Aims/hypotheses

* We will determine whether habitual exposure to high impacts influences peak bone mass (PBM), by investigating whether these impacts explain the relationship between habitual physical activity and PBM as measured by DXA

* We will examine whether effects of high impacts on PBM translate into greater bone strength, by studying relationships between high impacts and cortical bone strength as assessed by pQCT

* We will determine if a critical time exists in bone development when high impacts exert the greatest long term effects on PBM

* We will investigate whether relationships between exposure to high impacts and PBM represent a causal effect, as opposed to common dependence on intrinsic muscle strength (assessed by jumping mechanography), given physical activity affects muscle strength and muscle strength independently affects bone mass

* We aim to identify whether factors which influence PBM act via altered responsiveness to high impact activity

o We will determine whether individuals with low fat mass show weaker relationships between high impact activity and PBM. Whether such a relationship is explained by altered calorie intake will be investigated, along with the contribution of altered eating behaviour

o We will study whether interactions exist between high impact activity, PBM and constitutive factors affecting cortical bone development eg insulin, adiponectin, bone resorption

* What are the other benefits of high impact exercise for musculoskeletal health? We will investigate whether high impact exercise is associated with musculoskeletal pain, or aspects of hip shape implicated in the pathogenesis of osteoarthritis.