 After the baseline investigation is over, subjects will be randomized into one of two groups exercise or no exercise for 16 weeks. Strength training will be performed using standard free weights under the supervision of a trained physical therapist or member of the study staff. Each session begins with a 5 minute warm-up and a standard flexibility routine. The strength training routine will follow and will focus on the following muscle groups: knee extensors and hip extensors, ankle plantar flexors, elbow flexors, pectoralis and shoulder abduction. The aerobic training program will follow the general guidelines established by the American College of Sports Medicine. Training will be performed using a standard stationery bicycle. Aerobicendurance exercise will be performed with large muscle groups. The duration of the aerobic component of the training session will be 20 minutes during the first two weeks and 30 minutes thereafter. The intensity of the exercise will be 60 of maximal heart rate during the first two weeks and 75 thereafter. As the weeks progress, the repetitions and intensity of the exercise will be gradually increased. Subjects return to the GCRC at 8 weeks for a medical history, a physical exam, weight, and pregnancy test. Subjects return again at 16 weeks for a visit identical to that at baseline. 