Hammami and Khan raise two important points about our study [1]. In response to their first concern, we point out that the dominant-to-nondominant side difference in upper-arm and forearm circumference and muscle strength was always significantly greater in players than in controls (see Table 2 of our paper). When this criterion is used, the increase in bone mineral content is associated with the increase in upper-arm and forearm circumference and isometric muscle strength. The absolute values of these indices, however, were not higher in players than in controls, and we do believe that exercise (or "impact-type of mechanical loading") can increase bone mineral content without showing any significant increase in muscle bulk and strength. This finding has been confirmed in animal experiments in which a passive in vivo mechanical loading of limb bones has resulted in increased bone mineral content, bending strength, and other biomechanical characteristics of the loaded bone [2, 3]. Also, we must consider that our strength measurements were not specific for tennis stroke and that other tests might have yielded different results.

As described in our paper, none of the controls was involved in intense physical training (or activities affecting one arm only), and we therefore believe that the side-by-side comparison between players and controls was appropriate.

It was clear that in this study, years of training (or any other variable describing training history) did not confound the results of players analyzed by the biological age at which playing began. Considering the basic principles of the effects of training on any musculoskeletal tissue, this finding was not a surprise. Namely, the response of bones and muscles to exercise follows the typical saturation curve in which a previously untrained organ response—whether increased by increasing training frequency, intensity, or duration—does not lead to a similar increase in the response. Thus, we have reason to believe that the maximum benefit from tennis loading was achieved within 5 years of training, the minimum required for inclusion in our study. As stated in our Discussion section, an important undetermined issue remains the minimum amount of exercise and minimum number of years of playing required to achieve the observed increases in bone mineral concentration and whether more exercise, perhaps involving an entirely different type of additional loading, could further increase bone mass.