Ten years muscle-bone hypothesis: what have we learned so far?--almost a festschrift--

The importance of mechanical stimuli for bone is widely appreciated. Mechanostat theory proposes a negative feedback system to explain the adaptation of bone by homeostatic control of peak strains. However, no assumption is made as to which forces cause these strains. Biomechanical analyses suggest that the largest forces emerge from muscle contractions, rather than from body weight per se. Hence, the idea of a 'muscle-bone' unit emerged ten years ago, proposing that bones adapt to muscle strength. This muscle-bone hypothesis is well able to account for the accrual of bone mass and strength during childhood, and also to explain why certain types of exercise are able to prevent bone loss during immobilization. However, the hypothesis fails to explain why exercise becomes rather ineffective to increase bone strength after puberty. It is here proposed that joint size as a 'third agent' might solve the conundrum. More specifically, the assumptions are made that the peak forces determine joint size until the end of puberty, and that motor control limits joint reaction forces to critical limits during adulthood in order to prevent joint damage. Providing evidence in favour or against these conjectures will improve our understanding of the musculoskeletal system.