The primary outcomes of the study are bone mass accretion and changes in bone mineral density, which we will determine by dual energy x-ray absorptiometry at the beginning and end of the study. We will do activity assessments throughout the study period to determine whether participation in the gross motor activity group also increases spontaneous activity in these children. Anthropometric measurements and dietary information will allow us to statistically control for these potential confounders. We will obtain additional bone mass and physical activity measurements 12 months after completion of the program to determine if these interventions have long-term effects on bone mineral density and physical activity.