Muscle mass is more strongly related to hip bone mineral density than is quadriceps strength or lower activity level in adults over age 50 year.

This cross-sectional study examined whether reduced hip bone mineral density (BMD) is better explained by isokinetic knee extensor strength (KES), lower limb lean body mass (L-LBM), or Physical Activity Scale for the Elderly (PASE). Through population-based recruitment, 1543 adults without knee osteoarthritis were recruited. For men and women respectively, means+/-SD were age 60.8+/-8.0 and 61.1+/-7.9 yr; body mass index 29.6+/-4.6 and 29.1+/-5.4 kg/m(2); hip BMD 1.025+/-0.138 and 0.895+/-0.128 g/cm(2); KES 124.9+/-41 and 72.7+/-22.9 N.m; L-LBM 10.3+/-1.5 and 7.0+/-1.2 kg; and PASE 206.4+/-99.7 and 163.8+/-77.0. The relationship between BMD and KES in men (r(2)=0.21, p> or =0.002) and women (r=0.23, p<0.001) was significant before adjustment. However, this association was no longer significant after controlling for L-LBM. Even after controlling for age, race, and sex, the association between BMD and KES was better explained by L-LBM (partial R(2)=0.14, p<0.001) than by PASE (partial R(2)=0.00). Allometric scaling of KES to body size attenuated the association of BMD with KES (Std Beta=0.03). The significant association between BMD and L-LBM (Std Beta=0.36) remained stronger than that between BMD and weight (Std Beta=0.21). Therefore, muscle mass accounted for a greater proportion of the variance in hip BMD than KES or activity level and explained a significant proportion of the association between weight and BMD.