Correlation of bone density to strength and physical activity in young men with a low or moderate level of physical activity.

The objective of this study was to evaluate the relationship among bone mineral density (BMD), physical activity, muscle strength, and body constitution, in young men with a low or moderate level of physical exercise. Another aim was to investigate whether the head is unaffected by physical activity. The subjects consisted of 33 Caucasian healthy men, mean age 24.8 +/- 2.3 years. BMDs of the total body, lumbar spine (L2-L4), femoral neck, Ward's triangle and trochanter, humerus, and head were measured using dual-energy-X-ray absorptiometry (DXA). Bivariate correlations were measured among the different BMD sites and age, weight, height, body mass index (BMI), fat mass, lean body mass, amount of physical activity (hours/week), hamstrings strength, and quadriceps strength. Significant predictors were found for all BMD sites except the head. Using all these variables, only 6% of the variation in BMD of the head could be explained, whereas 46% (total body), 31% (humerus), 17% (lumbar spine), 38% (femoral neck, Ward's), and 41% could be explained for the trochanter. Physical activity and muscle strength were found to be independent significant predictors of BMD of the total body and the sites at the proximal femur. These results suggest that at the time of peak bone mass attainment, physical activity is an important predictor of the clinically relevant proximal femur in young men with a low or moderate level of physical activity. Furthermore, since head BMD was not related to the level of physical activity, we suggest that head BMD may be used as an internal standard, to control for selection bias, in studies investigating the effect of physical activity on bone mass.