Muscle mass is gained by combining a consistent workout regimen with a high-protein diet. When muscles are exercised to their limit, micro-tears are created in the muscle tissue and the body creates stronger muscle fiber to compensate and prepare for the next exertion. Eating a diet high in protein gives the body the raw material it needs to create muscle tissue.
Before beginning any exercise regimen, consult a physician or certified personal trainer. Diet and exercise myths abound, and it's easy to be disappointed by less-than-fast results. Make a schedule for workouts; the key is to create a balance between constantly forcing muscles to adapt and giving them enough time to rebuild. A good rule of thumb is to allow one day of rest for a given major muscle group after a workout.
The best way to gain muscle mass is by using resistance training exercises. Repeatedly lifting weights will force the body to develop more muscle as a response to perceived stress. After a workout, eat a high-protein meal, and eat a light snack about half an hour before the workout. The total protein intake should be about 0.8 grams per pound of body weight. Also, eat at least 2,500 calories per day, mostly protein, some carbohydrates and little fat.