP) pathway, (2) the
N2A-muscle ankyrin repeat protein (MARP) pathway,
and (3) the titin-muscle RING Wnger protein (MuRF)
(1) is part of a stretch-dependent myocardial signalling
pathway whose impairment contributes to the patho-
genesis of a subset of dilated cardiomypathies in
humans (Knoll et al. 2002) and is also induced by
skeletal muscle injury due to eccentric exercise
(Barash et al. 2004, 2005; Chen et al. 2002; Hentzen
et al. 2006).
(2) involves 3 homologous MARPs: CARP/MARP1,
Ankrd2/Arpp/MARP2, and DARP/MARP3. MARPs
show cytokine-like induction following cardiac injury,
muscle denervation, and eccentric exercise in vivo
(CARP/MARP1) (Aihara et al. 2000; Barash et al.
2004; Baumeister et al. 1997; Kuo et al.
1999); strain
in culture, immobilisation at stretched muscle length,
and eccentric exercise in vivo (Ankrd2/Arpp/
MARP2) (Barash et al. 2004, 2005; Kemp et al. 2000;
McKoy et al. 2005); or during recovery after meta-
bolic challenge (DARP/MARP3) (Ikeda et al. 2003),
(3) is likely to be involved in the regulation of ubiquitin-
proteasome-mediated myoWbrillar protein degrada-
tion (see “Atrophy signalling”). MuRF1 was found
to bind the C-terminal immunoglobulin domains of
titin (Centner et al. 2001), and in the nucleus MuRF1
can bind the transcription factor, glucocorticoid
modulatory element binding protein-1 (McElhinny
et al. 2002). Furthermore, mechanical tension and the
titin catalytic domain have been shown to regulate
the nuclear localisation of MuRF2 and serum
response factor (SRF) (Lange et al. 2005).
Recently, it has also been demonstrated in an in vivo rat
model that CARP and MLP are sensitive to both muscle
tissue stress and contraction mode, while Ankrd2/Arpp
is sensitive only to contraction mode (Barash et al. 2005).
This raises the possibility that strain can be sensed inde-
pendently of stress.
Further evidence for the molecular transduction of
passive tension comes from the » 1.5- and » 2-fold
increase in the serine/threonine kinase Akt [protein
kinase B (PKB)] activity following 5 and 10–20 min of
passive stretch, respectively, of the fast-twitch rat exten-
sor digitorum longus muscle but not in the slow-twitch
soleus muscle (Sakamoto et al. 2003). Once activated,
Akt phosphorylates an array of substrates, including
proteins that mediate protein synthesis, gene transcrip-
tion, cell proliferation, and survival (Vivanco and Saw-
yers 2002). In mammals, there are three forms of Akt,
(Akt1-3), encoded by distinct genes (Vivanco and Saw-
yers 2002). Expression of a constitutively active form of
Akt1 in skeletal muscle cells, either in vitro (Takahashi
et al. 2002) or in mice and rats (Bodine et al. 2001b; Lai
et al. 2004; Pallafacchina et al. 2002), causes hypertro-
phy. Conversely, Akt1 inhibits atrophy in vitro and in
mice (Sandri et al. 2004; Stitt et al. 2004). Based on their
Wnding that passive stretch has no eVect on Akt activity
in rat slow-twitch soleus muscle (in contrast to rat
extensor digitorum longus muscle), the authors sug-
gested that susceptibility to mechanical stretch is Wbre
type-speciWc (Sakamoto et al. 2003). This notion is also
supported by the recent Wnding that the relative impor-
tance of titin and the extracellular matrix for total pas-
sive tension can vary between diVerent adult rabbit
skeletal muscles (Prado et al. 2005). Slow-twitch rabbit
muscles exhibit low titin-based passive tension but this
tension is highly variable in fast-twitch muscles. Fur-
thermore, titin-based passive tension, but not extramyo-
Wbrillar passive tension correlates with the muscle type
(Prado et al. 2005).
Cellular sensing of mechanical stretch
Satellite cells are muscle precursor cells that lie between
the basal lamina and sarcolemma of skeletal muscle
Wbres (Mauro 1961). In normal adult muscle, satellite
cells are mitotically and metabolically quiescent (Schultz
et al. 1978). With appropriate environmental signals, sat-
ellite cells enter into the cell cycle, i.e. are “activated”, to
provide the precursors needed for new muscle formation
in growth and repair (Charge and Rudnicki 2004; Hill
et al. 2003; McKinnell et al. 2005). Results from in vitro
stretch assays demonstrate that mechanical stretch can
result in satellite cell activation (Anderson 2000; Ander-
son and Pilipowicz 2002; Tatsumi et al. 2001; Wozniak
et al. 2003). This mechanical stretch induces hepatocyte
growth factor (HGF) release from its tethering in the
extracellular matrix in a nitric oxide-dependent manner
(Tatsumi and Allen 2004; Tatsumi et al. 2002). Once
released, HGF binds to the c-met receptor which is
located on the plasma membrane of the satellite cells.
This interaction initiates a cascade of signalling events
that lead to DNA synthesis, and, thus, to satellite cell
Structural adaptation to strain perturbation
It has long been known that muscles adapt to a new
functional length by adding or removing sarcomeres in
series at the ends of the existing myoWbrils (Dix and
Eisenberg 1990; GriYn et al. 1971; Tabary et al. 1972;
Williams and Goldspink 1971). Immobilisation at long
muscle length results in an increase in the number of sar-
comeres in series. Conversely, immobilisation at short
muscle length leads to a decrease in the number of sarco-
meres in series. Furthermore, remodelling of the connec-
tive tissue following immobilisation has been
demonstrated multiple times in mice, rats, rabbits, and
cats (Goldspink 1985; Tabary et al. 1976; Tardieu et al.
1977, 1982; Williams and Goldspink 1984). However,
both the occurrence and the extent of remodelling seem
to depend on the connective tissue type (series elastic ele-
ment and/or parallel elastic element), species, age, muscle
length during immobilisation, and time period of immo-
In exercise physiology, serial sarcomere number mod-
ulation has been a neglected topic so far (Morgan and
Proske 2004). Only recently has serial sarcomere number
modulation been investigated in the context of exercise.
Direct evidence for exercise-induced modulation of serial
sarcomere number has come from treadmill-trained rat
vastus intermedius muscles, the postural knee extensors
(Lynn and Morgan 1994; Lynn et al. 1998). Rats were
trained by running on a climbing or descending treadmill
for approximately 10 min day
for 5 days. The latter
had previously been shown to cause muscle damage.
Subsequently, serial sarcomere analysis for single Wbres
was performed by laser diVraction. As a result, the
descending-trained rats had the largest sarcomere count,
the climbing-trained rats had the smallest count, and
sedentary rats had intermediate counts, although closer
to the climbing group. In another series of experiments,
rat vastus intermedius muscles were tested mechanically
while still in situ, i.e. attached to the bones, but with all
other muscles about the knee joint removed. As a result,
in descending-trained rats, the knee angle for optimum
torque generation corresponded to longer muscle lengths
than in climbing-trained rats. It follows from these
results that eccentric exercise (lengthening contractions)
leads to accretion of serial sarcomeres. Conversely, exer-
cise comprising only shortening contractions leads to a
decrease in the number of serial sarcomeres. Due to the
Wnding that 38% of the diVerence in sarcomere numbers
between decline- and incline-trained rats does not appear
as a diVerence in optimum angle, the authors suggest
that it has been taken up by shortened tendons (Lynn
et al. 1998).
Recently, the contraction type-dependent diVerential
serial sarcomere number adaptation has been conWrmed
by measuring vastus intermedius and vastus lateralis
muscle Wbre dynamics of up- and downhill-running rats
in vivo (ButterWeld et al. 2005). It was shown in that
study that vastus intermedius and vastus lateralis mus-
cles of uphill-walking rats undergo repeated concentric
contractions, and therefore they suVer no contraction-
induced injury. Conversely, the vastus intermedius and
vastus lateralis muscles during downhill walking
undergo repeated eccentric contractions (ButterWeld
et al. 2005). Accordingly, short muscle lengths for uphill
concentric-biased contractions result in a loss of serial
sarcomeres, while long muscle lengths for downhill
eccentric-biased contractions result in a gain of serial
sarcomeres (ButterWeld et al. 2005).
In humans, the optimum angle for torque generation
can be measured reliably, e.g. by isokinetic dynamome-
try. By that means, an angle-torque curve is measured
during maximum voluntary contraction with constant
velocity shortening. As determined by this measure, a
series of eccentric contractions (“hamstring lowers”) of
human hamstring muscles produced a signiWcant shift of
approximately 7° in optimum knee angle for torque gen-
eration to longer muscle lengths (Brockett et al. 2001).
The shift in optimum knee angle for torque generation
was parallelled by delayed-onset muscle soreness
(DOMS) in the hamstrings. The shift occurred immedi-
ately after exercise and persisted 8 days postexercise,
consistent with a training eVect. The mechanism by
which eccentric exercise produces muscle damage,
DOMS, and increased optimum length for torque gener-
ation has been postulated in the “popping sarcomere
hypothesis” (Morgan 1990
). The popping sarcomere
hypothesis states that stretch-induced muscle damage
results from nonuniform lengthening of sarcomeres,
when active muscle is stretched beyond optimum length.
If sarcomeres are beyond optimum length, then the lon-
gest sarcomeres will be the weakest and, so, will be
stretched more rapidly than the others. Thus, they will
become weaker, until rising passive tension compensates
for falling active tension. For at least some muscles this
corresponds to lengths beyond Wlament overlap. The
term “popping” is used to describe the uncontrolled, vir-
tually instantaneous lengthening of a sarcomere from a
length commensurate with its passive length to a length
where passive structures primarily support the tension.
Because the weakest sarcomeres are not at the same
point along each myoWbril, this nonuniform lengthening
leads to a shearing of myoWbrils, exposing membranes,
especially T-tubules, to large deformations. This is
thought to lead to loss of Ca
homeostasis, and, hence,
damage, either through tearing of membranes or open-
ing of stretch-activated channels (Allen et al. 2005). Sup-
port for the nonuniform lengthening of sarcomeres
comes from a recent study of myoWbrils from rabbit
psoas muscle and left ventricles of guinea pig during acti-
vation and relaxation (Telley et al. 2006a). However,
these authors also show that albeit half-sarcomeres of
contracting single rabbit psoas myoWbrils lengthen to
diVerent extents during a stretch, rapid elongation of
individual sarcomeres beyond Wlament overlap (pop-
ping) does not occur. Moreover, in contrast to predic-
tions of the popping sarcomere hypothesis, they
postulate that a stretch rather stabilises the uniformity of
half-sarcomere lengths and sarcomere symmetry (Telley
et al. 2006b).
With respect to eccentric exercise, it is postulated that
the structural adaptation consists of an increase in the
number of sarcomeres in series so that a given muscle
length corresponds to a shorter sarcomere length (Mor-
gan and Talbot 2002). Whether and how serial sarco-
mere adaptation in humans following eccentric exercise
is parallelled by changes in tendinous and/or muscle
belly connective tissue remains to be established.
A consequence of the popping sarcomere hypothesis
is that the unloaded shortening velocity of muscle Wbres
should increase with eccentric training. The reason for
this is that the unloaded shortening velocity of a Wbre is
the sum of the velocities of its sarcomeres. Thus, the
more sarcomeres in series, the faster the unloaded short-
ening velocity, provided that no alterations in MyHC
composition occur. However, this will have to be demon-
strated in future experiments, especially with respect to
the MyHC isoform gene switching associated with
stretch and force production (Goldspink et al. 1991).
Another consequence of the popping sarcomere hypoth-
esis is that signiWcant muscle damage also can occur with
endurance exercise, provided that the duration (mara-
thon running) or mode (downhill running) of exercise is
extreme. Therefore, it is predicted that under certain cir-
cumstances, endurance exercise can lead to serial sarco-
mere accretion with concurrent increase in unloaded
Wbre shortening velocity. On the contrary, “conven-
tional” endurance exercise, which is associated with a
bias towards shortening contractions at short muscle
lengths, will lead to a decrease in the serial sarcomere
number and, thus, to shorter muscle lengths. Short mus-
cle lengths come with a reduction in functional range of
motion (ROM). In general, a reduction in ROM is not
desirable in health-based settings that aim at increasing
musculoskeletal and cardiovascular function. Hence, in
order to increase or preserve a functional ROM by serial
sarcomere number modulation, eccentric resistance exer-
cise covering the functional articular range might be the
method of choice.
In conclusion, there is a substantial body of evidence
that muscle Wbres and satellite cells can sense changes in
length. Accordingly, active and passive excursions from
resting length are transduced into a molecular and cellu-
lar response with subsequent structural adaptation. How-
ever, with respect to active excursions from resting length,
the response at the molecular, cellular, and structural
level is dependent on the contraction mode. It follows
directly that muscle length change as well as contraction
mode are two mechano-biological determinants of exer-
cise-induced skeletal muscle length adaptation. Therefore,
these two mechano-biological determinants, among oth-
ers (described below), need to be speciWed in reports com-
ing from investigations into the plasticity of skeletal
muscle following (resistance) exercise. As a measure of
muscle length change we suggest to specify ROM (x
Table 1) during exercise and the number of length excur-
sions [i.e. the number of repetitions (x
, Table 1)]. It must
be pointed out, however, that ROM might not always be
indicative of fascicle length excursion. The reason for this
is that length changes of the muscle-tendon units do not
necessarily correspond to the length changes in the mus-
cle fascicles (Hoyt et al. 2005). This means that the mus-
cle-tendon-unit may lengthen, while the contracting
muscle is shortening or isometric. As a measure of con-
traction mode we propose to report the fractional distri-
bution of the three contraction types [shortening
(concentric), isometric, lengthening (eccentric)] per repeti-
tion in terms of occurrence and temporal requirement (x
Table 1). Also, the number of contractions should be
reported (x
and x
, Table 1). For example, did the exer-
cise comprise one set of several, only lengthening contrac-
tions or was one repetition composed of one shortening,
one isometric, and one lengthening contraction? How
much time did it take to perform one repetition and how
was this time distributed over the respective contraction
modes? The importance of the latter point for inducing
muscle hypertrophy and gains in strength has been dem-
onstrated in studies where the eVect of fast lengthening
contractions versus slow lengthening or slow shortening
contractions has been investigated (Farthing and Chili-
beck 2003; Shepstone et al. 2005).
Molecular and cellular determinants of radial muscle
Previous work showed that if the tibialis anterior in the
mature rabbit was electrically stimulated while held in
the stretched position by plaster cast immobilisation, it
increased in mass by 35% within 7 days (Goldspink et al.
1992). Thus, if the lengthened (stretched) rodent muscle
is additionally subjected to electrical stimulation, it
increases in girth as well as length. The distinct role of
active tension in generating radial growth is evidenced
by this Wnding. Conversely, when muscle contractile
activity is reduced by means of immobilisation (e.g. cast-
ing) or unloading (bed rest, space Xight), rapid muscle
loss (atrophy) occurs (Booth and Kelso 1973; Thomason
and Booth 1990). Muscle loss is accentuated when immo-
bilisation occurs at short muscle length and attenuated
when immobilisation occurs at long muscle length, i.e. in
a stretched position (Dupont Salter et al. 2003). There-
fore, muscle growth and muscle atrophy are two oppos-
ing phenomena that are mechanistically linked. Either
the activity or inactivity of a common set of molecules
controlling a few cellular pathways determines whether
the skeletal muscle tissue will respond to deWned stimuli
with increased protein synthesis and stimulation of cell
growth or with increased protein breakdown and
reduced cell proliferation (Glass 2003a, b, 2005; Nader
2005; Rennie et al. 2004; Sartorelli and Fulco 2004). In
essence, the maintenance of skeletal muscle mass is the
result of the dynamic balance between muscle protein
synthesis and muscle protein degradation. Thus, these
two opposite processes are believed to hold the key to
the understanding of the mechanisms involved in the reg-
ulation of skeletal muscle mass.
Resistance exercise in humans and relevant animal
models such as functional overload via synergist abla-
tion can produce a signiWcant increase in the mass of the
overloaded muscles. In contrast to endurance exercise,
resistance exercise is associated with high-intensity-
short-duration workloads. The high-intensity-short-
duration workloads placed on skeletal muscle during
resistance exercise are at or near maximal capacity, and
as such produce signiWcant perturbations to the skeletal
muscle Wbres and the associated extracellular matrix.
These perturbations can lead to signiWcant muscle dam-
age, especially if lengthening contractions (eccentric
exercise) with supramaximal loads are performed. How-
75% 1RM 6 1
0 s
2 s
2 s 24+5 s no 60% 24 h
75% 1RM 6 shortening
2 s
4 s
10 s 96+10 s yes 100%
72 h
2 per week
10 weeks
, load magnitude
, fractional and temporal distribution of the contraction
modes per repetition and duration [s] of one repetition
, duration of the experimental period ([d] or weeks)
, number of repetitions
, time under tension ([s] or [min])
, volitional muscular failure
, range of motion
, rest in-between sets ([s] or [min])
, recovery time in-between exercise sessions ([h] or [d])
, number of sets
, number of exercise interventions (per [d] or week)
, anatomical definition of the exercise (exercise form)
New set of descriptors Classical set of descriptors
Complete set of mechano-biological descriptors
, rest in-between repetitions ([s] or [min])
2 per week
10 weeks
Table 1 Mechano-biological descriptors of resistance exercise stim-
uli. In order to better discriminate between the signiWcant stimula-
tory cues leading to distinct muscular adaptations, we propose to
design and describe resistance exercise-related muscular perturba-
tions based on mechano-biological descriptors (a). Example of how
inaccurate exercise stimulus description might lead to wrong conclu-
sions (b). Based on the traditional resistance exercise descriptors (x
), two hypothetical subjects (denoted A and B) receive exactly the
same resistance exercise stimulus. However, when further missing
mechano-biologically signiWcant descriptors (x
) are taken into
account, it is obvious that the two conditions diVer. These two diVer-
ring conditions will lead to distinct mechano-chemical signal trans-
ductions in the subjects’ muscles, even if the two subjects had the
same potential to adapt (i.e. the same response matrix characteristics
[see Fig. 1]). Thus, the two diVerring mechano-chemical signal trans-
ductions will most likely lead to distinct muscular adaptations and
adaptational eVects. Consequently, such results could be misinter-
ever, while both resistance and endurance exercise can
result in muscle injury, resistance exercise is more likely
to be associated with increases in Wbre cross-sectional
area and mass. The reasons for the response’s speciWcity
point to diVerences in the integration of hormonal, meta-
bolic, mechanical, neuronal, and immune responses,
which are all likely involved in the distinct transcrip-
tional responses that characterise endurance and resis-
tance training.
Molecular determinants of skeletal muscle hypertrophy
and atrophy
Exercise-induced hypertrophy mediators upstream of Akt
Resistance exercise-induced muscle hypertrophy results
when muscle protein synthesis exceeds muscle protein
degradation. In contrast, muscle atrophy is the result
from increased muscle protein degradation over muscle
protein synthesis. The integration of both anabolic and
catabolic signals that lead to the increase or decrease in
skeletal muscle mass (Fig. 2) is believed to occur at the
molecular nodal point Akt (Nader 2005). Thus, activated
(phosphorylated) Akt is both an eVector of anabolic sig-
nals and a dominant inhibitor of catabolic signals. Acti-
vation of Akt is mediated by the insulin-like growth
factor 1 (IGF-1)/phosphatidylinositol-3 kinase (PI3K)
pathway. The IGF-1/PI3K pathway is triggered by
increased muscle loading and subsequent expression of
the gene encoding IGF-1 in both animal models (DeVol
et al. 1990) and humans (Bamman et al. 2001). On the
basis of their mRNA transcripts, three human muscle
IGF-1 isoforms have been identiWed so far: IGF-1Ea
(“liver type” isoform), IGF-1Eb, and IGF-1Ec (MGF)
(Goldspink 2005; Hameed et al. 2003). In overloaded
rodent muscle there are two clearly identiWed transcripts,
IGF-1Ea and IGF-1Eb, of which IGF-1Eb has been
termed MGF (Goldspink 2005). Rodent MGF diVers
slightly from the human MGF sequence as it contains a
52 base pair insert in exon 5 (Goldspink 2005). Other
terms such as “mIGF-1”, which corresponds to the IGF-
1Ea isoform, have also been used to describe the diVerent
isoforms (Musaro et al. 2001). However, only MGF
appears to be activated by mechanical signals (Yang
et al. 1996).
These muscle-speciWc isoforms of IGF-1 are believed
to be suYcient to induce hypertrophy through either
autocrine or paracrine mechanisms (DeVol et al. 1990).
Transgenic mice engineered to overexpress systemic or
liver type IGF-1 under the control of a muscle-speciWc
promoter have skeletal muscles that are twofold greater
in mass than those seen in normal mice (Coleman et al.
1995; Musaro et al. 2001). Binding of the cytokine IGF-1
induces a conformational change in the muscle IGF-1
receptor (IGFR) tyrosine kinase, resulting in its
phosphorylation and the subsequent phosphorylation of
insulin receptor substrate 1 (IRS-1). In turn, this results
in the activation of PI3K. Finally, activation of PI3K
results in the production of phosphatidylinositol-3,4,5-
triphosphate and activation of Akt via 3-phosphoinosi-
tide-dependent protein kinase 1 (PDK1). However,
whether IGF-1 acts as an extracellular cue in muscle
biology depends on its availability for muscle IGFR.
Indeed, the availability of IGF-1 for muscle IGFR is
controlled by IGF-1-binding proteins (IGFBPs). Binding
of IGF-1 to IGFBPs can lead either to stimulation or
inhibition of IGF-1 eVects.
Hypertrophy mediators downstream of PI3K and Akt
Two pathways downstream of PI3K and Akt are
believed to mediate hypertrophy (Glass 2005; Nader
2005): the Akt/mammalian target of rapamycin (mTOR)
pathway, and the Akt/glycogen synthase kinase 3 beta
(GSK3) pathway. Both pathways lead to marked
hypertrophy through activation of the protein synthetic
machinery. Activation of mTOR by phosphorylated Akt
Fig. 2 SimpliWed model for the relationship between muscle Wbre
size and the balance between anabolic and catabolic stimuli. Muscle
size (girth and/or length) is set by the balance between activity-in-
duced hypertrophy (anabolic) (blue) and counteracting atrophy
(catabolic) (red) signals. In normal muscle, subjected to some
amount of tear and wear, hypertrophy and atrophy signals are in
balance (a). Resistance exercise perturbs the balance by inducing
hypertrophy signals over atrophy signals (b), or by inhibiting atro-
phy signals (c), or both (d), thus driving hypertrophy. This model
does not take into account changes in the contractile and metabolic
proWle that may occur following resistance exercise
Hypertrophy signalling Atrophy signalling
Hypertrophy signalling
Atrophy signalling
Hypertrophy signalling
Atrophy signalling
Hypertrophy signalling
Atrophy signalling
Set muscle
Muscle size
Set muscle
Set muscle
Set muscle
results in an increase in protein translation by two mech-
anisms: Wrst, mTOR activates 70 kDa ribosomal S6 pro-
tein kinase (S6K1/p70
), a positive regulator of protein
translation; second, mTOR inhibits the activity of
PHAS-1 (also known as 4E-BP1), a negative regulator of
the protein initiation factor eIF-4E. Conversely, phos-
phorylation of Akt results in the inactivation of GSK3.
GSK3 blocks protein translation initiated by the eIF2B
protein. Therefore, GSK3 inhibition may induce hyper-
trophy by stimulating protein synthesis independent of
the mTOR pathway.
Other growth-signalling pathways in skeletal muscle
Other signal transduction pathways shown to be acti-
vated in response to various forms of muscle contraction
include those involving the mitogen-activated protein
kinase (MAPK) signalling pathways (Long et al. 2004).
The MAPK-signalling pathways constitute a network of
phosphorylation cascades that link cellular stress to
changes in transcriptional activity. Relevant to the pres-
ent review is the observation that exercise leads to the
activation of at least three MAPK-signalling pathways,
i.e. extracellular signal-regulated kinases (ERK)1/2, p38
-terminal kinase (JNK), in skeletal
muscle (Aronson et al. 1998; Boppart et al. 1999; Wide-
gren et al. 2000). Further, AMP-activated protein kinase
(AMPK) activity has been shown to be increased during
contractions and exercise both in rodents and humans
(Winder 2001). However, the relevance of the AMPK-
signalling pathway has recently been questioned (Brooks
2005; Wadley et al. 2006). With respect to calcineurin sig-
nalling in working skeletal muscle, the reader is referred
to chapter “Molecular and cellular determinants of con-
tractile and metabolic tuning”.
Atrophy signalling
As with protein synthesis, degradation of cellular pro-
teins is an essential process for the maintenance of myo-
cellular homeostasis. However, in some speciWc
situations, when protein degradation exceeds protein syn-
thesis, skeletal muscle mass loss occurs. This process of
mass loss is termed atrophy. Skeletal muscle atrophy is a
serious consequence of various conditions such as micro-
gravity, hindlimb suspension, immobilisation, and
numerous diseases, including cancer and AIDS (Baracos
2001; Booth and Kelso 1973; Miro et al. 1997; Thomason
and Booth 1990). Muscle loss is parallelled by profound
transcriptomic (Bey et al. 2003; Cros et al. 2001; St-
Amand et al. 2001; Stein et al. 2002; Stevenson et al.
2003; Wittwer et al. 2002) and proteomic (Isfort et al.
2000, 2002a, b; Toigo et al. 2005) changes. Over the years,
several studies have identiWed at least Wve diVerent sys-
tems involved in the degradation of proteins during mus-
cle atrophy (Jackman and Kandarian 2004; Kandarian
and Jackman 2006). These are the lysosomal system, the
calpain system, the caspase or apoptotic protease system,
the ubiquitin proteasome system, and the nuclear factor
kappa B (NF-B) system. At present, it remains unclear
what the relative contribution of these systems to the
atrophy process are, and which speciWc roles they may
play during each particular disease state or context in
which muscle atrophy develops. However, among the
various systems involved in muscle proteolysis during
atrophy, the ubiquitin-proteasome system is thought to
play a major role (Jagoe et al. 2002). In addition to ubiq-
uitin, three distinct enzymatic components are required,
an E1 ubiquitin-activating enzyme, an E2 ubiquitin-con-
jugating enzyme, and an E3 ubiquitin-ligating enzyme
(Glickman and Ciechanover 2002). The E3 ubiquitin
ligases are the components which confer substrate speci-
Wcity. In multiple models of skeletal muscle atrophy, the
expression levels of two genes increased signiWcantly:
Muscle Ring Finger 1 (MuRF1) (Bodine et al. 2001a) and
Muscle Atrophy F-box (MAFbx) (Bodine et al. 2001a)
[also called Atrogin-1 (Gomes et al. 2001)]. Both MuRF1
and MAFbx/Atrogin were shown to encode E3 ubiquitin
ligases and to be speciWcally expressed in skeletal muscle
(Bodine et al. 2001a). However, the upregulation of
MuRF1 and MAFbx/Atrogin requires the nuclear trans-
location and activity of a family of transcription factors
termed Forkhead box O (FOXO). Indeed, in the context
of skeletal muscle atrophy, an increase in FOXO1
mRNA in addition to several other atrophy-related genes
was reported (Lecker et al. 2004). Also, FOXO3 activa-
tion was demonstrated to be suYcient to induce atrophy
(Sandri et al. 2004). However, recent evidence shows that
FOXO transcription factors are excluded from the
nucleus when phosphorylated by Akt, and translocate to
the nucleus upon dephosphorylation. Thus, muscle atro-
phy is prevented by FOXO inhibition through nuclear
exclusion by phosphorylation through Akt. This Wnding
highlights Akt’s role as a molecular checkpoint for the
integration of both anabolic and catabolic signals that
lead either to the increase or decrease in skeletal muscle
mass. However, although there is a distinct set of genes
which are inversely regulated by hypertrophy and atro-
phy (Latres et al. 2005), distinct transcriptional pathways
are activated in skeletal muscle atrophy. These distinct
transcriptional pathways are not necessarily the converse
of those seen during hypertrophy. Thus, it seems that
atrophy is not simply the converse of hypertrophy.
Muscle mass enhancement by inhibition
of negative regulators
Myostatin, also known as growth and diVerentiation
factor 8 (GDF-8), is a transforming growth factor–
(TGF-) family member. It inhibits the progression of
myoblasts from G1- to S-phase of the cell cycle through
upregulation of p21, the only cyclin-dependent kinase 2
(Cdk2) inhibitor (McCroskery et al. 2003). Myostatin
also inhibits myoblast diVerentiation by downregulation
of MyoD/Myogenin expression (Langley et al. 2002).
Consequently, myostatin acts as a negative regulator of
skeletal muscle mass in (1) cattle, (2) mice, and (3)
humans. (1) Naturally occurring mutations in the myo-
statin gene are responsible for the “double-muscling”
phenotype, which is characterised by a dramatic
increase in muscle mass of certain breeds of cattle
(McPherron and Lee 1997). (2) Myostatin-null mice
show an increase in muscle mass due to muscle hyper-
plasia and hypertrophy (McPherron et al. 1997). (3)
Recently, a child with muscle hypertrophy was found to
have a loss-of-function mutation in the myostatin gene
(Schuelke et al. 2004). This individual showed a quadri-
ceps cross-sectional area 7.2 standard deviations above
the mean for age- and sex-matched controls and the
ability to hold two 3 kg dumbbells in “horizontal sus-
pension with arms extended” at the age of 4.3 years
(Schuelke et al. 2004). As suggested, other less dramatic
changes in the myostatin gene (or heterozygosity for the
splice site mutation) may confer enhanced athletic
prowess in a less conspicuous manner (McNally 2004).
However, the child’s mutation has not been found in
any other individual, and is therefore not a polymor-
phism-driving normal human variation (Gordon et al.
2005). Furthermore, genetic association studies with
myostatin polymorphisms have consistently failed to
demonstrate any statistically signiWcant relationship
with any human muscle trait (Ferrell et al. 1999; Ivey
et al. 2000; Thomis et al. 2004).
In summary, skeletal muscle mass depends on the
dynamic balance of protein synthesis versus protein
breakdown. Whether muscle Wbre protein synthesis out-
weighs protein degradation depends on the activity of
intracellular hypertrophy- and atrophy-inducing media-
tors (Fig. 2). The activity of intracellular hypertropy- and
atrophy-inducing mediators is coordinated at molecular
checkpoints within the myoWbre. These molecular check-
points integrate anabolic and catabolic signals that are
triggered by (patho-)physiological conditions. Resistance
exercise is a physiological condition that aims at induc-
ing hypertrophy signalling while repressing atrophy sig-
nalling (Fig. 2), Wnally leading to myoWbre hypertrophy.
Resistance exercise is associated with high active tension
that is imposed on skeletal muscle. As shown, active ten-
sion through muscular contraction is per se a potent ana-
bolic stimulus for myoWbre hypertrophy. However, the
levels of active tension required to induce graded hyper-
trophic eVects or to prevent atrophy are most likely to
diVer. Therefore, such graded tensional eVects must be
investigated at the molecular and cellular level, if speciWc
exercise regimens, e.g. for the prevention or treatment of
sarcopenia, are to be developed. Consequently, the level
of active tension that is imposed on skeletal muscle dur-
ing resistance exercise is a further signiWcant mechano-
biological determinant of skeletal muscle size adaptation
, Table 1). As such, it should be quantiWed in resis-
tance exercise reports. However, the quantiWcation of the
load magnitude poses some problems, since usually, load
magnitude is reported in terms of the one-repetition-
maximum (1RM), e.g. 75% 1RM. It is beyond the scope
of this review to discuss issues related to the 1RM.
SuYce it to say that in a scientiWc setting we do not con-
sider the 1RM an appropriate measure to determine the
magnitude of the tensional load for exercise. In a scien-
tiWc setting, we suggest to construct maximal voluntary
torque (MVT)-angle curves, whenever possible. Based on
these MVT-angle curves, the respective choices with
respect to tension magnitude can be legitimated. More-
over, MVT intramuscular imbalances (joint angles of
disproportionate torque) can be detected and pre-/post-
MVT-angle curves can be compared with respect to
MVT as well as optimum angle for torque generation
see “Structural adaptation to strain perturbation”). In
most other settings it might be more practical to report
the load magnitude in terms of 1RM [e.g. % 1RM (x
Table 1)]. Importantly, information about the 1RM
should always be combined with information about the
time under tension (TUT) (x
, Table 1) until failure. That
is, how many seconds the exercise can maximally be sus-
tained prior to volitional failure (x
, Table 1). This will
additionally give important information about the meta-
bolic changes occurring with training (see “Molecular
and cellular determinants of contractile and metabolic
tuning”). However, load magnitude per se is not a mea-
sure of muscular loading. Only an anatomically perfect
technique will allow the eYcient “delivery” of the load to
the muscle under investigation. It follows that a sound
anatomical deWnition of the exercise in terms of joint
positions, movement velocity (movement control), etc.
should be an integral part of the exercise stimulus
descriptions (x
, Table 1). It is imperative to know if the
muscle was under permanent tension and how much of
the load eVectively “reached” the target muscle.
Cellular determinants of muscle hypertrophy
and atrophy
As mentioned above, satellite cells are lineage-commit-
ted adult muscle stem cells, located between the basal
lamina and the sarcolemma of myoWbres. Satellite cells
contribute to postnatal muscle growth and muscle
regeneration after injury (Charge and Rudnicki 2004;
Dhawan and Rando 2005; McKinnell et al. 2005;
Wagers and Conboy 2005). Upon myotrauma, quiescent
satellite cells become activated, proliferate, and ulti-
mately fuse to existing damaged muscle Wbres or among
themselves to form new myoWbres. Satellite cells are
activated in response to hypertrophic stimuli, such as
those occurring during muscle mechanical overload
(Darr and Schultz 1987; Moss and Leblond 1971; Schi-
aVino et al. 1976). In several animal models of compen-
satory hypertrophy (Hanzlikova et al. 1975; Snow 1990)
or after resistance training in humans (Kadi et al. 1999a,
b, 2004; Roth et al. 2001), the total number of activated
satellite cells is substantially increased. The mechanisms
leading to satellite cell activation during muscle hyper-
trophy are not entirely understood. It is postulated that
extensive physical activity, such as resistance training or
muscle overloading (chronic stretch, agonist muscle
ablation, tenotomy), inXicts muscle injury (Allen et al.
2005; Armstrong et al. 1991; Faulkner et al. 1993;
Gibala et al. 1995). Consequently, muscle injury, similar
to more severe muscle damage, may initiate a process of
regeneration. An indirect proof of muscle damage after
mechanical stress is given by an increase of serum mark-
ers such as muscle creatine kinase, an enzyme that is
usually restricted to the myoWbre cytosol. Muscle injury
initiates an inXammatory response with the attraction of
nonmuscle mononucleated cells, such as neutrophils and
macrophages, into the damaged zone (Fielding et al.
1993). Subsequently, several growth factors are released
either by the inWltrating cells or by the damaged myoW-
bres themselves. These growth factors may ultimately
regulate satellite cell proliferation and diVerentiation.
Indeed, several cytokines have been described that mod-
ulate proliferation and diVerentiation of satellite cells in
vitro or during regeneration after (exercise-induced)
muscle injury (Charge and Rudnicki 2004; Vierck et al.
2000). As mentioned above, HGF is considered to be a
key regulator of satellite cell activity during muscle
regeneration (Allen et al. 1995; BischoV 1997). HGF is
secreted by damaged tissue during the early phase of
muscle regeneration in amounts proportional to the
extent of muscle injury (Sheehan and Allen 1999; Tats-
umi et al. 1998). It seems that HGF directly regulates
satellite cell activation. As described, a large body of evi-
dence supports the importance of IGF-1 in the genesis
of skeletal muscle hypertrophy. IGF-1 can promote
both proliferation and diVerentiation of cultured satel-
lite cells, and these Wndings have been conWrmed in ani-
mal models (Charge and Rudnicki 2004). Experiments
showed that muscle-localised expression of IGF-1Ea
(“mIGF-1”) prevented, through an increase of the
regenerative potential of satellite cells, the age-related
loss of muscle mass (Musaro et al. 2001). Also, satellite
cells derived from mice overexpressing IGF-1Ea display
an increased proliferative potential (Chakravarthy et al.
2000b). Increased proliferative potential seems to be
mediated by activation of the IGF-1/PI3K/Akt path-
way, which results in the inactivation (phosphorylation)
of FOXO1 (Machida et al. 2003). Inactivation of
FOXO1 downregulates the activation of the p27
promoter (Chakravarthy et al. 2000a). Therefore, the
molecular pathways activated by IGF-1 in the muscle
Wbres to promote increased protein translation appear
also to be activated in satellite cells. However, IGF-1
action on satellite cells seems to be IGF-1 isoform-spe-
ciWc with apparently diVerent expression kinetics
(Goldspink 2005). After exercise and/or damage, the
IGF-1 gene is Wrst spliced towards MGF but after a day
or so becomes completely spliced towards the systemic
IGF-1 isoforms, which in human muscle are IGF-1Ea
and IGF1-Eb (Goldspink 2005; Haddad and Adams
2002; Hill and Goldspink 2003; Hill et al. 2003; Yang
and Goldspink 2002).
Concepts of myocellular enlargement
Muscle Wbres, i.e. multinucleated muscle cells, develop
during embryonic diVerentiation, when mononucleated
myoblasts Wrst proliferate and then fuse to form myotu-
bes that become innervated. Following myoblast fusion,
no further mitotic divisions occur within the myotubes
or muscle Wbres. Thus, under normal biological condi-
tions, adult skeletal muscle is an extremely stable tissue
with little turnover of nuclei (Decary et al. 1997; Sch-
malbruch and Lewis 2000). These Wndings about the
postmitotic and multinucleated nature of muscle Wbres
have led to the concept of a DNA unit or myonuclear
domain (Allen et al. 1999; Cheek 1985; Hall and Ralston
1989). The myonuclear domain is the theoretical amount
of cytoplasm supported by a single myonucleus. How-
ever, the concept of a myonuclear domain is a theoretical
one since regulation of the expression and distribution of
individual proteins within the muscle Wbre is dependent
on a number of diVerent variables related to the nature
of each protein. Nonetheless, since each muscle Wbre is
made up of many myonuclear domains, muscle Wbre
radial or longitudinal hypertrophy could conceivably
result from either an increase in the number of domains
(by increasing myonuclear number) or by an increase in
the size of existing domains (Edgerton and Roy 1991)
(Fig. 3). Research to date has strongly supported the
former concept by showing that satellite cell activation is
required for muscle hypertrophy. The requirement of
satellite cell activation was Wrst demonstrated by an
approach in which mild
-irradiation was employed to
block satellite cell proliferation. In response to functional
overload, myonuclear number or muscle size was not
increased in irradiated rat and mice muscles (Adams
et al. 2002; Rosenblatt and Parry 1992). However, recent
reports indicate that the size of myoWbres can increase
without the addition of new myonuclei (Kadi et al. 2004;
Wada et al. 2003; Zhong et al. 2005). It was found that
following 30 and 90 days of resistance exercise, the Wbre
area controlled by each myonucleus gradually increased
throughout the training period and returned to pretrain-
ing values during detraining (Kadi et al. 2004). No alter-
ations in the number of myonuclei were detected.
Moreover, it has been shown that under normal physio-
logical conditions myonuclear domain size might vary
throughout mouse lifespan (Wada et al. 2003) and that
myonuclear domain size is not constant during rat soleus
muscle atrophy (Zhong et al. 2005).
Whether the increase in mass during hypertrophy
results from an increase in the size of each Wbre (hyper-
trophy) or by an increase in Wbre number (hyperplasia),
has been under debate. Although the evidence has been
somewhat contradictory, there has been some suggestion
that an increase in Wbre number may occur in some ani-
mals under certain experimental conditions. Indeed, a
review of several investigations into skeletal muscle
growth concluded that in several animal species certain
forms of mechanical overload increases muscle Wbre
number (Kelley 1996). However, it has been suggested
that some reports have misinterpreted the intricate
arrangements of elongating Wbres as increases in Wbre
number (Paul and Rosenthal 2002). Indeed, studies
reporting an increase in the number of muscle Wbres used
avian or cat muscles (Kelley 1996). Both avian and cat
muscles have multiple endplate bands and Wbres that do
not insert into both tendons but terminate intrafascicu-
larly (Paul 2001). Thus, it remains to be determined
whether the radial growth of muscles with intrafascicu-
larly terminating Wbres in larger mammals arises from
new Wbre formation as assumed previously, or from
elongation of existing Wbres as recently proposed (Paul
and Rosenthal 2002). However, most human muscle fas-
cicles, despite their great length, consist of Wbres that
extend continuously from one tendon to the other with a
single nerve endplate band. Therefore, muscle hypertro-
phy in the adult human apparently can be accounted for
predominantly by hypertrophy of existing Wbres via
addition of newly constructed myoWbrils to the contrac-
tile apparatus. Accordingly, it has been suggested that
hyperplasia does not occur in humans following resis-
tance exercise (MacDougall et al. 1984; McCall et al.
1996). However, hyperplasia still remains a thinkable
mechanism of muscle enlargement. Thus, more conclu-
sive evidence might come from future investigations into
the plasticity of human muscle Wbre number. Fibre split-
ting or branching is also a characteristic feature of mus-
cle regeneration (Charge and Rudnicki 2004). Fibre
splitting is commonly observed in muscles from patients
suVering neuromuscular diseases, in hypertrophied mus-
cles, and in ageing mouse muscles, all of which are asso-
ciated with abnormal regenerative capacity (Bockhold
et al. 1998; Charge et al. 2002; SchiaVino et al. 1979). It
has been hypothesised that Wbre splitting occurs due to
the incomplete fusion of Wbres regenerating within the
same basal lamina (Blaivas and Carlson 1991; Bourke
and Ontell 1984).
Summarising, mechanical stress through high-inten-
sity resistance exercise (especially, but not exclusively
supramaximal eccentric exercise) inXicts myotrauma.
Upon myotrauma, quiescent satellite cells become acti-
vated: (1) through anabolic cytokines that are released
by the perturbed extracellular matrix; (2) by inWltrating
cells involved in the inXammatory response; (3) by the
damaged myoWbres; (4) in an autocrine manner by the
satellite cells themselves. Following activation, the satel-
lite cells proliferate, and ultimately fuse to existing mus-
cle Wbres or among themselves for tissue repair/
regeneration. Thus, besides myoW
bre hypertrophy due to
increased protein synthesis/decreased protein degrada-
tion (see Cellular determinants of muscle hypertrophy
and atrophy), satellite cell-based myoplasmic enlarge-
ment is a further mechanism in adult skeletal muscle
hypertrophy. It is consistent with the concept of the myo-
nuclear domain, where the satellite cells provide the
additional DNA for the establishment of additional
myonuclear domains during myoplasmic enlargement. It
follows that muscle damage is a further signiWcant mec-
hano-biological determinant of skeletal muscle size
adaptation. Thus, load magnitude (as a measure of extra-
cellular matrix and satellite cell perturbation) as well as
the number of lengthening contractions (as a measure or
eccentric damage) need to be speciWed in resistance exer-
Fig. 3 Hypothetical model of
skeletal muscle Wbre cytoplas-
mic enlargement. Schematically
depicted are satellite cells (red/
orange) lying beneath the basal
lamina and sarcolemma of mul-
tinucleated (dark blue) skeletal
muscle Wbres (cross and longitu-
dinal sections). According to the
concept of the myonuclear do-
main (see text for details), mus-
cle Wbre hypertrophy could
conceivably result from either
an increase in the size of existing
domains (dark blue shading)
(a!b and a!b for radial and
longitudinal hypertrophy,
respectively) or by an increase in
the number of domains by addi-
tion of new myonuclei (dark
blue) provided by the satellite
cells (red) (b!c / a!c and
b!c / a!c for radial and
longitudinal hypertrophy,
cise reports (x
, x
, x
, Table 1). Additionally, successful
recovery (repair) from muscle injury depends on the bal-
ance between the degenerative and regenerative pro-
cesses. Importantly, degenerative and regenerative
processes are time-dependent. Thus, if resistance exercise
perturbations are delivered at very short time intervals,
the degenerative processes prevail. If the degenerative
processes prevail, muscle mass is lost. As a consequence,
studies in which subjects are resistance-trained over a
determinate period of time (x
, Table 1) should report
recovery times in between the exercise sessions (x
Table 1) and number of exercise interventions per week
, Table 1), as a function of exercise-induced muscle
damage (exercise intensity). The importance of these
three factors (x
, x
, x
, Table 1) has been demonstrated
in studies, in which the duration of elevated protein turn-
over following resistance exercise sessions was investi-
gated (Chesley et al. 1992; MacDougall et al. 1995;
Phillips et al. 1997).
Molecular and cellular determinants of contractile
and metabolic tuning
Muscle Wbre has multiple, complex functional gene
groupings adapting independently to environmental
stimuli (Spangenburg and Booth 2003). The molecular
regulation of these functional gene groupings is Wbre
type-speciWc and results in the Wbre type’s phenotypical
characteristics. Such characteristics comprise, e.g. con-
tractile protein isoforms, mitochondrial volume, myoglo-
bin levels, capillary density, and oxidative enzyme
capacity. Each of these characteristics could be consid-
ered a functional gene domain within the respective Wbre
type (Spangenburg and Booth 2003). Not only can these
functional modules be regulated within diVerent myoW-
bres but also MyHC protein expression can be heteroge-
neous within a single Wbre (Pette and Staron 2000;
Talmadge et al. 1996), resulting in “hybrid” (Baldwin
and Haddad 2001) or “polymorphic” (Caiozzo et al.
2003) Wbres. It is believed that contractile activity follow-
ing neural activation induces changes in common regula-
tory factors within a subpopulation of genes (i.e. gene
“modules”) to modify the muscle Wbre phenotype
(Spangenburg and Booth 2003). More speciWcally, neural
activation of skeletal muscle results in the release of ace-
tylcholine from the neuromuscular junction and depolar-
isation of the plasma membrane, which activates force
production by a process known as excitation–contrac-
tion coupling. The frequency and duration of stimulation
determine the amplitude and duration of the Ca
sients and, as a result, the level of force output by the
muscle. Thus, both the amplitude and duration of the
transient in skeletal muscle are determined by the
motor unit (MU) Wring frequency. The increases in
amplitude, as well as the duration for which these ampli-
tudes are achieved, are thought to encode signals that
will be recognised by diVerent downstream Ca
dent pathways. The key signalling pathways downstream
of the elevation in intracellular Ca
that translate this
signal into a transcriptional response include the Ca
calmodulin(CaM)-dependent phosphatase calcineurin
(Cn), Ca
/calmodulin-dependent kinase II (CaMKII),
/calmodulin-dependent kinase IV (CaMKIV), and
-dependent protein kinase C (PKC) (Chin 2005). In
turn, these Ca
-dependent key signalling pathways will
determine the set of genes expressed, thus providing a
mechanism for tightly coupling the extent of muscle exci-
tation to regulation of transcription (i.e. excitation–tran-
scription coupling) (Chin 2005). Many Ca
target genes have been identiWed in skeletal muscle.
Downstream Ca
-sensitive target genes of varied
expression levels between Wbres include the nicotinic ace-
tylcholine receptor (nAChR), glucose transporter 4
(GLUT4), sarcoplasmic reticulum (SR) Ca
(SERCA1), MyHC isoforms, oxidative enzymes, as well
as genes that regulate mitochondrial biogenesis. Apart
from the transcriptional regulation, it has been suggested
that muscle Wbre contractile characteristics might also be
regulated by posttranslational modiWcation of contrac-
tile proteins (Canepari et al. 2005). In addition to the role
of Cn signalling in the determination of muscle Wbre type
characteristics, this phosphatase is known to play an
important role in muscle hypertrophy (Dunn et al. 1999;
Michel et al. 2004). Cn dephosphorylates the transcrip-
tion factor nuclear factor of activated T cells (NFAT),
enabling its nuclear translocation and DNA binding.
The Cn–NFAT pathway has been linked to Ca
induced skeletal muscle hypertrophy, at least in cultured
skeletal muscle (Semsarian et al. 1999). With regard to
skeletal muscle hypertrophy in animal models, the role of
Cn remains controversial. Current thought suggests that
hyperactivation of Cn alone is not suYcient to induce
skeletal muscle hypertrophy but that the activation of
accessory parallel signalling pathways for growth is
required (Michel et al. 2004).
The size principle of motor recruitment—consequences
for resistance exercise
As mentioned above, amplitude and duration of Ca
transients in skeletal muscle as a function of MU Wring
frequency are decoded at the molecular level, resulting in
expression changes of Wbre type-speciWc functional gene
modules. For mammals, it has been shown that for many
activities there is a graded level of muscle recruitment
that is driven by the diVerent thresholds of the motor
neurones: “the size principle of motor recruitment
(Denny-Brown and Pennybacker 1938; Henneman et al.
1965, 1974). The size principle predicts that MU recruit-
ment is determined by force requirements. The majority
of units are, in fact, recruited voluntarily in the order of
increasing size (Monster and Chan 1977; Tanji and Kato
1973). Typically, small MUs are type I units, and unit
size increases with progression through the Wbre types:
I < IIA < IIX (according to the MyHC isoform classiW-
cation system in humans). Therefore, when low force is
required, only type I MUs will be active. Only when force
is high will recruitment demand involvement of the
larger MUs. It follows that slow muscle Wbres are acti-
vated for low-force contractions and fast muscle Wbres
are additionally activated to supply greater force
demands. Several researchers have shown that MU
recruitment is completed by »50% of maximum volun-
tary contraction (MVC) in small muscles [adductor polli-
cis (Kukulka and Clamann 1981), and Wrst dorsal
interosseus (De Luca et al. 1982a, b, 1996; Milner-Brown
et al. 1973)], and 70–80% of MVC in large muscles
[biceps (Kukulka and Clamann 1981), deltoid (De Luca
et al. 1982a, b), and tibialis anterior (De Luca et al. 1996;
Erim et al. 1996)]. The size principle applies when either
slow-ramp force is exerted or constant low forces are
compared with constant higher forces. The size principle
of orderly recruitment is also preserved during exercise
(Gollnick et al. 1974a, b; Vollestad and Blom 1985; Vol-
lestad et al. 1984, 1992), whether the comparison is of
sustained contractions at diVerent intensities, or at diVer-
ent times during exercise at the same submaximal force
sustained until exhaustion (Adam and De Luca 2003).
However, during some forms of voluntary or reXex
contraction, motor neurone size may not be the sole fac-
tor determining excitation threshold: (1) Specialisation
of the synaptic input among the motor neurones, and
joint position have been shown to aVect recruitment
order in some cases; (2) MU rotation strategy has been
demonstrated for sustained isometric contraction of
biceps brachii at 10 but not at 40% of maximum; (3) In
certain very rapid or sudden corrective movements—
such as accelerations or sudden changes in direction—
high-threshold units that do not participate in walking
or even in running might be selectively recruited, as
shown for the human short extensor muscle of the toes
(Grimby 1984). With respect to ballistic movements,
there is some controversy over the extent to which selec-
tive recruitment may occur (if at all) (Zehr and Sale
1994); (4) It has been suggested that selective recruitment
of large MUs occurs during lengthening contractions
(Nardone and Schieppati 1988; Nardone et al. 1989); (5)
In certain cases of altered motor tasks, recruitment order
might vary. Based on these results, it has been suggested
that diVerential recruitment simply represents the exis-
tence of distinct, task-related subpopulations of motor
units, rather than “violations” of the size principle
(Burke 2002; Cope and SokoloV 1999). Hence, when the
same motor task is undertaken in exactly the same way,
the order in which MUs are recruited remains Wxed. This
is of special importance, since it supports the notion that
recruitment order is maintained during muscular fatigue.
In particular, it has been shown that submaximal fatigu-
ing contractions in the vastus lateralis muscle of humans
lead to the monotonic decline in the recruitment thresh-
old of all MUs and the progressive recruitment of new
MUs, without change in the recruitment order. Thus, as
the force capacity of continuously active muscle Wbres
declines progressively, increased excitation is required to
keep the muscle output constant. The increased excita-
tion produces the recruitment of additional MUs. The
recruited MUs thus become active at a lower torque level
than their initial threshold, and the recruitment thresh-
old continues to decrease in subsequent contractions as
the force production of the active MUs continues to
decrease (Adam and De Luca 2003). Indeed, it could be
shown that the reduction in peak tetanic torque is line-
arly correlated with the decrease in the mean MU
recruitment threshold at corresponding endurance times
(beginning, middle, and end of fatiguing contractions)
(Adam and De Luca 2003). However, these data do not
prove a causal relationship between changes in muscle
force output and changes in MU recruitment. Nonethe-
less, it can reasonably be assumed that the drop in peak
tetanic torque that comes with muscular fatique corre-
sponds to an increase in the relative force requirement to
sustain the target torque level.
Based on these Wndings, we propose a theoretical
model of muscle fatigue and MU recruitment during
resistance exercise progression (Fig. 4). Provided that
exercise is performed to volitional muscular failure, the
three diVerent load magnitudes in Fig. 4a–c will lead to
similar, i.e. “complete” MU recruitment (Fig. 4d), and,
thus, to a similar stimulation of protein synthesis.
Indeed, preliminary studies aimed at delineating the
dose–response relationship between the intensity of exer-
cise and the rates of muscle protein synthesis have shown
that when the same total amount of ATP is turned over
and recruitment is complete, exercise at 60, 75, and 90%
of the 1RM results in exactly the same stimulation of
muscle protein synthesis (Bowtell et al. 2003). Addition-
ally, increases in tension above 65% cause no further
stimulation in muscle protein synthesis (Bowtell et al.
2003). However, the diVerent TUT until muscular failure
imply diVerent MU recruitment dynamics. As a conse-
quence, distinct metabolic loads are inXicted on the exer-
cising muscle in Fig. 4a–c. In Fig. 4a, progression
through the Wbre types I ! IIA ! IIX occurs fast (30 s).
In contrast, in Fig. 4c, progression through the Wbre
types occurs relatively slowly (180 s). Consequently, in
Fig. 4c the Wbres’ capability in providing ATP through
oxidative metabolism is more pronouncedly challenged
than in Fig. 4a. This is due to the fact that with low resis-
tive loads in % of the maximum voluntary torque
(MVT), the threshold for complete recruitment is
reached later. Thus, if a biceps exercise is assumed for the
examples in Fig. 4, only in Fig. 4a the recruitment is pre-
dicted to be complete at start. In Fig. 4b, c, complete
recruitment will be reached later. It follows directly, that
the two strategies of MU recruitment and MU Wring
rate-coding for the increase in force output will vary to
diVerent extents for Fig. 4a–c. Consequently, the distinct
pattern of MU recruitment and rate-coding in Fig. 4a–c
will have a diVerent impact on the previously described
excitation–transcription coupling (Chin 2005).
In conclusion, we deduce that the TUT (x
, Table 1),
volitional muscular failure (x
, Table 1), number of sets
, Table 1), rest in-between sets (x
, Table 1), and rest
in-between repetitions (x
, Table 1) are further mechano-