Muscles begin to lose mass, a process called atrophy, after only a few days of disuse, according to the American Academy of Orthopaedic Surgeons. However, how fast you lose muscle mass depends upon many factors. During immobilization, muscles that have been used frequently lose mass quicker than those that are not. A person's age also influences the rate of muscle atrophy.
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Rate of Muscle Loss
In healthy young men, the gastrocnemius muscle -- the large calf muscle in the back of the lower leg -- loses approximately 8.4 percent of its mass and 22.9 percent of its strength after 14 days of immobilization, according to Benjamin T. Wall et al. in "Disuse Impairs the Muscle Protein Synthetic Response to Protein Ingestion in Healthy Men." In another study, healthy retirement-age people who spent 10 days on bedrest lost approximately 2.2 pounds of weight from their lower extremities compared to less than 1.1 pounds of weight loss in young adults after 28 days of bedrest, according to William J. Evans in "Skeletal Muscle Loss: Cachexia, Sarcopenia, and Inactivity."