Differential effects of reduced muscle use (hindlimb unweighting) on skeletal muscle with aging.

The changes in hindlimb muscle mass, fiber area and contractile tension that occurred with two weeks of reduced weight-bearing (HU) were of greater magnitude in 6-month-old as opposed to 36-month-old rats. The pattern of change following HU for young and old animals differed, which may indicate that multiple mechanisms are responsible for the observed changes. The majority of old rats had difficulty with ambulation following unweighting, suggesting that the functional consequences of reduced weight-bearing are greater in old than in young animals. One hour of weight-bearing during HU attenuated the decline in SOL fiber atrophy, muscle mass and Po, but had no apparent effect on the GAST, PL or EDL. The reduction in Po with HU was not due, except for the old PL, to changes at the neuromuscular junction.