Muscle fiber number in immobilization atrophy.

Eight Sprague-Dawley rats 10 to 12 weeks of age had one limb immobilized by plaster in knee flexion and ankle plantar flexion for four weeks. The soleus muscles from the control and immobilized limbs were excised and frozen. The technique used insured that cross sections subsequently cut and stained would contain all the fibers extending from the origin to the insertion aponeuroses. The total number of muscle fibers per muscle was counted from 25 to 50 photomicrographs made for each muscle cross section. Paired comparisons of total muscle fibers in control and immobilized muscles revealed no significant differences. The total number of fibers in control and immobilized soleus muscles was 2812 +/- 521 and 2930 +/- 403, respectively. Therefore, muscle fiber loss does not occur in disuse atrophy produced by external immobilization as has been reported.