Sarcopenia Age-Related Muscle Wasting and Weakness: Mechanisms and Treatments P40 ppt

376 J.A. Faulkner et al.
With maximum activation, the forces developed are greatest during lengthening
contractions, intermediate during isometric contractions, and least during shortening
contractions. The explanation for the greater force during lengthening contractions
than during isometric contractions is that during isometric contraction only the
cross-bridges that are in their driving stroke generate tension, but when a maximally
activated muscle is stretched, additional strongly-bound cross-bridges that have not
progressed into their ‘driving stroke’ resist the ‘lengthening’ of the skeletal muscle, are
strained and generate force. Consequently, the force developed during a lengthening
contraction can exceed that developed during an isometric contraction by as much
as twofold. The high forces developed during lengthening contractions are partially
responsible for the high susceptibility of muscles to contraction-induced injury
during this type of contraction. In fact, only the lengthening contractions are capable
of producing a contraction-induced injury.
3 Age-Related Muscle Wasting and Muscle Weakness
The ‘wasting’ or ‘atrophy’ of a skeletal muscle refers to a loss in the mass of the
skeletal muscle, a condition that arises from a reduced usage of skeletal muscles at
any age. The reduction in the daily usage may arise from: (a) sickness and imposed
bed-rest, (b) disuse of a specific muscle due to immobilization by casting, or to the
placement of an injured arm in a sling. In addition, by 70–80 years of age an
outright loss of skeletal muscle fibers occurs that is estimated, based on data from
vastus lateralis muscles, to be as high as 50% of the fibers (Lexell et al. 1988). The
loss in the number of muscle fibers contributes significantly to the concurrent loss
of muscle mass and myofibrillar protein. In contrast to atrophy, ‘weakness’ of a
muscle reflects an inability of a muscle to generate the normal or expected force
when activated. As people age, particularly into advanced old age, the vast majority
of humans, both men and women, become less physically active and invariably
show signs of both muscle wasting and muscle weakness. Particularly in old age,
the combined impact of decreased physical activity and muscle wasting and
weakness lead to the debilitating condition of frailty (Hadley et al. 1993). The
increase in physical frailty with old age has serious consequences in terms of the

health and longevity of the elderly. Physical frailty invariably leads to a further
decrease in physical activity as well contributing to respiratory and cardiovascular
problems (Hadley et al. 1993). Despite the magnitude of the problem, even in the
elderly, these conditions are at least partially reversible by re-establishing an
increased level of physical activity, but such programs must be carefully designed
with a slow progression and close supervision by highly trained exercise leaders.
Although some amelioration of muscle atrophy is achievable through exercise,
the component of muscle atrophy that is due to the loss of muscle fibers appears
inevitable and irreversible. Consequently, the magnitude of the improvements
attainable with physical training of the frail elderly must be realistic and kept in
perspective with the limitations of the participants.
377Role of Contraction-Induced Injury in Age-Related Muscle Wasting and Weakness
4 Late-Onset Muscle Soreness
The phenomenon of a contraction-induced injury to skeletal muscle fibers was first
recognized inadvertently by Theodore Hough, during experiments on the fatigue of
finger flexor muscles (Hough 1901, 1902). Hough’s subjects performed a highly
fatiguing muscle contraction protocol using a pulley-system that enabled lifting and
lowering a weight with flexion and extension of the middle finger. Some of the
participants complained of pain in the forearm between 8 and 12 h after the comple-
tion of the protocol, with the soreness increasing and reaching its highest level 48
or even 60 h afterward. In these experiments, it was not recognized that the sore-
ness was initiated by the lowering of the weight. The phenomenon of muscle sore-
ness encountered in the Hough studies was ignored for almost 80 years, and then
re-surfaced as ‘delayed onset muscle soreness’ in the early 1980s. Late onset
muscle soreness has been observed after a number of different protocols that
involved the lowering of a weight or the ‘stretching’ of the activated skeletal mus-
cle fibers and a number of inventive protocols were developed to investigate the
factors involved in the lengthening contractions that initiated the delayed soreness
of the muscle. These early protocols involved repeatedly stepping up with one leg
and down with the other leg on and off a fairly high stool (Newham et al. 1983a,

b), raising and lowering a weight with forearm flexion and extension (Newham
et al. 1987), and resisting the reverse-rotation of the pedals of a bicycle ergometer
(Friden et al. 1983). Needle biopsy samples of both arms and legs indicated that
these protocols of lengthening contractions invariably caused morphological evi-
dence of injury to skeletal muscle fibers (Fig. 2a–c).
Lengthening contractions produce a decrease in maximum strength and assays
of blood samples indicate a peak in plasma creatine kinase several days after the
initial injury (Fig. 3a). Subjective assessments of pain indicate that the exact timing
of the onset of muscle soreness varies somewhat with the individual and with the
type of exercise, but typically peaks after ~2 days and is resolved within 5 days.
The recovery of strength and reestablishment of pre-injury levels of circulating
creatine kinase take anywhere from 1 to 2 weeks depending on the severity of the
injury and repeated bouts of training with lengthening contractions reduce the
occurrence of late onset muscle soreness (Newham et al. 1983a, b). The experi-
ments on volitional lengthening contractions performed by human subjects were
soon followed up with more definitive experiments on mice and rats (Armstrong
et al. 1983; McCully and Faulkner 1985). The experiments on small mammals
substantiated the time course of the injury to muscle fibers and that the magnitude
of the injury was greatest approximately 3 days after the lengthening contraction
protocol with complete recovery requiring 3–4 weeks (Fig. 3b). A number of fac-
tors have been cited as the likely causes of the late-onset muscle soreness. The most
plausible of these factors are the actual damage to muscle fibers and connective
tissue and inflammation (Cheung et al. 2003; Friden et al. 1983, 1986; Newham
et al. 1983a, b; Jones et al. 1986; Schwane and Armstrong 1983). From the begin-
ning, Hough (1902) cited the ruptures within the muscles as the cause of the
Fig. 2 Electron micrographs from an EDL muscle of a young mouse after a protocol of 75
lengthening contractions. (a) A longitudinal section of a single fiber at high magnification taken
immediately after a severe lengthening contraction protocol. Note that some sarcomeres have
actually shortened down to a 1.40 mm length, whereas the weaker sarcomeres have been damaged
severely through a stretch out to a 3.80 mm that has displaced the thick filament to one end of the

sarcomere or the other. This segment of this fiber will undergo the degenerative and regenerative
stages shown in Fig. 6. This photomicrograph depicts a part of a single fiber in Stage 2. (b) A
longitudinal section of a myofiber 10 min after a lengthening contraction protocol showing areas
of focal damage (*) within single or small groups of sarcomeres. In some sarcomeres, the damage
appears to be in the A-band region, with Z-lines remaining intact, whereas in other sarcomeres the
damage involves the Z-lines. (c) Transverse sections of a muscle 3 days after the protocol. Muscle
fibers range from those with intact myofibrils (M3 and M4) to those with degenerating myofibrils
(M1) or devoid of cytosolic constituents (M2). Fiber M2 has phagocytes (P) within the basement
membrane (arrows). C is a capillary (Figure 2b and c reproduced from Faulkner et al. 1995 with
permission of Oxford University Press)
Hours
01 2310614
Maximum Value (%)
0
20
40
60
80
100
Muscle pain
Plasma creatine kinase
Maximum isometric strength
Days
Time After Initial Injury
Hours
01 23610 14
Maximum Value (%)
0
20
40

60
80
100
EDL muscles
TBA muscles
Days
Time After Initial Injury
Maximum isometric force of
a
b
Fig. 3 Data are given for several indices of contraction-induced injury measured prior to and at
selected time periods following a protocol of lengthening contractions administered to (a) the
elbow flexor muscles of human beings and (b) the ankle dorsiflexor muscles of mice. The values
indicated on the abscissa are the times in “hours” and “days” after the initiation of the contraction
protocols. (a) Eight human subjects (age 24–43 years) performed maximal lengthening
contractions of the elbow flexor muscles once every 15 s for 20 min. (b) The dorsiflexor muscle
group of mice was exposed to a maximal lengthening contraction every 5 s for 30 min during
plantar flexion of the ankle with the foot in a “shoe” apparatus. Data are shown for the maximum
isometric forces developed by the tibialis anterior (TBA) and extensor digitorum longus (EDL)
muscles measured in vitro following the injury protocol (n = 4−9 for each data point). All values
are expressed as percentages of the maximum value for each variable. For isometric strength and
maximum isometric force, the maximum values were achieved by all subjects prior to the exercise
and are taken as 100%. For muscle pain and plasma creatine kinase, each subject did not reach his
or her maximum values on the same day. Therefore, the peak values for these variables do not
correspond to 100%. Values are given as means ± standard errors. When no error bars are shown,
they are contained within the symbol (Modified from data in Newham et al. 1987; Faulkner et al.
1989; with permission. Reprinted from Faulkner et al. 1993; with permission of the American
Physical Therapy Association. This material is copyrighted, and any further reproduction or
distribution is prohibited)
380 J.A. Faulkner et al.

soreness, although he had no direct evidence for this. Later needle biopsy studies
of humans definitively demonstrated ultrastructual disruptions within muscle fibers
associated with late-onset muscle soreness.
5 The Cause of the Contraction-Induced Injury
The concept of a contraction-induced injury that occurred only when skeletal
muscle fibers were activated to produce high forces and then stretched was slow to
evolve. Early investigations of lengthening contractions focused primarily on the
absorption of the work done on the muscle and the ‘heat of lengthening’ (Abbott
et al. 1951). A major advance occurred in the understanding of the physiological
cost of positive and negative work with the Abbott et al. (1952) study utilizing the
modified bicycle-ergometer that enabled both positive and negative work to be
performed. Knuttgen and his colleagues (Knuttgen and Saltin 1972; Knuttgen et al.
1982) also modified a bicycle ergometer to enable subjects to pedal against the load
and perform lengthening contractions with either the arms or the legs. The fourfold
difference observed between the energy cost during the shortening compared with
the lengthening contractions is rather amazing (Fig. 4) and the complex physiologi-
cal implications of this difference in energy cost are still not understood. The focus
of the research on lengthening contractions gradually shifted to the effects of the
lengthening contraction protocols on muscle pain and damage. The prevailing view
initially was that as long as a given protocol of contractions was sufficiently
intense, select populations of fibers would be injured. Armstrong (1990) expressed
this view at a Symposium on Muscle Injuries, when he wrote that “muscular
Work (kg m/min)
1500 1000 500
0 500 1000 1500
Oxygen consumption (l./min)
0.5
1.0
1.5
2.0

2.5
3.0
3.5
Free-wheeling (mean)
Resting (mean)
Fig. 4 Variation in the rate of oxygen consumption with the rate of work in pedaling for both
positive and negative work (Reproduced from Abbott et al. 1952 with permission of Wiley)
381Role of Contraction-Induced Injury in Age-Related Muscle Wasting and Weakness
exercise commonly results in injury to fibers in active muscles, particularly when
the exercise is relatively intense, is of long duration, and/or includes lengthening
contractions”.
The hypothesis that “eccentric” exercise (exercise that involves lengthening
contractions of muscles) preferentially damages fibers (Newham et al. 1987) was
explored using comparable protocols of lengthening, shortening and isometric
contractions of isolated muscles of mice (McCully and Faulkner 1985). With
experiments on in situ single muscles of mice or rats (McCully and Faulkner 1985;
Brooks and Faulkner 1990; Brooks et al. 1995) or single permeabilized fibers
obtained from muscles of mice or rats (Macpherson et al. 1996; Brooks and
Faulkner 1996; Lynch et al. 2008), precise protocols of lengthening contractions
were designed to investigate the underlying mechanisms responsible for the injury
associated with lengthening contractions. Such experiments demonstrated
conclusively that injury was only observed following lengthening contractions
regardless of the intensity of the shortening or isometric contraction protocol
(McCully and Faulkner 1985). Furthermore, the magnitude of the injury induced by
a given protocol of lengthening contractions was found to be a function of the force
developed during the lengthening contraction, the magnitude of the stretches
imposed, and the number of repetitions of the lengthening contractions in a given
protocol (Brooks et al. 1995; Lynch et al. 2008; McCully and Faulkner 1986).
Contraction-induced injury is thus most likely to occur during activities that
involve a severe lengthening of a maximally activated muscle, such as lowering a

very heavy object, or with multiple lengthening contractions of smaller groups of
motor units as in distance running (Komi 2000). Running at relatively high speed,
even on the level, involves stretching of the quadriceps muscles on the landing
(Komi 2000), and running faster or longer distances than a runner is accustomed to
may result in contraction-induced injury to fibers in the muscles involved. In any given
activity, untrained participants are much more likely to experience a contraction-
induced injury than trained subjects. Despite the protection provided by training,
even trained athletes may sustain a contraction-induced injury during transition
periods when training loads or work-outs are increased or modified.
After single lengthening contractions (Brooks and Faulkner 1990; Li et al. 2006)
or a protocol of many lengthening contractions (McCully and Faulkner 1986), the
severity of a contraction-induced injury is most accurately assessed by the deficit
in force generation (Fig. 5). An immediate force deficit occurs when a maximally
activated fast skeletal muscle fiber of a rat is stretched through a single 20% strain
(Macpherson et al. 1996; Lynch and Faulkner 1998; Panchangam et al. 2008) or an
in situ skeletal muscle is stimulated maximally and stretched through a 20% strain
for three 5-min contraction periods separated by 5 min (McCully and Faulkner
1985). The single 20% lengthening contraction of the single fiber produced a 17%
force deficit in fast fibers of rats (Macpherson et al. 1996; Panchangam et al. 2008),
whereas the 450 lengthening contractions of extensor digitorum longus muscles of
the mice produced a 60% force deficit immediately afterward (McCully and Faulkner
1985). Force deficits invariably cause a more severe initial injury in muscles of old
compared with young or adult animals. When activated maximally and exposed to
382 J.A. Faulkner et al.
a single stretch through 30% of fiber length, a small 8–10% force deficit was
observed for in situ extensor digitorum longus (EDL) muscles of young and old
mice, but 40% and 50% strains produced large force deficits with the muscles of
the old experiencing twofold greater force deficits than those of the young and adult
mice (Fig. 5a and b). For single permeabilized fibers from fast muscles of rats,
Strain (% L

f
)
01020304050
Force Deficit (%)
0
20
40
60
80
100
a
b
Work (J/kg)
050 100 150 200 250 300
Force Deficit (%)
0
20
40
60
80
100
Young Mice
(Brooks et al., 1995)
Adult Mice
Old Mice
Fig. 5 The force deficits following single stretches of maximally activated muscles.Data are
presented for single stretches varying in magnitude but not velocity(V = 2 L
f
s
−1

) for pooled young
and adult mice (
•
) and old mice (
•
) in (a) and in situ EDL muscles of young (Ñ), adult (
°
) and old
(
°
) mice in (b). The work input during the stretch is normalized by muscle wet mass (J kg
−1
),
strain is expressed as a percentage of optimum fiber length (L
f
), and the force deficit observed 1
min after the stretch is expressed as a percentage of the isometric force developed just prior to the
stretch. Each symbol in (b) indicates a data point from a single stretch. The coefficients of deter-
mination for the regression relationships for data from adult mice (continuous line) and old mice
(dashed line) are 0.59 and 0.77, respectively. The slopes of the relationships, 0.20 for muscles in
adult mice and 0.39 for muscles in old mice, are significantly different. Data for young mice (r
2
= 0.73;
slope − 0.13) are reproduced from Brooks et al. 1995. Data in (a) are presented as means ± S.E.M.
Sample size is from 3 to 12 for each point. *Significant difference (P <− .05) in the mean force
deficits between the two groups (Reprinted from Brooks and Faulkner 1995)
383Role of Contraction-Induced Injury in Age-Related Muscle Wasting and Weakness
force deficits immediately after single strains of 10% or greater were approximately
twofold larger for single fibers from muscles of old compared with those from adult
animals (Brooks and Faulkner 1996; Lynch et al. 2008). In combination, the whole

muscle and single fiber experiments indicate a greater susceptibility of muscles in
old animals to injury that is due at least in part to a mechanically compromised
sarcomeric structure that is less able to withstand stretch.
6 Progression of the Injury
The severity of the contraction-induced injury is a direct function of how severely
single fibers are injured and how many fibers are injured sufficiently to initiate the
cascade of events associated with a secondary injury. This cascade of events involves
phases of contraction-induced injury to skeletal muscles that can be broadly
categorized as: (1) the initial lengthening contraction that triggers the injury; (2) an
autogenic stage that includes degradation by proteolytic and lipolytic systems
indigenous to the fibers, (3) a phagocytic stage from 4 to 6 h through 2–4 days
including an inflammatory response, and (4) a regenerative stage beginning at 4–6
days and extending to 10–14 days depending on the severity of the injury (for review
see Tidball 1995). These four phases match well with the seven phases depicted in
Fig. 6, with Phases (c) and (d) the phagocytic stage and (e) and (f) depicting the
regenerative phase. During lengthening contractions, the actual injury to sarcomeres
in a myofibril appears to occur when thick filaments of single sarcomeres are
displaced to one end of the sarcomere and some or all of the filaments fail to
interdigitate properly within the myofibril when the sarcomere attempts to return to
its resting length (Fig. 2a). Usually the injury occurs to a highly localized cluster of
sarcomeres within a single fiber. Damage to the muscle fiber compromises the
fiber’s ability to maintain proper calcium homeostasis. The prolonged increase in
intracellular calcium levels in damaged muscle fibers activates the m-calpain
protease system. M-calpain and related proteases perform the initial disassembly of
damaged myofibrils (Jackman and Kandarian 2004). Once the sarcomere has been
disassembled, the damaged proteins are broken down into their constitutive amino
acids by the ubiquitin-proteasome system. Within a few days following injury, protein
synthesis pathways are activated and new sarcomeres are synthesized.
Following severe protocols of lengthening contractions, the large force deficits
displayed by muscles from both young and old mice indicate that throughout the

cross-sections of individual fibers a substantial number of sarcomeres have been
injured and that portions of these fibers will undergo additional degeneration of the
total cross-section of the injured fibers (Rader et al. 2006). The additional steps
include: a sealing off of the damaged area accompanied by the infiltration of
inflammatory cells, phagocytosis of the damaged tissues, and subsequent activation
of satellite cells and regeneration of entirely new segment of fiber (Fig. 6). Satellite
cells are muscle precursor cells that reside between the sarcolemma and the basal
lamina in skeletal muscle fibers. Satellite cells normally exist in a quiescent state, but
upon injury the satellite cells are activated, migrate to the site of injury, proliferate,
384 J.A. Faulkner et al.
and fuse with the damaged fiber to replace the nuclei lost as a result of the injury.
Mechanical disruption of the endomysium causes the release of inactive hepatocyte
growth factor (HGF) (Tatsumi and Allen 2004). The HGF is activated within the
injured tissue (Tatsumi et al. 2006) and binds to the c-met receptor on the plasma
membrane of the resident satellite cells, which are thus activated from their quiescent
state and migrate to the site of injury.
As satellite cells migrate to the site of injury, they also undergo several rounds of
proliferation. The initial proliferation of satellite cells is brought about by an increase
in the expression of the basic helix-loop-helix (bHLH) transcription factor MyoD.
MyoD is one of four members of myogenic regulatory factor (MRF) family that also
a
b
c
d
e
f
g
Fig. 6 Schematic diagram of the sequence of events for a typical muscle fiber following a severe
LCP. Within several hours following focal injury, the plasma membrane is damaged, an influx of
calcium activates proteases intrinsic to the muscle fiber, and myofibrils hypercontract, resulting in

a zone of necrosis. The freely permeable basement membrane remains intact. By 1 day, the
hypercontracted myofibrils degenerate while vesicles accumulate to seal off the viable portions
from the necrotic segments of the fiber. Neutrophils infiltrate at this time. Between 2 and 5 days,
macrophages infiltrate, releasing more cytotoxic substances such as ROS that break down
damaged tissue further, as well as previously uninjured tissue, resulting in a secondary injury.
Satellite cells migrate to the site of injury. At 5–30 days, satellite cells proliferate and fuse across
the necrotic segment so that recovery takes place (Reproduced with modifications based on a
previously published figure (Bischoff 1994) with permission of the McGraw-Hill Companies.
Figure also published in Rader et al. 2006 with permission Wiley)
385Role of Contraction-Induced Injury in Age-Related Muscle Wasting and Weakness
include Myf-5, myogenin and MRF-4. The MRFs induce the “myogenic program”
in these proliferating satellite cells, causing the cells to begin to express skeletal
muscle contractile proteins. Once in proximity of the damaged region of the muscle
fiber, satellite cells fuse with each other to form multinucleated structures called
myotubes. Myotubes fuse with the damaged muscle fiber and restore the nuclei lost
after the initial injury. Some proportion of the satellite cells that underwent prolifera-
tion do not form myotubes, but instead resume a sub-basal lamina position, return to
the quiescent state, and repopulate the satellite cell pool.
In addition to satellite cells, fibroblasts and inflammatory cells are attracted to the
site of injury within the muscle. These cells assist in the removal of cellular debris
and in the repair of the extracellular matrix (ECM). If there is a severe disruption of
the ECM, fibroblasts respond with an overproduction of ECM resulting in the clinical
condition of fibrosis, or scar tissue accumulation (Huard et al. 2002). The prevention
of scar tissue accumulation is an important goal in the initial treatment of muscle
injuries, as this scar tissue is disruptive to the normal function of muscle tissue and,
once formed, is relatively permanent (Järvinen et al. 2005). Clear evidence shows that
recovery from contraction-induced injury is impaired in muscles of old compared
with adult animals (Brooks and Faulkner 1990; McArdle et al. 2004), but the basis for
the regeneration defects remain an active area of investigation (Carlson et al. 2009;
Conboy et al. 2003). Moreover, the impaired regenerative potential of skeletal muscle

in old animals is associated with an increase in tissue fibrosis (Brack et al. 2007).
7 Contribution of Lateral Transmission of Force
to Contraction-Induced Injury
A contraction-induced injury to a muscle fiber occurs when a segment, or segments,
within the fiber contains groups of sarcomeres that are weaker than the sarcomeres
in series with them (Fig. 6). The weaker sarcomeres normally receive lateral sup-
port from the adjacent sarcomeres in the myofibrils surrounding them through
intermediate filament proteins, including desmin, located at the z-discs (Fig. 7a).
The desmin anchors each of the z-discs of a myofibril to the z-lines of each of the
surrounding myofibrils so that the force generated by each myofibril is transmitted
laterally, providing stability for all of the myofibrils within a fiber. For the myofi-
brils that are immediately adjacent to the sarcolemma of a fiber, the z-discs are
anchored into the sarcolemma by costameres (Fig. 7a). The costameres (Fig. 7)
include the dystrophin-associated glycoprotein (DAG) complex, a portion of which
extends into the ECM. The DAG appears to be situated in a position suitable for the
transmission of the force laterally through the sarcolemma into the ECM. The lat-
eral transmission of force continues without decrement through the intermediate
filaments at each z-disc from myofibril to myofibril throughout the muscle fiber
(Fig. 7b) and then through costameres from fiber to fiber throughout the muscle.
This concept is supported by the successful demonstration of the lateral transmis-
sion of force from a maximally activated single fiber partially dissected free in a