Muscle tissues are likely to atrophy (shrink and weaken) if they are not used for too long a time.

Thus, illness or injuries that cause paralysis or an extended period of immobility for the body or a part of it must be followed by a medically supervised program of physical therapy to rebuild the weakened muscles.

Causes
Some muscle atrophy occurs normally with aging. Other causes may include: Alcohol-associated myopathy
Amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease)
Burns
Dermatomyositis and polymyositis
Guillain-Barre syndrome
Injury
Long-term corticosteroid therapy
Malnutrition
Motor neuropathy (such as diabetic neuropathy)
Muscular dystrophy
Not moving (immobilization)
Osteoarthritis
Polio
Rheumatoid arthritis
Spinal cord injury

Home Care

An exercise program (under the direction of a therapist or doctor) is recommended to help treat muscle atrophy. This may include exercises in water to reduce the muscle workload, and other types of rehabilitation.
People who cannot actively move one or more joints can do exercises using braces or splints.