MUSCLE ATROPHY AND WEAKNESS WITH AGING - CONTRACTION-INDUCED INJURY AS AN UNDERLYING MECHANISM

In old compared with young animals, muscle mass is decreased by 30% to 40%, and maximum force and power are decreased to an even greater ext ent. The age-related declines in muscle mass and muscle function are s imilar to those bat occur with decreased physical activity. Despite be similarities, we conclude that the losses in muscle mass, force, and power are not due solely to old animals being less active, but rather accrue from intrinsic age-related changes in muscles and in muscle fib ers that appear to be immutable and irreversible. The intrinsic change s are associated with denervation of fast fatigable fibers and motor u nits and motor unit remodeling, which may be initiated by contraction induced injury. The mechanisms remain unresolved for the weakness, the fatigability, the high susceptibility to contraction-induced injury, and the impaired recovery from injury demonstrated by the skeletal mus cles of old animals.