Objective: To determine in female tennis and squash players the effect of biological age (that is, the starting age of playing relative to the age at menarche) at which tennis or squash playing was started on the difference in bone mineral content between the playing and non-playing arms.
Design: Cross-sectional study.
Setting: Finnish tennis and squash federations.
Participants: 105 female Finnish national-level players and 50 healthy female controls.
Main outcome measures: Differences in bone mineral content in playing and nonplaying (dominant to nondominant) arms (proximal humerus, humeral shaft, radial shaft, and distal radius) were compared in the players and controls and among six groups of players. Players were divided into groups according to the biological age (years before or after menarche) at which their playing careers 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 years after.
Results: Compared with controls (whose mean +/- SD differences 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 difference 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 mineral content were significant (P < 0.001 to P = 0.005), with the group means clearly decreasing with increasing starting biological age of playing. The difference was two to four times greater in the players who had started their playing careers before or at menarche (lowest mean difference in bone mineral content, 10.5% +/- 7.2%; highest difference, 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 height) did not change these trends.
Conclusions: Bones of the playing extremity clearly benefit from active tennis and squash training, which increases their mineral mass. The benefit of playing is about two times greater if females start playing at or before menarche rather than after it. The minimal level and minimum number of years of activity necessary to produce these results, the extent to which this benefit is sustained 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.