EFFECT OF STARTING AGE OF PHYSICAL-ACTIVITY ON BONE MASS IN THE DOMINANT ARM OF TENNIS AND SQUASH PLAYERS

Objective: To determine in female tennis and squash players the effect of biological age (that is, the starting age of playing relative to t he age at menarche) at which tennis or squash playing was started on t he difference in bone mineral content between the playing and nonplayi ng arms. Design: Cross-sectional study. Setting: Finnish tennis and sq uash federations. Participants: 105 female Finnish national-level play ers and 50 healthy female controls. Main Outcome Measures: Differences in bone mineral content in playing and nonplaying (dominant to nondom inant) arms (proximal humerus, humeral shaft, radial shaft, and distal radius) were compared in the players and controls and among six group s of players. Players were divided into groups according to the biolog ical age (years before or after menarche) at which their playing caree rs began: more than 5 years before; 3 to 5 years before; 2 to 0 years before; 1 to 5 years after; 6 to 15 years after; and more than 15 year s after. Results: Compared with controls (whose mean +/- SD difference s in bone mineral content were 4.6% +/- 4.6%, 3.2% +/- 2.3%, 3.2% +/- 3.8%, and 3.9% +/- 4.3% at the previously noted anatomical sites), the players had a significantly (P < 0.001) larger side-to-side differenc e in every measured site (15.5% +/- 8.4%, 16.2% +/- 9.8%, 8.5% +/- 6.6 , and 12.5% +/- 7.1%). Among players, the group differences in bone mi neral content were significant (P < 0.001 to P = 0.005), with the grou p means clearly decreasing with increasing starting biological age of playing. The difference was two to four times greater in the players w ho had started their playing careers before or at menarche (lowest mea n difference in bone mineral content, 10.5% +/- 7.2%; highest differen ce, 23.5% +/- 7.2%) than in those who started more than 15 years after menarche (lowest difference, 2.4% +/- 4.8%; highest difference, 9.6% +/- 4.9%). Adjustment for potential confounding factors (age and heigh t) did not change these trends. Conclusions: Bones of the playing extr emity clearly benefit from active tennis and squash training, which in creases their mineral mass. The benefit of playing is about two times greater if females start playing at or before menarche rather than aft er it. The minimal level and minimum number of years of activity neces sary to produce these results, the extent to which this benefit is sus tained after cessation of intensive training, and the degree to which these results can be extended to other forms of physical activity and other bone sites should be studied further.