Definition: Appendicular skeletal muscle mass (mass divided by height squared) more than 2 standard deviations below the mean for a young healthy reference population.  Measured by dual x-ray absorptiometry (DXA)

Mass
With age, lean body mass declines as fatty mass increases due to:   

• Decrease in muscle fiber (cell) mass and fiber number
• Reduction in both type IIa and type IIb fibers (fast twitch/power)
• Type I fibers are retained (slow twitch/postural or endurance)

Function     
Strength declines with age:

• Peak torque declines in both concentric and eccentric muscle action
  Concentric- muscle shortens
  Eccentric- muscle resists lengthening

Muscle fatigability increases with age:

• Decreased oxidative enzyme activity and peak VO2

Structure of the whole muscle
Decline in type II fibers
Loss of contractile proteins:

• Actin & Myosin which shorten the fibers

Increase in connective tissue and fat
Weight gain in adults is associated with an increase in absolute and percent fat mass. Lean body mass decreases with aging but body weight does not change due to the accumulation of body fat.

The loss of lean body mass partially results from the loss of individual muscle fibers, or motor units.

Progressive loss of motor units:

• Motor Unit= 1 nerve innervates multiple muscle fibers

The increase in connective tissue and fat mass and corresponding decrease in muscle fibers reduce the amount of muscle tissue available to perform the body's normal metabolic and locamotive functions.


References for this page (3,22,41,5)