It is now thought that the critical property of bone is strength rather than weight, and that control of bone strength is mainly exercised through the effect of the mechanical loads brought to bear on bone. Muscle contraction places the greatest physiological load on bone, and so the strength of bone must be adapted to muscle strength (the functional muscle-bone unit). The Utah paradigm of skeletal physiology [J Hum Biol 1998;10:599–605] provides a model of bone development that describes how bone structure is regulated by local mechanical effects that can be adjusted by the effects of hormones. The DONALD (Dortmund Nutritional and Anthropometric Longitudinally Designed) study analysed the interaction between the muscle and bone systems in males and females before and during puberty. This study found that differences between the genders in bone adaptation during puberty are at least partly driven by the influence of oestrogen in females. Testosterone seems to have no direct relevant effect on bone during puberty, but may be implicated in the greater amount of muscle mass achieved in boys compared with girls.