SKELETAL-MUSCLE WEAKNESS IN OLD-AGE - UNDERLYING MECHANISMS

Maintenance of muscle mass and strength contributes to mobility which impacts on quality of life. Although muscle atrophy, declining strengt h, and physical frailty are generally accepted as inevitable concomita nts of aging, the causes are unknown. Clarification of the mechanisms responsible for these changes would enhance our understanding of the d egree to which they are preventable or treatable. The decline in muscl e function between maturity and old age is similar for muscles of many different animals including human beings, and is typified by the decr eases of similar to-35% in maximum force, similar to 30% in maximum po wer, and 20% in normalized force (kN.m(-2)) and power (W.kg(-1)) of ex tensor digitorum longus (EDL) muscles in old compared with adult mice. Much of the age-associated muscle atrophy and declining strength may be explained by motor unit remodeling which appears to occur by select ive denervation of muscle fibers with reinnervation by axonal sproutin g from an adjacent innervated unit. Muscles in old mice appear more su sceptible to injury than muscles in young or adult mice and have a dec reased capacity for recovery. The process of age-related denervation m ay be aggravated by an increased susceptibility of muscles in old anim als to contraction-induced injury coupled with impaired capacity for r egeneration.