Effects of immobilization on rat hind limb muscles under non-weight-bearing conditions

The effect of stretched and unstretched immobilization of a hind limb on the concentration and the metabolism of proteins in the hind-limb muscles of rats was investigated. The animals were divided into three groups: (1) weight-bearing controls, (2) tail-cast-suspended, and (3) suspended, with one hind limb immobilized with the ankle in dorsiflexion (30-40 deg angle) and the other freely moving. It was found that unloading the hind limbs for 6 days by tail cast suspension caused soleus to atrophy and reduced growth of the gastrocnemius and plantaris muscles; unloading resulted in a higher degradation rate and lower synthesis rate in both in vitro and in vivo. Chronic stretch of the unloaded soleus not only prevented its atrophy but led to significant hypertrophy, relative to weight-bearing controls, with increases in both the sarcoplasmic and myofibrillar protein fractions. Immobilizing one ankle in dorsiflexion prevented the inhibition of growth in the plantaris and gastrocnemius muscles due to unloading.