With disuse, e. g. that resulting from tail-suspension, skeletal muscle shows a decrease in both mass and strength, with concomitant atrophy. Type IIc fibers are observed during muscle fiber generation, regeneration and fiber type transformation. Thus, the recovery process of atrophied muscle fibers can be investigated by observing changes in type IIc fiber distribution. In this study, the behavior of type IIc fibers was examined in rats during recovery following 5 weeks of tail-suspension. Male rats were assigned to three groups : control (C), tailsuspension (S), and pair-weight (P), in which body weight was adjusted to that of the S group by controlled food consumption.
The results were as follows : 1) After 5 weeks of tail-suspension, the weight of the m. soleus in the S group was less than 40% of that in the C group. 2) Weights of all muscles removed in the S group increased rapidly until 2 weeks into the recovery period, and returned to the level in the P group by 5 weeks. 3) Type IIc fiber distribution in the m, soleus in the S group was increased by about 35% after 5 weeks of tail-suspension, and had not returned to the level in the C group by 5 weeks of recovery. 4) Phanocytosis and central nuclei were observed in the atrophic muscle fibers of the S group.