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How to keep muscle mass
Modify your training routine.
Regardless of why you had to cut back or stop your training routine, there are still exercises you can do to help you maintain your strength. If time is an issue, consider doing a more complete circuit 1-2 times a week.
If you had to stop training because of an injury, keep working the uninjured areas of your body. A doctor or physical therapist may be able to help you figure out appropriate exercises to maintain muscle without aggravating your injury.