Morphological Responses of Human Skeletal Muscles to Decreased Mechanical Loading

Abstract

This paper summarized previous findings on the morphological responses of human skeletal muscles in decreased mechanical loading conditions, i.e., in bed rest, limb immobilization and space flight, and to discuss how intensity and duration of muscle contractions is needed for preventing muscle atrophy in these conditions. The anatomical cross-sectional areas or volumes of lower limb muscles in a given unloading condition linearly decrease with increasing the time of unloading if the duration is less than about 60 days. Within this duration, space flight induces a greater atrophic change in the lower limb muscles as compared to bed rest and limb immobilization. In addition, the difference in the type of decreased mechanical loading produces different atrophic changes between muscles and between the positions of a muscle group. Resistance exercises with a load for force development at 70% ~ of maximum voluntary contraction or one repetition maximum is effective as a countermeasure to muscle atrophy during exposure to unloading condition. On the other hand, there is evidence suggesting that even if a resistive exercise cannot produce hypertrophy in ambulatory condition, it will be an effective measure for preventing muscle atrophy during unloading condition. With regard to the minimal effective training volume as countermeasure, further investigation is needed.