We investigated the effect of exercise on muscle atrophy and histo-pathological changes induced by hindlimb suspension (HS) as an experimental model of disuse in 4-, 10-, and 20-months old rats. Maximum tetanic tension (Pmax) of the soleus muscle (SOL) decreased significantly with HS but the degree of tension decline did not differ between three age groups (70, 76, 75% in 4-, 10- and 20-months old, respectively). This decrease was more pronounced than a decrease in muscle wet weight (39-41%) and fiber cross-sectional area (55-61%). In SOL of HS, core-like lesions and ragged-red fibers appeared in addition to muscle fiber atrophy. Also, interstitial space increased with HS. These changes were observed at any age. Therefore, the tension decline due to HS was responsible for muscle fiber atrophy and pathological changes. Isometric exercise (30 minutes per day, 6 days per week) during HS counteracted tension decline,but less effective in 20-months old than 4-months old. Muscle fiber atrophy and pathological changes decreased with isometric exercise. Pmax per total fiber cross-sectional area of suspended-exercised groups was similar to that of control group in 4-months old rats but was lower in 20-months old rats. Consequently, it was assumed that the factors other than suppression of muscle fiber atrophy were associated with age-related difference in effect of exercise on tension decline.

DESCENTE SPORTS SCIENCE Vol.19/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE