Unloading the rat hindlimb results in a decrease in mass, especially in those muscles that normally have a load-bearing function. The present study was designed to evaluate the effect of intermittent periods of weight support in ameliorating this atrophic response. Adult male Sprague-Dawley rats were assigned to either a control (CON), a hindlimb suspended (HS), or a hindlimb suspended plus intermittent weight support (HS-WS) group. HS-WS rats were walked slowly on a treadmill at 0.2 m/s and a 19% incline for 10 min, every 6 h. After 7 d, the in situ mechanical properties of the soleus (Sol) and medial gastrocnemius (MG) were studied. Body weights of HS and HS-WS rats were 9 and 13% lower than CON. The SOl weight relative to body weight was 21 and 9% lower in HS and HS-WS than CON. Maximum tetanic tension relative to muscle mass was significantly lower in HS than CON, whereas HS-WS had values similar to CON. The MG weight relative to body weight was significantly lower in both suspended groups. The maximum tetanic tension relative to muscle weight was significantly elevated in HS-WS compared to CON, suggesting that weight support may have preferentially maintained the contractile protein component of the muscle. Contraction times were 25% faster (p less than 0.05) in the Sol and unchanged in teh MG of HS rats. For each muscle, the fatigue properties were similar in all groups. These data indicate that a low-force, short-duration exercise regime results in a significant functional recovery in the "slow" Sol, whereas the "fast" MG is less affected.(ABSTRACT TRUNCATED AT 250 WORDS)