EFFECT OF LONG-TERM UNILATERAL ACTIVITY ON BONE-MINERAL DENSITY OF FEMALE JUNIOR TENNIS PLAYERS

High peak bone mass in early adulthood is an important protective fact or against osteoporotic fractures in later life, but little is known a bout the effects of exercise on growing bone. The purpose of this cros s-sectional study was to determine at which state of maturity (Tanner stage), the areal bone mineral density (BMD) differences between the p laying and nonplaying arms of junior tennis players become obvious, an d to clarify in each developmental stage which training and background variables, if any, could explain the interindividual differences in b ones) response to mechanical loading. Ninety-one 7- to 17-year-old fem ale tennis players and 58 healthy female controls were measured. In ea ch Tanner stage, differences in BMD in playing and nonplaying (dominan t and nondominant) arms (proximal humerus, humeral shaft, and distal r adius) and BMD of the lumbar spine and nondominant distal radius,were compared between the controls and players. Within each Tanner stage of players, the associations between training and background variables a nd BMD differences were analyzed with Spearman rank correlation coeffi cients. In players, BMD differences between the playing and nonplaying arms were significant (P < 0.05- < 0.001) in all Tanner stages, with the mean difference ranging from 1.6 to 15.7%. In controls, these domi nant-to-nondominant arm differences were clearly smaller (ranging from -0.2 to 4.6%), but significant at some measured sites. In comparison with the relative side-to-side arm differences between the players and controls (i.e., examination of the training effect), the mean differe nce was not obvious and significant until the adolescent growth spurt (i.e., the girls in Tanner stage III with a mean age of 12.6 years). I n the lumbar spine, significant BMD differences between players and co ntrols were not found until Tanner stage IV (mean age 13.5 years; 8.7% , P < 0.05) and V (mean age 15.5 years; 12.4%, P < 0.05). In a nonload ed site of the skeleton (nondominant distal radius), no significant BM D differences were found between the players and controls in any Tanne r stage. In the correlation analysis, the Tanner I and II players (mea n ages 9.4 and 10.8 years) showed no significant associations between any of the predictive variables and the side-to-side BMD differences, while in Tanner stages III, IV, and V, such associations could be foun d; the total amount of training hours during the playing career and th e number of training sessions per week showed a significant and system atic correlation (r(s) ranging from 0.43 to 0.80) with the side-to-sid e BMD differences in several measured bone sites. In conclusion, this study suggests that in a majority of female junior tennis players, the benefit of unilateral activity on bone density does not become clearl y evident until the adolescent growth spurt or Tanner stage m. The tot al amount of training during the player's career and the current train ing frequency (sessions per week) seem to best explain the training ef fect on bone tissue, leaving, however, room for speculation on the sti ll unknown factors that modulate the loading response of a growing bon e.