CHILD MOBILITY: ROLE OF STRENGTH, BODY FAT & ENERGY COST

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Strength is the most important determinant of mobility status among children with motor impairments. Obesity, a common problem among disabled children, is a second important factor negatively influencing mobility status in disabled children. this study focuses on strength and body fat as determinants of energy cost of ambulation and wheelchair locomotion in children with variable degrees of mobility impairment due to myelomeningocele (MM), spinal cord injury (SCI), cerebral palsy (CP), and Duchenne Muscular dystrophy (DMD). Specifically, the primary aims of the proposal will address two hypotheses: 1) excess body fat increases the energy cost of ambulation and wheelchair locomotion over and beyond the increased costs associated with lean tissue deficits and weakness, and 2) higher energy cost of locomotion is associated with reductions in daily total energy expenditure (TEE) and physical activity; reduced physical activity will be associated with increased adiposity. The study population consists of 375 male and female children ages 6 to 20: 90 MM, 45 SCI, 120 CP, 30 DMD, and 90 controls. Measurements include: 1) anthropometrics, 2) whole body and regional body composition by dual-energy x-ray absorptiometry (DXA), 3) quantitative isometric and isokinetic strength at the elbow and knee, 4) resting metabolic rate (RMR) by indirect calorimetry, 5) energy cost of ambulation and/or wheelchair locomotion (ml O2/Kg meter and Kcal/Kg meter) via a portable telemetric metabolic, measurement system, and 6) TEE and physical activity index (TEE/RMR ratio) by continuous three day heart rate (HR) monitoring. The energy cost of locomotion will be accessed at baseline and with added weight via a pelvic belt equivalent to a 10% increase in % body fat and a 20% increase in % body fat. We will determine the relative importance of weakness and decreased ratio of lean tissue to total body mass in producing increased energy cost of locomotion, reduced physical activity (as evidenced by the TEE:RMR ratio) and a risk of increased adiposity for disabled children. Objective measures of both energy economy of locomotion and physical activity are functionally relevant outcomes. Establishment of a relationship of both impaired strength and increased adiposity to increased energy cost, and hence, reduced physical activity, will provide a basis for varied interventions to improve functional mobility. Outcome studies assessing interventions directed at improving the energy efficiency of locomotion will need to also consider body composition changes over time if there are indeed significant incremental energy costs during locomotion associated with increased adiposity.