Figure 2. Comparison of muscle mass and soleus fiber cross-sectional area changes in adult and old rats subjected to hindlimb unloading (HU) and reloading. Adult (9 mo) and old (29 mo) male rats underwent HU for 14 days or underwent HU for 14 days and then were allowed to resume normal weight bearing activity for either 1, 3, 7, or 14 days. Average masses of the soleus (A), plantaris (C), medial gastrocnemius (D), extensor digitorum longus (E), and tibialis anterior (F) muscles of adult (open squares) and old (filled squares) rats at the various HU and reloading time points (n=6-7/group). (B) Changes in fiber cross-sectional area (CSA) were measured in the soleus of adult (open squares) and old (filled squares) rats after 14 days of HU and after 3, 7, and 14 days of reloading. Fiber CSA was determined from laminin-stained cross sections (n=5-6/group). Values are mean ± SEM, *p<0.05 vs adult control, #p<0.05 vs old control, φp<0.05 vs adult at same time point.