Ja. Faulkner et al., MUSCLE ATROPHY AND WEAKNESS WITH AGING - CONTRACTION-INDUCED INJURY AS AN UNDERLYING MECHANISM, The journals of gerontology. Series A, Biological sciences and medical sciences, 50, 1995, pp. 124-129
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.