LOCAL BONE-MINERAL DENSITY, MUSCLE STRENGTH, AND EXERCISE IN ADOLESCENT BOYS - A COMPARATIVE-STUDY OF 2 GROUPS WITH DIFFERENT MUSCLE STRENGTH AND EXERCISE LEVELS

The primary objective of the present study was to evaluate the impact of physical activity and muscle strength on bone mineral density (BMD) of the tuberositas tibiae in adolescent boys. Two groups with differe nt exercise levels were compared. The high activity group consisted of 20 subjects (age 15.9 +/- 0.3) from a junior ice hockey team. The ref erence group consisted of 24 volunteers (age 15.9 +/- 0.3) not trainin g for more than 3 hours per week. The groups were matched for age, wei ght, and pubertal stage. BMDs (g/cm(2)) of the tuberositas tibiae and proximal tibia were measured using dual energy X-ray absorptiometry. Q uadriceps strength was significantly higher in the high activity group (P < 0.01). Univariate correlations were measured between tuberositas tibiae BMD and pubertal stage, weight, height, BMI, fat mass, lean bo dy mass, quadriceps strength, and hamstrings strength in the high acti vity group and the reference group, respectively. Quadriceps strength was estimated to be the best significant predictor of BMD of the tuber ositas tibiae in the reference group. A multivariate analysis confirme d this result. In the high activity group, there was no significant pr edictor of BMD of the tuberositas tibiae. There was no significant dif ference in BMD at this site when comparing the two groups. However, fi ve of the boys in the high activity group had a former history of Mb O sgood-Schlatter with a significantly lower BMD of the tuberositas tibi ae than the rest of the boys in that group. After exclusion of these b oys, the remaining 15 boys were matched against 20 boys from the refer ence group using the previous criteria. These 15 boys then showed a si gnificantly higher BMD of the tuberositas tibiae (P < 0.05) but not of the proximal tibia than the 20 boys in the reference group. In conclu sion, this study demonstrates site-specific increments of tuberositas tibiae BMD in adolescent ice hockey players unless they are affected b y the negative effects on BMD by Mb Osgood-Schlatter. These increments seem primarily to be associated with forceful muscle contractions rel ated to high quadriceps strength and not greater weight-bearing loadin g. Muscle strength seems to positively affect BMD of the tuberositas t ibiae in adolescents, but only up to a certain level, above which addi tional muscle strength has no effect.