High bone mass and altered relationships between bone mass, muscle strength, and body constitution in adolescent boys on a high level of physical activity.

We have recently demonstrated a general relationship between bone mass, muscle strength, and several body constitutional parameters in adolescent boys with a moderate exercise level. The present study was conducted to evaluate these previously described relationships in adolescent boys subjected to high physical activity and also to compare the bone mass of the same group with that of adolescents on a moderate level of physical activity. The reference group consisted of 24 boys, age 15.9 +/- 0.3 years, not training for more than 3 h per week. The ice hockey players consisted of 20 boys, age 15.9 +/- 0.3 years, from an ice hockey junior team training for about 10 h per week. The groups were matched according to age, pubertal stage, and weight. Areal bone mineral density (BMD) was measured in total body, head, humerus, spine, femur, and proximal femur using dual energy X-ray absorptiometry. BMD was significantly higher in the humerus (p < 0.01), femur (p < 0.05), and proximal femur (p < 0.05) in the high activity group. Furthermore, physical activity (h/week) was an independent predictor of humerus BMD (p < 0.01) and proximal femur BMD (p < 0.05), among all subjects investigated (n = 44). Isokinetic muscle strength of the quadriceps and hamstrings muscles was significantly higher in the high activity group (p < 0.05). In the reference group, there was a general strong independent relationship between BMD, muscle strength, and different body constitutional parameters. In the high activity group, muscle strength of the thigh independently predicted BMD of humerus and spine. Furthermore, in the same group, weight, BMI, and fat mass independently predicted only spine BMD. In conclusion, the higher BMD demonstrated in the ice hockey players seems to be site-specific and may well be associated with the type and magnitude of loading from predominantly ice hockey. High physical activity seems to weaken the relationship between BMD, muscle strength, and body constitution demonstrated in adolescent boys on a low or moderate level of physical activity.