Effects of hypokinesia/hypodynamia on contractile and histochemical properties of young and old rat soleus muscle.

To determine if the atrophic process was different in the young and in the aged slow-twitch soleus muscle, in which the aging process is clearly apparent, hind limb hypokinesia/hypodynamia was induced. After 3 weeks, we measured speed-related indices, tension indices, and tension producing capacities. Fiber type composition and fiber cross-sectional area were also investigated. After the treatment, body weight and muscle weight decreased, the most important loss being measured in the young group. The ratio of muscle weight to body weight was not altered by hypokinesia/hypodynamia but this ratio was greater in the 3-month-old control group than in the 22-month-old group. Contraction time and half-relaxation time decreased in both groups. In the control group, the old soleus muscle was slower to contract and relax than in the young muscle. Tension indices and tension producing capacities decreased. These included twitch tension, maximally developed tension, force generated per gram muscle, and specific tension. The mean muscle area and muscle weight of the young soleus decreased more than that of the old soleus. The fiber cross-sectional area was reduced in both groups whereas fiber type composition remained the same after hypokinesia/hypodynamia. The suspension atrophy appeared to be independent of age in the soleus at the contractile level whereas atrophy was greater in the young muscle at the morphometric level.