In contrast to alpine skiing and soccer, the nonweight-bearing environment of swimming may have elucidated an adaptational response necessary to increase the strength to weight ratio of the skeleton. This could allow for the optimization of the skeleton that is beneficial for a swimmer, where the skeleton can withstand applied forces in their sport and training, while simultaneously limiting the weight of the skeleton. Although it is possible that optimization of the skeleton has occurred in swimmers due to their loading environment, it is also possible that

swimmers are naturally equipped with this type of bone structure, and are therefore more likely to continue in their sport. It has previously been shown that genetics account for

approximately 60–80% of the variance in bone structure [57], [58] and [59], and it seems very likely that self-selection bias exists for MK0683 bone parameters on a larger scale that correlate highly with body size and shape, for example total cross-sectional area of a bone. However, regarding other parameters such as Ct.BMD in this sample, particularly after adjusting for body size, it seems more plausible that an adaptational response has occurred, and any other self-selection bias would not depend on specific bone traits, but instead neuromuscular and fitness traits. For example, it MAPK Inhibitor Library molecular weight seems more likely a child who has better coordination, easier access to sporting activity, gains enjoyment from the sport, and has particular advantages pertaining to large-scale

structure (e.g. height), may be directed into particular sports, but not solely because of inherited bone traits. Nevertheless, we cannot disregard the possibility of self-selection bias, and therefore must consider it as a potential reason for observable 3-mercaptopyruvate sulfurtransferase differences in bone traits across sporting activities. We note important limitations of this study. First, the cross-sectional design does not allow for evaluation of causal relationships between loading occurring during sporting activity and bone quality, and this data may also be affected by selection bias. Due to this possibility, our findings should be considered hypothesis generating, and as such, they provide a foundation for future prospective studies. Second, our health history questionnaire revealed a history of menstrual cycle disturbances in four female subjects (one alpine skier, three controls) and these may have lead to alterations in bone metabolism in these participants. However, we did not adjust for history of amenorrhea/oligomenorrhea in our analysis, as these subjects were not identified as outliers for bone parameters. Third, we did not measure vitamin D intake nor did we obtain serum samples of serum 25(OH)D. Thus, we cannot rule out the possibility that seasonal variation in vitamin D levels may have influenced our findings. Fourth, HR-pQCT scanning is limited to the distal radius and distal tibia, sites of minimal or no muscle insertion points.