Bone mineral density varies throughout the skeleton of athletes dependent on their sport: Allometric modelling identifies the "effective" forces associated with body mass

Objectives: Bone mineral density (BMD) varies throughout the skeleton with the differences influenced by the type of sport, body mass and participant's age. What is not so well understood is how body mass influences BMD across different sites. Design: Proportional allometric modelling on cross-sectional data. Methods: Male athletes (n = 106) from 8 different athletic groups and controls (n = 15) were scanned by dual X-ray absorptiometry (DXA) and analysed for total body composition and BMD of the ribs, arms, thoracic spine, lumber spine, pelvis and legs. Results: ANOVA identified significant differences in BMD between “sites”, “sports” and site-by-sport interaction (all p < 0.001). By introducing body mass and age as covariates, the “sites” differences disappeared. The explanation is due to the systematic difference in body-mass exponents (k _i), with lowest positioned sites (e.g. legs) having the steepest slopes, and the elevated sites (e.g. arms having the shallowest slopes). To illustrate, the arm-mass exponent was approximately 0.33 indicating that for a 75 kg male, arm BMD responds to forces associated with body mass = (75) ^0.33 = 5 kg. For the same individual, the leg-mass exponent was closer to 0.66 suggesting that leg BMD responds to forces associated with body mass = (75) ^0.66 = 25 kg. Conclusions: The model for BMD identified that body mass ^{k _i} plays a crucial role in determining the effective forces (both gravitational/ground reaction and compressional forces) operating throughout the skeleton, where k _i systematically increases where the lower the bone is situated, a mechanism that also explains why activities involving running benefit the leg BMD compared with weight-supported activities (e.g., rowing).