This study will use a latent variable (and negative control) approach to investigate association between adolescent physical activity (PA) profiles and hip bone mineral density (BMD) at age 25. The main analysis will examine relationship between PA patterns over adolescence and age 25 hip BMD by applying (2nd order) latent growth models to repeated measures of PA intensity (light, moderate, vigorous) derived from Actigraph accelerometers at ages 11, 13 and 15. An additional analysis will be performed to explore association between gravitational impacts from PA, assessed at age 17 from Newtest acceleromters, and hip BMD at age 25.