Development of rat muscle during short- and long-term hindlimb suspension.

Histochemical and contractile properties of developing rat soleus (Sol) and plantaris (P) muscles were studied after hindlimb suspension to determine the effects of reduced activity levels on muscle development. Suspension (S) began at age 18 days and lasted for 14, 28, and 206 days, and results were compared with age-matched controls. Body weights were normal until 14 days and Sol growth was inhibited more than P, weighing 38 and 47% of controls at 46 and 224 days compared with 68 and 59% in P. The Sol did not develop into a slow-twitch (ST) muscle as evidenced by faster times to peak tension and half-relaxation times, faster times to develop 50% of maximum tetanic tension (Po) and a mean of 33% fewer ST fibers. Twitch tension and Po were lower in S-Sol and S-P, but force/cross-sectional area was unchanged. Fiber areas were smaller, but no structural changes characteristic of disuse atrophy were found. Fiber type populations were unchanged in P, and contractile properties were only minimally affected, demonstrating the greater importance of activity for ST muscles during development.