It can be caused by immobility, aging, malnutrition, medications, or a wide range of injuries or diseases that impact the musculoskeletal or nervous system.

Disuse causes rapid muscle atrophy and often occurs during injury or illness that requires immobilization of a limb or bed rest.

Malnutrition first causes fat loss but may progress to muscle atrophy in prolonged starvation and can be reversed with nutritional therapy.

Finally, diseases of the muscles such as muscular dystrophy or myopathies can cause atrophy, as well as damage to the nervous system such as in spinal cord injury or stroke.

Given the implications of muscle atrophy and limited treatment options, minimizing immobility is critical in injury or illness.

[1] The predominant symptom is increased weakness which may result in difficulty or inability in performing physical tasks depending on what muscles are affected.

[2] Skeletal muscle serves as a storage site for amino acids, creatine, myoglobin, and adenosine triphosphate, which can be used for energy production when demands are high or supplies are low.

[4] Disuse is a common cause of muscle atrophy and can be local (due to injury or casting) or general (bed-rest).

There are many proposed mechanisms of sarcopenia, such as a decreased capacity for oxidative phosphorylation, cellular senescence or an altered signaling of pathways regulating protein synthesis,[9] and is considered to be the result of changes in muscle synthesis signalling pathways and gradual failure in the satellite cells which help to regenerate skeletal muscle fibers, specifically in "fast twitch" myofibers.

[10] Sarcopenia can lead to reduction in functional status and cause significant disability but is a distinct condition from cachexia although they may co-exist.

[14] More widespread damage such as in traumatic brain injury or cerebral palsy can cause generalized muscle atrophy.

This involves complex cell signalling that is incompletely understood and muscle atrophy is likely the result of multiple contributing mechanisms.

Protein needs may vary dramatically depending on metabolic factors and disease state, so high-protein supplementation may be beneficial.

[26][27][28] Based upon a meta-analysis of seven randomized controlled trials that was published in 2015, HMB supplementation has efficacy as a treatment for preserving lean muscle mass in older adults.

[29] More research is needed to determine the precise effects of HMB on muscle strength and function in various populations.

[29] In severe cases of muscular atrophy, the use of an anabolic steroid such as methandrostenolone may be administered to patients as a potential treatment although use is limited by side effects.

"[3] Loss of lean body mass is also associated with increased risk of infection, decreased immunity, and poor wound healing.

[31] In humans, prolonged periods of immobilization, as in the cases of bed rest or astronauts flying in space, are known to result in muscle weakening and atrophy.

Bears are an exception to this rule; species in the family Ursidae are famous for their ability to survive unfavorable environmental conditions of low temperatures and limited nutrition availability during winter by means of hibernation.

[citation needed] During hibernation, bears spend 4–7 months of inactivity and anorexia without undergoing muscle atrophy and protein loss.

[35] Immobilization of one of the hindlegs of mice leads to muscle-atrophy as well, and is hallmarked by loss of both muscle mass and strength.