Bone geometry and density in the skeleton of pre-pubertal gymnasts and school children
Introduction
On a daily basis, loading from the muscles exerts the greatest forces on the skeleton. In elite gymnasts, the forces exerted on the lower limb can be 10 times greater than that of body weight [1]. A bone will adapt to this loading in order to maintain efficiency in providing structural, functional support to the skeleton without injury or fracture [2], [3]. The adaptation of the bone to loading will be to increase its size, change geometry and increase the amount of mass within the periosteal envelope.
Participation in gymnastics has been demonstrated to have beneficial effects on the skeleton [4], [5], [6], [7], [8]. The high loads experienced by gymnasts could explain the more generalised increases in areal bone mineral density (aBMD) throughout the skeleton [4], [5], [9], in comparison to sports such as racquet sports or football (soccer) where only the most involved limb has adapted to loading [10], [11], [12]. Most studies report differences in BMD or bone mineral content (BMC) between gymnasts and controls; few have investigated differences in the geometry of bones in gymnasts compared to controls.
In this paper, we present the baseline peripheral quantitative computed tomography (pQCT) and dual energy X-ray absorptiometry (DXA) data from a double blind randomised controlled trial (RCT) to investigate whether children who regularly participate in competitive gymnastics training respond better to calcium supplementation than pre-pubertal children. We hypothesise that gymnasts will have greater aBMD, volumetric BMD (vBMD), and bone geometry than age-matched sedentary controls using pQCT and DXA.
The specific pre-specified hypotheses of the baseline analyses were that compared to controls, gymnasts would have higher bone stress-strain index [13], [14] (SSI; a measure of diaphyseal bone's bending and torsional strength), greater total bone cross-sectional area, cortical content and muscle cross-sectional area (proxy for muscle force) of the radius and tibia, measured by pQCT. At the distal metaphyseal sites of the radius and tibia, gymnasts would have greater bone cross-sectional area and total and trabecular vBMD. We also used DXA to investigate differences between the lumbar spine and whole skeleton in gymnasts versus controls; body composition was also studied using DXA. By measuring different skeletal sites and studying diaphyseal bone geometry, we aim for these data to provide greater insight into how children's bones have altered their morphology in order to withstand the loads imposed upon them.
Section snippets
Participants
Participants were recruited, to a 12-month RCT of calcium supplementation, from gymnastics centres and schools within Northwest England. The inclusion criterion was pre-pubertal children aged 5–14 years. The exclusion criteria were (1) any chronic childhood condition, inherited disorder, or taking medication, known to affect bone mass, (2) family history of osteoporosis, kidney stones or nephrocalcinosis, (3) personal history of kidney stones or nephrocalcinosis, (4) allergy to dairy products
Results
Anthropometric data, calcium intake and duration of physical activity per week in 86 pre-pubertal participants, 44 gymnasts (27 females: 17 males) and 42 controls (22 females: 20 males) are shown in Table 1. After adjustment for age and sex, gymnasts were significantly shorter (mean difference = 4.4 cm, p = 0.002) and lighter (mean difference = 3.3 kg, p = 0.003) than controls. Bone age was not significantly different between the two groups (p = 0.72). Baseline recreational physical activity
Discussion
We investigated differences in the pre-pubertal skeleton of elite gymnasts to controls. We also compared differences between sexes and skeletal sites (axial and peripheral). The nature of the differences between the two groups appeared to be site specific and depended upon the type of bone (cortical or trabecular) that was studied.
The gymnasts were shorter than the controls, we therefore chose to adjust our data for height. We had anticipated that the gymnasts would be shorter than the controls
Acknowledgments
We are extremely grateful to all the children and parents who participated in the trial and to the schools and gymnastic coaches who assisted recruitment. We would also like to thank Dr S New, University of Surrey, and her BSc nutrition students who have analysed all of the food diaries from the trial. Finally, we would like to thank Action Medical Research for funding the trial.
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