Background: Muscle tissue has an exceptional ability to regenerate, however, unresting damage to the muscles by intense and frequent exercises occasionally causes prolonged muscle fatigue, soreness, and underperformance in sports. Taking rest is generally considered to be crucial for regular training to avoid the accumulation of muscle damage. We hypothesized that differences in the resting intervals between two periods of exercise may result in histological differences in muscle regeneration. Method: An eccentric contraction model of mouse gastrocnemius muscle was made using percutaneus electrical stimulation. The mice received eccentric exercises twice with resting intervals of 0, 12, 24 hours, 2, and 3 days. The authors investigated the ratio of myofibers with central nuclei to whole myofibers histologically (the centronuclear cell ratio; CNCR) at 14 days after the second exercise as an index of the muscle regeneration. Results: The CNCR of the group that exercised one-time was 29.5%. In the groups exercised twice, it increased from 31.8% with an interval of 0 hours to a peak of 43.9% with 24 hours, then decreased to 32.8% with an interval of 3 days. The ratios of the groups with intervals of 12 and 24 hours were higher than those with one-time exercise and those with the intervals of 0 hours, 2 days, and 3 days. Conclusions: The resting interval between two periods of eccentric exercises affected the histology of muscle regeneration. The amount of muscle damage and/or the recovery process of damaged muscles should vary depending on the length of resting interval between strenuous exercises. An appropriate interval for rest must be necessary in order to avoid further muscle damage.
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