Variations over time in proximal femoral strength in young adult men are not explained by areal bone mineral density alone

The decline in areal bone mineral density (aBMD) in men may start as early as age 19, but it is debated whether proximal femoral strength follows the same pattern. We measured femoral neck aBMD in a cross-sectional cohort of 1101 men aged 18-28 yr with two-dimensional dual energy X-ray absorptiometry (DXA). Body height, weight, jump height, grip strength, and maximum knee extension torque muscle strength were measured with standard tests, whereafter body mass index was calculated. We adopted a validated method to automatically obtain three-dimensional finite element (FE) models from 2D DXA images and predict proximal femoral strength. We compared femoral neck aBMD and FE-predicted proximal femoral strength in relation to age. Linear mixed-effect models were used to evaluate the association between age (whole years), femoral neck aBMD, femoral neck area, jump height, grip strength, maximum knee extension torque, and FE-predicted proximal femoral strength. A negative correlation (r = −0.10, p = .001) was observed in femoral neck aBMD and age between ages 18 and 28 while no statistically significant correlation (r = −0.02, p = .55) between age and FE-predicted proximal femoral strength was found. There was a positive correlation between FE-predicted proximal femoral strength and the subjects’ height (r = 0.22, p < .001) and weight (r = 0.35, p < .001). Higher jump height (r = 0.13, p < .001), grip strength (r = 0.23, p < .001), and maximum knee extension torque (r = 0.31, p < .001) were associated with higher FE-predicted proximal femoral strength. Femoral neck aBMD, femoral neck area, and body height explained ~78% of the variations in proximal femoral strength in the linear mixed-effect models. The inter-subject variations in proximal femoral strength were associated with femoral neck aBMD, femoral neck area, body height, and higher outcomes in jump height, grip strength, and maximum knee extension torque.