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If you do not exercise at all, your heart muscle, and all the muscle of the body will gradually weaken. Muscle of both types requires exercise (and for the heart this means sustained exercise at a slightly higher rate than your resting heart rate, called aerobic exercise). Of course with your history you need to consult your doctor to obtain an exercise program that is gradual. Cardiac rehab would be very helpful if you have not had this in the past. They can teach you to monitor your heart rate and how to know when and how much to exercise.
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Thank you! Here is a source for you that may provide some tips to get him started: