The medical term for muscle wasting or shrinking from disuse is muscle atrophy. It occurs when muscle tissue loses mass, often after immobilization, bed rest, injury, or prolonged inactivity.
Disuse atrophy reflects a shift in the muscle’s normal protein balance: protein breakdown exceeds protein synthesis. A casted limb may become visibly smaller, and weakness often accompanies the loss of muscle mass. With appropriate activity and rehabilitation, disuse-related atrophy can sometimes be partly or largely reversed.
Atrophy is a broad medical term, so muscle loss can also result from aging, malnutrition, nerve damage, certain medications, or disease. Age-related muscle loss is commonly called sarcopenia, while disease-related wasting may involve cachexia. These terms overlap with atrophy but are not interchangeable in every context.
The alternatives describe different processes. Hypertrophy means an increase in muscle size; hyperplasia means an increase in cell number; and dystrophy refers to a group of muscle diseases, some of which can cause atrophy.