Muscle atrophy is defined as a decrease in the mass of the muscle; it can be a partial or complete wasting away of muscle, and is most commonly experienced when persons suffer temporary disabling circumstances such as being restricted in movement and/or confined to bed as when hospitalized. When a muscle atrophies, this leads to muscle weakness, since the ability to exert force is related to mass. Modern medicine's understanding of the quick onset of muscle atrophy is a major factor behind the practice of getting hospitalized patients out of bed and moving about as active as possible as soon as is feasible, despite sutures, wounds, broken bones, and pain.
Muscle atrophy results from a co-morbidity of several common diseases, including cancer, AIDS, congestive heart failure, chronic obstructive pulmonary disease, renal failure, and severe burns; patients who have "cachexia" in these disease settings have a poor prognosis. Moreover, starvation eventually leads to muscle atrophy.
Disuse of the muscles, such as when muscle tissue is immobilized for even a few days – when the patient has a primary injury such as an immobilized broken bone (set in a cast or immobilized in traction), for example – also leads rapidly to disuse atrophy. Minimizing such occurrences as soon as possible is a primary mission of occupational and physical therapists employed within hospitals working in co-ordination with orthopedic surgeons.
Neurogenic atrophy, which has a similar effect, is muscle atrophy resulting from damage to the nerve that stimulates the muscle, causing a shriveling around otherwise healthy limbs. Also, time in a near-zero g environment without exercise leads to atrophy, partially due to the smaller amount of exertion needed to move about, and because muscles are not used to maintain posture. In a similar effect, patients with a broken leg joint undergoing as little as 3 weeks of traction can lose enough back and buttocks muscle mass and strength as to have difficulty sitting without assistance, and experience pain, stress, and burning even after a short 10-minute exposure, when such positioning is contrived during recovery.