A comparison of the effects of unloading in young adult and aged skeletal muscle

Purpose: The objective of this investigation was to determine whether morphological adaptations to unloading are different in young adult and aged skeletal muscle. Methods: Sixteen young adult (8-month) Fischer 344 rats were randomly assigned to either a control or hindlimb suspension (HS) group. Sixteen aged (22-month) rats were similarly assigned to either control or HS conditions. After 4 wk, animals were euthanized and soleus and EDL muscles were histochemically analyzed. Results: In controls, neither the soleus nor EDL displayed a.-c-related differences in fiber size or composition. Unloading elicited fiber atrophy of the soleus in both age groups but to a greater extent (P < 0.05) in aged rats. Only in aged solei were HS-induced fiber type conversions (Type I --> II) detected. In the EDL, unloading caused atrophy only among the aged. Conclusion: These data suggest that aged muscle experiences greater detriment as a result of unloading. This may have important consequences in the aged because they are more likely to be restricted to bed rest or limb immobilization due to falls and other afflictions.