Regaining muscle after being sick, starving or abstaining from activity for long periods of time is possible, but care should be taken to ensure adequate nutrition is also consumed. Too many calories can result in rapid weight gain, primarily in the form of fat. A healthful, balanced diet with calories sufficient for the amount of work done throughout the day and in weight-training workouts is acceptable. Light weight training, at least initially, will help build strong muscles quickly as well.
Start slowly and engage in light weight training two to three days a week. This should include using light weights for biceps curls, triceps extensions, lateral raises, shoulder presses and chest presses. In addition to the weight routine, perform squats, lunges, and calf raises to help build lower-body muscles. Allow a 24-hour rest period between each workout.
Walk briskly for 30 to 60 minutes five to seven days a week. This will help improve cardiovascular health as well as encourage muscle growth in the lower body, abdomen and back. Keep a brisk pace---3 to 4 mph is a good target speed. Keep the abdominal muscles tight while walking, hold the arms with elbows bent at 45-degree angles, and gently twist your torso slightly with each step to work the abdominal muscles, back muscles and help keep the pace.
Eat a balanced diet for muscle growth and regrowth. Ensure calories consumed do not exceed the calories expended through exercise and daily existence: calories burned simply by breathing, thinking and digestion, for example.
If muscle mass was lost because of starvation, re-feeding slowly is essential to avoid developing weight problems. If the muscle mass was lost because of temporary inactivity, such as during pregnancy or while recovering from an injury, it is likely that adequate nutrition was provided during those times, and re-feeding is not an issue.
Increase weights used in weight training after three or four weeks, as the muscles begin to re-develop and the light weights become too light to make sufficient progress. Weight increases should be in two-pound increments; moving too fast to heavier weights can lead to injury and slowed progress.