How long does it take for muscle to atrophy if you are bedridden?

Age and disease: It depends on the age of the patient and the underlying condition. Elderly are affected more severely. As a rough guide an elderly person loses about 10% of muscle function per day of immobility. If there are other factors, e.g., fever, nutritional impairment, trauma, and some medications, e.g., steroids, the muscle atrophy is worsened.