Rat hindlimb muscle responses to suspension hypokinesia/hypodynamia

Hypokinetic/hypodynamic (H/H) whole body suspension of rats eliminates hindlimb load bearing functions while permitting continued use of the forelimbs. Responses of hindlimb muscles were assessed in terms of absolute and relative weights during 1 and 2 weeks of H/H suspension. Muscle mass loss was in the order soleus greater than gastrocnemius = plantaris greater than extensor digitorum longus (EDL). The soleus, a postural antigravity muscle composed mainly of slow twitch fibers, was most sensitive, losing 35% and 45% of its weight during the first and second weeks, respectively. The gastrocnemius and plantaris showed losses during the first week but no significant loss during the second week. The EDL showed little or no weight loss. During post suspension recovery all muscles showed a weight gain. H/H suspended rats failed to grow; following removal from suspension they gained weight linearly, comparable to controls. Products of muscle metabolism including urea, ammonia, and 3-methylhistidine increased in the urine during H/H suspension and were significantly reduced approaching control levels during recovery. This suspension model offers considerable promise for comparison with H/H responses during weightlessness.