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					Age-Related Changes
 in Skeletal Muscle
           By
   Dr. Michael Banoub
   Skeletal muscle undergoes major structural and
    functional adaptations in response to physical
    inactivity, just as it does in response to disease,
    nutritional status, and obesity.

   Age-related reductions in muscle mass are a
    direct cause of declines in muscle strength and
    disability in the older adult
      Reductions in Muscle Mass
   Aging is associated with decreases in total
    muscle cross sectional area, to approximately
    40% between the ages of 20 and 80 years

   This accompanied by increases in noncontractile
    structures such as fat and connective tissue.
The maintenance of muscle mass occurs as a result
 of the balance between protein synthesis and
 degradation.

The rates of skeletal muscle protein synthesis
 decline with age and may also contribute to
 muscle atrophy
Reduction in Muscle Fiber Number
The decline in muscle cross sectional area is due to
 decreases in total fiber number, especially type II
 fast-twitch fibers. The loss of muscle fibers is
 followed by a replacement with fat and fibrous
 tissue
     Changes in Muscle Fiber Size
   The size of the individual fast-twitch type II
    fibers decreases with age 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.
     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, 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.

                   Why?
                 Axon


There is an age-related reduction in the
 numbers of large and intermediate
 myelinated fibers but no significant
 reduction of the small nerve fibers.
      Age-Related Changes in Motor
                Neurons
   Aging beyond 60 years is associated with a reduction in
    number of lumbosacral spinal cord motor neurons.
    These decreases appear to be caused by losses of the
    largest alpha motor neurons and their myelinated axons
    in lumbar ventral roots
   The surviving segmental neurons increase in branching
    complexity and exhibit additional collateral growth
    increased load due to increased innervation ratios to the
    muscle fibers they innervate.
The collected evidence strongly suggests that as
 age increases beyond 60 years, muscle undergoes
 continuous denervation and reinnervation, due
 to an accelerating reduction of functioning
 motor units Initially, reinnervation can
 compensate for this denervation. However, as
 this neurogenic process progresses, more and
 more muscle fibers become permanently
 denervated and subsequently replaced by fat and
 fibrous tissue
     Age-Related Changes in Muscle
             Performance
   Arm, leg, and back strength decline at an overall
    rate of 8% per decade, starting in the third
    decade of life.
   Healthy men and women in their seventh and
    eighth decades of life demonstrate average
    reductions of 20% to 40% in maximal isometric
    strength in various muscles.
   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 muscles.

   The age-associated reduction of quadriceps
    muscle strength is such that the average 80-year-
    old is at or near the minimum level of strength
    required to rise from a chair.
Metabolic Pathways: Glucose Uptake
   Aging is associated with decreased glucose
    tolerance and a greatly increased incidence of
    noninsulin-dependent        diabetes     mellitus
    (NIDDM).
   That is linked to the age-associated changes in
    body composition and activity levels. Improved
    fitness as a result of aerobic exercise improves
    glucose tolerance, and exercise prevents the
    onset of NIDDM.
Insulin resistance is primarily due to defects in skeletal
  muscle with age. there is an age-associated decline in
  the insulin-receptor signaling system at the cellular level
  which has a significant impact on function.
       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

				
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posted:11/12/2011
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