Lean mass explains the association between muscular fitness and bone outcomes in 13‐year‐old boys

Aim This study investigated the associations between fitness indices and bone outcomes in young males. Methods Data were collected between autumn and winter 2014–2015 on 121 males with a mean age of 13.1 ± 0.1 years: 41 swimmers, 37 footballers, 29 cyclists and 14 nonathletes. Participants were recruited from athletic clubs and schools across South West England. Lean mass, areal bone mineral density and hip structural estimates were measured using dual‐energy X‐ray absorptiometry. The relationships between bone outcomes and the vertical jump, standing long jump and the 20‐m shuttle run test were analysed using three regression models: model 1 was adjusted by age and stature, model 2 added vigorous physical activity and model 3 then added lean mass. Results The boys’ performance in the vertical jump and standing long jump was positively associated with the majority of bone outcomes in models 1 and 2, but most of these disappeared in model 3. The 20‐m shuttle run test was positively associated with most bone outcomes in all three models. Lean mass played a key role in the association between muscular fitness and bone outcomes. Conclusion Vigorous physical activity did not explain the associations between fitness and bone outcomes, but lean mass did.