Effects of weight bearing intervals on disuse atrophy of rat soleus muscle.

The purpose of this study was to investigate the effects of weight bearing at varying intervals in suppressing the progression of disuse muscle atrophy, while setting the total daily weight bearing time constant. Disuse muscle atrophy was induced by 2-week hindlimb suspension. Thirty male Wistar rats ( weight : 215 ± 8 g ) were divided into 6 groups ( 5 rats/group ) : control (CON), suspension alone (SUS), two sessions of 30-minute weight bearing at intervals of 4, 8, and 12 hours during suspension, and one session of 60-minute weight bearing at intervals of 24 hours during suspension. Weight bearing was done each day during the daytime. Histochemical staining, followed by morphometrical analysis using NIH Image, demonstrated that the cross-sectional area of type I fiber in SUS was 44% of that in CON, while in the weight bearing groups ranged from 62 to 70%. The proportion of type I fibers was lower in SUS and tended to increase as the interval of weight bearing become longer, indicating the effects of weight bearing at intervals of 12 and 24 hours. For both types I and II, the distribution of muscle fiber size indicated that weight bearing at longer intervals was more effective in keeping the cross-sectional areas of muscle fibers closed to that in CON. In conclusion, when suppressing the progression of disuse atrophy of rat soleus muscle by weight bearing of one hour per day, the results suggest that the weight bearing intervals is important factor.