High impact activity is related to lean but not fat mass: findings from a population-based study in adolescents

Background Objective measures of physical activity calibrated against energy expenditure may have limited utility in studying relationships with musculoskeletal phenotypes. We wished to assess an alternative approach using an accelerometer calibrated according to impact loading. Methods Of the 17-year olds from the Avon Longitudinal Study of Parents and Children (ALSPAC), 732 wore Newtest accelerometers while performing day-to-day activities for a mean of 5.8 days. Outputs were categorized as light, moderate, high and very high impact, based on the thresholds identified in 22 adolescents during graded activities. In subsequent regression analyses, activity data and fat mass were normalized by log transformation. Results The number of counts relating to high impact activity was similar to 2% that of light impact activity, and 33% greater in boys when compared with girls. High impact activity was more strongly related to lean mass [light: 0.033 (95% CI -0.023 to 0.089), moderate: 0.035 (95% CI -0.010 to 0.080) and high: 0.044 (95% CI 0.010 to 0.078)] (beta = SD change in outcome per doubling in activity, height adjusted, boys and girls combined). In contrast, lower impact activity was more strongly related to fat mass [light: -0.069 (95% CI -0.127 to -0.011), moderate: -0.060 (95% CI -0.107 to -0.014) and high: -0.033 (95% CI -0.069 to 0.003)]. In a more fully adjusted model including other activity types and fat/lean mass, lean mass was related to only high activity (boys and girls combined), whereas fat mass was related to only moderate activity (girls only). Conclusions Using an accelerometer calibrated according to impact loading revealed that high impact activity is related to lean but not fat mass.