Effects of aging on unloading-induced skeletal muscle atrophy and subsequent recovery in rats

Aged individuals with physiological muscle atrophy (sarcopenia) are likely forced into an unloading condition of inactivity such as bed rest and limb-immobilization. Accumulated experimental evidence, obtained with rats, indicates no definite aging effects on the extent of unloading-induced muscle atrophy. However, various suggestions can be deduced from experimental evidence as guidelines for the development of a combined intervention of exercise, nutrition and physical therapies that effectively maintain skeletal muscle as healthy as possible in aged individuals under unloading condition. The deduced guidelines could be summarized as 1) during the unloading period, atrophy and degeneration of especially slow muscle fibers should be attenuated, 2) in the early stage of the reloading period, massive muscle damage should be avoided, and 3) in the recovery period, re-growth of especially slow muscle fibers should be accelerated. This review also emphasizes that experimental evidence is still lacking, indicating the urgent need to stimulate research interest in the combined effects of aging and immobilization so as to improve health longevity in aging societies.