Abstract

The purpose of the study was to compare the effects of 2 different recovery modes from hindlimb suspension-induced hypodynamia on soleus muscle growth and composition and size changes of different fiber types within this muscle. Following 28 d of tail-suspension, rats were sedentarily recovered (HS), or were exercised 5 d/wk for one month (HR). Atrophied soleus muscle and reduced soleus wt/body wt ratio (both 60 of control) had returned to control values by day 7 of recovery in both suspended groups despite the fact that whole body wt gain was significantly reduced in HR compared to HS. Atrophied soleus Type I fiber mean cross-sectional area in both HR and HS demonstrated similar and significant increases during recovery. Increases in Type IIa and IIc fiber area during this period were significant only in HR. While the area of muscle composed of Type I fibers increased in both hypodynamic groups during recovery, the reduction in area percentage of muscle made up of Type IIa fibers was again only significant in HR. As to Type IIc fiber percentage, a biphasic response was seen in the soleus of both HS and HR during recovery. Type IIc fiber area increased by day 7 (HR) and day 14 (HR, HS) of recovery but returned to control values by day 28. Large pale-centered targetoid fibers within the same muscle, previously associated with denevation, were observed in HR and HS. The results indicate that exercise recovery following atrophy produces additional changes in soleus fiber size and composition not seen in paired sedentary rats. Verf.-R.