Physical Therapists can help underweight patients begin a program of weight training. The purpose of weight training is to increase muscle mass, thereby strengthening the entire body for better overall health. Other exercises, including aerobic exercise or running are generally not recommended as these activities may cause patients to lose additional weight. It’s important for a Physical Therapist to design the best possible weight and muscle-gaining exercise program for patients. Because being underweight is generally associated with other health conditions, exercise is best undertaken with the supervision of a professional.