Structural strength development at the proximal femur in 4- to 10-year-old precompetitive gymnasts: a 4-year longitudinal hip structural analysis study.

Gymnastics, a high-impact weight-bearing physical activity, has been shown to be highly osteogenic. Previously in this cohort, bone mass development (bone mineral content accrual [BMC]) was shown to be positively associated with low-level (recreational) gymnastics exposure (1 to 2 hours per week); however, BMC is only one single component of bone strength. Bone strength is influenced not only by bone mineralization but also bone geometry, bone architecture, and the imposing loads on the bone. The aim of this study was to investigate whether low-level gymnastics training influenced the estimated structural geometry development at the proximal femur. A total of 165 children (92 gymnasts and 73 non-gymnasts) between the ages of 4 and 6 years were recruited into this study and assessed annually for 4 years. During the 4 years, 64 gymnasts withdrew from the sport and were reclassified as ex-gymnasts. A dual-energy X-ray absorptiometry (DXA) image of each child's hip was obtained. Values of cross-sectional area (CSA), section modulus (Z), and cortical thickness (CT) at the narrow neck (NN), intertrochanter (IT), and shaft (S) were estimated using the hip structural analysis (HSA) program. Multilevel random-effects models were constructed and used to develop bone structural strength development trajectories (estimate ± SEE). Once the confounders of body size and lifestyle were controlled, it was found that gymnasts had 6% greater NN CSA than non-gymnasts controls (0.09 ± 0.03 cm(2) , p < 0.05), 7% greater NN Z (0.04 ± 0.01 cm(3) , p < 0.05), 5% greater IT CSA (0.11 ± 0.04 cm(3) , p < 0.05), 6% greater IT Z (0.07 ± 0.03 cm(3) , p < 0.05), and 3% greater S CSA (0.06 ± 0.03 cm(3) , p < 0.05). These results suggest that early exposure to low-level gymnastics participation confers benefits related to geometric and bone architecture properties during childhood and, if maintained, may improve bone health in adolescence and adulthood.