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Exercise itself creates metabolic changes. During a workout, muscle glycogen is depleted and the breakdown of muscle protein increases.  But in an effort to restore balance, the body produces enzymes and hormones that favor glucose uptake and the synthesis of glycogen and protein by the muscle cells when the workout ends.  Sports medicine research using stable isotope tracer techniques shows that enzyme levels remain elevated for only about 30-45 minutes.  After that time, the levels return to normal and the unique opportunity ends.