This review summarizes current information on structural and functional changes that occur during muscle atrophy and hypertrophy. Most published studies consider an increase in total mass of a muscle as hypertrophy, whereas a decrease in total mass of a muscle is referred to as atrophy. In hypertrophy, the rate of synthesis is much higher than the rate of degradation of muscle contractile proteins, leading to an increase in the size or volume of an organ due to enlargement of existing cells. When a muscle remains in disuse for a long period, the rate of degradation of contractile proteins becomes greater than the rate of replacement, resulting in muscle atrophy. This defect may occur as a result of lack of nutrition, loss of nerve supply, micro-gravity, ageing, systemic disease, prolonged immobilization or disuse. An understanding of the specific modifications that occur during muscle atrophy and hypertrophy may facilitate the development of novel techniques, as well as new therapies for affected muscles.

These results suggest that short periods of low-intensity, low-stimulation frequency (20 Hz) ES of muscle during periods of inactivity could maintain changes in both morphologic and histologic properties of the slow-twitch muscle fibers (soleus). Short periods of low-intensity, high-stimulation frequency (30 Hz) ES of muscle during periods of inactivity could maintain changes in both morphologic and histologic properties of the fast-twitch muscle fibers (extensor digitorum longus).