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Age Related Changes in Skeletal Muscle

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					Aging changes of the
  Musculoskeletal
      System
       Age-related reductions in muscle mass are
a direct cause of declines in muscle strength with
aging.
       This reduction in muscle strength is a
major cause of disability in the older adult since
strength and power are major components of
gait, balance, and the ability to walk.
Age-Related Structural Changes
      in Skeletal Muscle
       1-Reductions in Muscle Mass
        Aging is associated with decreases in total muscle cross-
sectional area, amounting to approximately 40% between the ages of 20
and 80 years.
        Reductions in leg muscle cross-sectional area have been
observed to begin in early adulthood and accelerate beyond 50 years of
age.
        This reduction in muscle cross-sectional area is accompanied
by increases in non-contractile structures such as fat and connective
tissue.
        Thus, the girth or volume measurements of muscle, used
commonly in the clinic, may not show the actual reduction in the
contractile proteins responsible for force generation (actin and
myosin).
        The rates of skeletal muscle protein synthesis decline with
age and may also contribute to muscle atrophy and repair process after
injury.
2-Reduction in Muscle Fiber Number
          The total number of muscle fibers is significantly
reduced with age, beginning at about 25 years and progressing at
an accelerated rate thereafter.
          The decline in muscle cross-sectional area is most likely
due to decreases in total fiber number, especially type II fast-
twitch glycolytic fibers.
          The loss of muscle fibers is followed by a replacement
with fat and fibrous tissue and a gradual increase in non-muscle

tissue.
  3-Changes in Muscle Fiber Size
        The size of the individual fast-twitch type II fibers decreases
with age (vastus lateralis, tibialis anterior, and biceps brachii), whereas the
slow-twitch type I fiber size does not change.
        For example, in the third or fourth decade of life, the mean cross-
sectional area of individual fast-twitch type II fibers in the quadriceps
femoris muscle exceeds that of slow-twitch type I fibers by approximately
20%.
        By the age of 85 the area of individual fast-twitch type II fibers
is less than 50% of that of slow-twitch type I fibers.
            In addition, small, angulated fibers and grouped atrophy are
commonly seen in muscles of older men and women. These morphological
changes are similar to changes that are observed in skeletal muscle
diseases.
  4-Motor Unit Number and Size
        There is a decrease in total number of motor units
with age. The average motor neuron loss from the second to
tenth decade is approximately 25%.
        The decrease in motor unit number is accompanied
by an increase in size or innervation ratio, such that on
average, each motor neuron innervates more muscle fibers
in the older adult.
        The increase in size of the motor unit is found
primarily in the muscles of the lower limb, particularly in
persons 60 years old, and more in distal than in proximal
muscles.
                        5-Axon
        The loss of motor neurons is accompanied by a
reduction in both the numbers and diameters of motor axons.
        There is an age-related reduction in the numbers of
large and intermediate myelinated ventral root fibers but no
significant reduction of the small nerve fibers.
        Quantitative electromyography (EMG) has shown
changes in both duration and amplitude of motor unit action
potentials with increasing age.
        Axonal nerve conduction velocities of all motor nerve
fibers are slowed with aging.
        This suggests that with aging, the alterations in
conduction velocities could reflect a variety of changes in the
nerve fibers, such as segmental demyelination, and a reduced
internodal length.
        In addition, the mean soma size of motor neurons is
reduced with age with accumulation of lipofuscin.
  6-Altered Motor Unit Remodeling
       Motor unit remodeling is the natural cycle of
turnover of synaptic connections occurring at the
neuromuscular junction by the process of denervation,
axonal sprouting, and reinnervation of the muscle.
       In aging, however, it appears that motor unit
remodeling is altered.
       Motor unit remodeling is altered such that type II
fibers are selectively denervated and reinnervated by
collateral sprouting of axons from fibers of the slow
motor units.
       Type II fibers, which become reinnervated by slow
motor unit axons, actually become type I fibers.
       The fast motor unit axons degenerate when they no
longer innervate muscle fibers.
Age-Related Changes in
 Muscle Performance
                          1-Strength
        Age-related decreases in strength have been well-
documented. Healthy men and women in their seventh and eighth
decades of life demonstrate average reduction of 20% to 40% in
maximal isometric strength in various muscles.
        Arm, leg, and back strength decline at an overall rate of 8%
per decade, starting in the third decade of life.
        Loss of muscle strength in leg muscles is greater than loss in
arm muscles between the ages of 30 and 80 (40% compared with
30%).
        Weight-bearing muscles showed greater changes than non-
weight-bearing muscle.
It is conceivable that age related weakness may be caused by:

1) Decreased central drive and thus a decrease in ability to

voluntarily activate a muscle.

   The threshold of excitability of the corticospinal tract increases
progressively with age and is significantly higher in the elderly.

2) Loss of muscle mass and therefore in the number of force

generating cross bridges interacting between actin and myosin; a

decrease in the force developed by each actin and myosin cross-

bridge.

3) Changes in the intrinsic properties of the remaining skeletal

muscle fibers.
                          2- Power
       The power output is governed by:
(1) The velocity of shortening and
(2) The force-generating capacity of the muscle.
         Aging reduces the peak forces that can be generated by
muscles at fast contraction speeds, resulting in a decrease in peak
power output. Peak power output declines by about 20% with
age.
         The decrease in power output results from motor unit
remodeling, which reduces the fast to-slow-fiber ratio.
         This reduced capacity for rapid-force generation might
also limit the ability to respond quickly to a loss of balance and
                       3-Endurance
         A decline in muscular endurance is a feature of old age that
contributes to functional loss and disability.
         Alterations in muscle, both contractile and metabolic, with
advanced age may contribute to the decrease in muscle endurance.
         The alterations include :
  1-Reduced blood supply and capillary density and
  2-Impairment of glucose transport and therefore of substrate
availability,
  3-Lower mitochondrial density, decreased activity of oxidative
enzymes, and decreased rate of phosphocreatine repletion.
     4-Blood Flow and Capillarity
         Sustained muscular performance requires a proper balance
between energy supply and demand.
         Capillary density decreases with age and appears to be due to an
actual reduction in the total number of blood capillaries, as both capillary-
to-fiber ratio and the number of capillaries in contact with each muscle
cell is lower in the aged.
         The reduction in capillarity has major importance for the ability of
muscle to sustain power output over time.
         In summary, maintenance of muscle mass and strength may be
critical for maintaining independent function in the elderly.
         Muscle mass and muscle strength decline with advancing age, and
the underlying mechanisms responsible for the altered muscle performance
are unclear but most likely involve multiple levels.

				
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