A developmentally dynamic physical activity intervention has been designed and pilot tested-one that engages the caregiver as a partner. Using techniques and tools as far-ranging as Dual X-ray Absorbiometry (DXA), smart phones, doubly labeled water, and lightweight, wireless accelerometers developed specifically for this purpose, the working hypothesis is that the one-year intervention will augment lean body mass (primary outcome variable) and improve bone mineralization and the ratio of lean to fat tissue (secondary outcome variables). The study will gauge the impact of the physical activity intervention on the balance between circulating anabolic mediators (insulin-like growth factor-I and growth hormone binding protein) and inflammation-associated cytokines (interleukin-6, and interleukin-1 receptor antagonist), which antagonize muscle and bone growth.