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- Materials and Methods
The amount of bone gained during childhood and adolescence impacts greatly on lifetime skeletal health, and it is well accepted that physical activity during growth increases bone acquisition.[1-4] Gymnastics training results in a unique mechanical loading to the skeleton and, therefore, provides an excellent model for assessing the effects of weight-bearing physical activity on bone development. Previous studies in young adolescent competitive female gymnasts have shown that they have 8% to 23% greater dual-energy X-ray absorptiometry (DXA)-derived areal bone mineral density (aBMD, cm/g2) at the total body, lumbar spine, and hip, respectively.[6-9] A recent review of prepubertal gymnastics participation found that gymnasts had denser bones in the lower body compared with non-gymnasts; however, the authors suggested that the DXA-derived aBMD of gymnasts may have underestimated the effect of gymnastics participation on bone strength because the gymnasts within the review displayed a trend toward increased bone mineral content (BMC) and had a smaller bone area than non-gymnasts in the lower body. Although bone strength generally trends in the same direction as aBMD and BMC, this is not always the case; bone strength has been shown to significantly improve with exercise training independent of changes in aBMD. In addition, these parameters are not themselves properties that govern strength. Bone strength is determined by structural dimensions (e.g., bone geometry) and material properties. Thus, bone strength may be better assessed by determining structural strength measures such as cross-sectional area (CSA) and section modulus (Z), which provide information about the structural dimensions of bone.
During the growing years, bone can respond to increased mechanical load by the apposition of bone to the endosteal as well as periosteal surface or by diminished resorption at the endosteal surface.[11, 13] Such geometric adaptations would positively affect bone structural strength. Hip structural analysis (HSA) is a program that allows for the estimation of geometric properties at the hip. Elite prepubertal female gymnasts have been found to have significantly greater strength indices at the hip as assessed by HSA; however, competitive gymnastics is a high-level competitive sport and participation is limited to a select number of skilled individuals. Recreational gymnastics, on the other hand, is attainable by most children and does not require a high level of training. Recreational gymnastics involves the development of spatial and body awareness, muscular strength, and neuromuscular coordination. A paucity of information, however, remains about the effect of recreational-level gymnastics participation and bone structural strength during childhood.
Our group has previously reported that the gymnasts in the present study had greater size-adjusted total body and femoral neck (hip) BMC compared with non-gymnasts (3% and 7%, respectively), although, as previously stated, aBMD and BMC may underestimate the influence of gymnastics participation on bone structural strength. There is also a paucity of research examining the effect of gymnastics participation at both the competitive and recreational level on bone strength parameters in males. Therefore, the aim of the present study was to compare bone structural strength, as assessed through geometric indices, at the hip in young children with a current or past participation history in recreational gymnastics against non-gymnastic controls. We hypothesized that young male and female gymnasts would have greater geometric indices of bone structural strength at the hip compared with children with no past history of gymnastics participation.
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- Materials and Methods
The aim of this study was to investigate whether recreational-level gymnastics participation was associated with the development of estimated structural geometry at the proximal femur in young males and females, and whether current or past participation history influenced this structural strength development. It was observed that those individuals who participated in recreational-level gymnastics had significantly greater bone structural strength at all sites of the proximal femur compared with individuals previously involved in training and nongymnastic controls. This is the first study, to our knowledge, to investigate the effects of recreational-level gymnastics participation on bone structural strength at the proximal femur in both males and females during early childhood.
Gymnastics participation has been previously shown to be associated with a 3% to 28% benefit in aBMD and BMC at the total body, lumbar spine, and hip when compared with non-gymnast.[6-9, 14] This magnitude of benefits is similar to the effects documented in the current study. Our results show that those individuals currently involved in recreational gymnasts had approximately 3% to 7% greater CSA and Z at the proximal femur compared with both ex-gymnasts and non-gymnast controls, when adjusted for age, height, weight, vitamin D, calcium intake, physical activity, sex, and the hours of gymnastics training. These observed effects are similar to those previously reported for BMC in the same cohort. This finding would suggest that low levels of continued recreational gymnastics participation provide benefits not only to BMC but also to geometric bone structural strength at the proximal femur. Thus, despite the low levels of gymnastics exposure, the dynamic loading nature of the jumping and tumbling activities associated with the recreational gymnastic training may serve as adequate stimulus for initiating bone structural strength adaptations, supporting previous assertions by Laing and colleagues. Laing and colleagues found that 4- to 8-year-old females participating in recreational gymnastics had significantly greater lumbar spine aBMD and forearm bone area than non-gymnast controls, suggesting that beginner-level gymnastics skills were adequate for stimulating gains to bone mass and area.
These advantages were not observed in the ex-gymnasts involved in the current study. The longitudinal models revealed that those individuals defined as ex-gymnasts did not possess any advantages to CSA and Z at any site of the proximal femur compared with non-gymnast controls, after adjusting for age, height, weight, vitamin D, calcium intake, physical activity, sex, and the hours of gymnastics training. These results would suggest that previous involvement in recreational gymnastics does not confer any advantages to childhood bone geometry. These conclusions, however, may be influenced by the length of time ex-gymnasts were involved in gymnastics. Greater gymnastic exposure, in either number of hours trained or years of training, have been documented to increase total and regional aBMD, suggesting a dose-response relationship between loading and bone mass.[27, 28] Given that the ex-gymnasts were involved in the recreational gymnastics training for <1 to 2 years (data not shown), the shortened exposure time to the recreational gymnastics may be insufficient to precipitate detectable bone structural changes.
The non-gymnasts controls were also a highly active group, evidenced by the non-significant difference in physical activity scores between groups; their similar activity levels to the ex-gymnasts may have resulted in contemporaneous structural strength benefits to a frequently loaded region such as the proximal femur. Supporting this conjecture is the finding that gymnastic exposure appears to provide smaller effects at the femoral shaft site compared with the more proximal NN and IT regions. This may be influenced by the way the skeletal load is perturbed by gymnastic loads at these regions, with greater bending effects being elicited at the two proximal regions and more axial effects at the shaft. These site-specific differences, however, may be better evident at sites uniquely loaded by the recreational gymnastic training, such as the distal radius. Recently, Erlandson and colleagues using peripheral quantitative computed tomography (pQCT), in the same cohort, reported that ex-gymnasts had 5% to 11% greater adjusted cortical bone content, cortical area, and polar strength strain index at the forearm compared with non-gymnasts controls, but these variables were not significantly different at the weight-bearing tibia. Similarly, Ward and colleagues compared bone strength parameters in the upper and lower limbs of young children (8 to 9 years of age) using pQCT to investigate differences between the peripheral axial skeleton of precompetitive gymnasts and age-matched controls. They observed that at the mid radius, gymnasts had significantly greater bone total area, periosteal circumference, cortical area, and bending strength than age-matched controls; however, no significant differences were observed between gymnasts and controls for any bone measures at the 65% tibia. Therefore, the recreational gymnastics in the current study may provide site-specific skeletal benefits to the ex-gymnasts that were not currently assessed. Additionally, retired gymnasts studies have observed that although competitive gymnastics may continue to confer skeletal advantages many years after gymnastic senescence, these benefits experience a similar rate of natural decline over time. Thus, any advantages that may have been accrued by the ex-gymnasts during their short exposure to recreational gymnastics may have ceased to be evident by the end of the current study's investigation period. Further research is necessary to support these speculations and to elaborate on the longevity of any site-specific skeletal advantages and potential dose-response relationship resulting from recreational gymnastics exposure, particularly in these young growing populations.
Although there is a growing body of literature in male gymnasts, most studies investigating the effects of gymnastics training on bone strength measures have focused on female populations. Of the limited male research, it has been observed that male gymnasts have greater calcaneus BMD, total body and femoral neck BMC, and proximal femur CSA and cross-sectional moments of inertia (CSMI).[5, 14, 32] These observations parallel the findings of the current study, which suggests that continued participation in recreational gymnastics bears significant advantages to bone structural strength at the proximal femur in both males and females. In the current study, it was also observed that sex was a significant predictor in the multilevel models for all bone structural strength measures except IT Z, identifying males as having greater CSA at the NN, IT, and S, and greater Z at the NN and S sites. This would suggest that alongside the positive effects of recreational gymnastics participation at this young age to both sexes, sex-specific advantages to bone structural strength may also be apparent regardless of gymnastic participation. This supports the sex-specific effects reported in previous studies that have documented greater cortical thickness, total body BMC, and femoral neck BMC in young male precompetitive gymnasts.[14, 30] These sex-specific differences may be owing to increases in endocortical and periosteal apposition seen in prepubertal boys. Given that type of exercises and their sex-specific adherence were not assessed, follow-up research is required to support the supposition of sex-specific benefits of gymnastics training.
Despite the novel findings of the current study, there are a number of study shortcomings. The observational mixed longitudinal design does not allow for any cause-effect assessments of current or past gymnastics participation on bone parameters. Controlled prospective studies before the initiation of gymnastics training are necessary to definitively address whether cumulative recreational gymnastic exposure is responsible for the observed greater bone structural strength. Physical activity was assessed for all participants in this study; however, the highly active nature of non-gymnasts, who participated in other recreational sports such as soccer, T-ball, basketball, and karate, may reduce or even mask the potential effects of the recreational gymnast participation. Further, the HSA procedure is also associated with a variety of limitations. DXA images are often noisy and blurred, resulting in the difficulty of locating precise edge margins. In addition, the assessment of CSA and Z assumes the mineralization of adult bone. This assumption, however, results in an underestimation of CSA and Z in pediatric populations.[19, 21] Thus, any effects observed may undervalue true differences. Finally, HSA is sensitive to small changes in the positioning of femur.[20, 21] All DXA scans were performed by qualified technologists familiar with proper positioning of the proximal femur to ensure hip scans were performed with care to limit these potential errors; nevertheless, it is difficult to position the hip consistently in repeated measures over time.
In summary, when compared with other physically active children, males and females participating in recreational gymnastics had significantly greater CSA and Z development at the proximal femur. Previous history of any recreational gymnastics participation may provide skeletal benefits at the load-bearing proximal femur; potential advantages may be site and exposure time specific; however, further longitudinal investigations of this cohort are necessary to substantiate these notions.