DIMENSIONS AND ESTIMATED MECHANICAL CHARACTERISTICS OF THE HUMERUS AFTER LONG-TERM TENNIS LOADING

This study evaluated the effects of longterm unilateral physical activ ity (tennis) on the playing arm humerus. Total lengths of both humeri, site-specific widths, and the bone mineral contents (BMC) at the prox imal, middle, and distal parts of the bones were measured using dual-e nergy X-ray absorptiometry (DXA). Bone mineral apparent density (BMAD) , cortical wall thickness (CWT), cross-sectional moment: of inertia (C SMI), and section modulus (Z) mel-e approximated from the DXA data for describing the bone's mechanical characteristics more concretely. The study population consisted of 67 healthy, competitive tennis players (17 young men with a mean age [+/- SD] of 25 +/- 5 years, 30 young wom en with a mean age of 19 +/- 3 years, and 20 older women with a mean a ge of 43 +/- 5 years) and 57 sedentary controls (16 young men with mea n age of 25 +/- 5 years, 25 young women with a mean cage of 21 +/- 3 y ears, and 16 older women with mean age of 39 +/- 6 years). All the pla yers had competitive playing histories greater than 4 years. Tile youn g male and female players had started their playing careers in childho od (men at the he of 10 +/- 3 years, women 9 +/- 2 years), while the o lder female players started the training at adulthood (29 +/- 6 years) . The playing-to-nonplaying or dominant-to-nondominant arm differences in humeral length ranged from +0.2 to +1.4%, the difference being sig nificant in young male players (+1.4%), young female controls (+1.1%), and older female players (+0.7%). When comparing players' relative si de-to-side length differences with those of the controls, no significa nt differences were found. Significant side-to-side differences in hum eral width were observed in all groups except male controls. Compared with the controls, the relative side-to-side width differences were si gnificantly larger at the proximal humerus of the young male players ( controls +1.2%, players +3.7%) and the distal humerus of young female players (controls -0.2%, players +1.6%). Compared with the controls, t he players' relative side-to-side differences in BMC (range, +7.6 to 25.2%), BMD (+5.8 to +22.5%), BMAD (+5.5 to +20.4%), CWT (+6.9 to +45. 2%), CSMI (+7.8 to +26.4%), and Z (+3.0 to +21.7%) were significantly larger in all measured humeral sites except BMAD in the distal humeri of the older female players. These relative side-to-side differences w ere dearly and significantly larger in the young players (+11.7 to +45 .2%) than in the older-players (+3.0 to +12.4%). In conclusion, long-t erm intensive tennis playing, especially if started in childhood or ad olescence, clearly increases the humeral BMC, BMD, and CWT but seems t o have only a minor effect on the width of this particular bone. In th is respect, there seems to be no sw difference. However, dong with the increases in mineral mass and density, the changes in bone width are important in increasing the bending stiffness and strength of the hume rus. In older players, the relative side-to-side differences are at th e same level or only slightly larger than those in their age-matched c ontrols, This suggests that even intense physical loading of a mature bone is only marginally better in increasing the bone mass, bone densi ty, and CWT of the target bone than the normal daily use of the domina nt extremity.