Purpose This 2-year longitudinal study examined the development of upper-extremity bone mineral density (BMD), bone mineral content (BMC), and lean mass (LM) asymmetry magnitudes in male and female youth tennis players. Methods Dominant and nondominant upper-extremity BMD, BMC, and LM values of 49 male and 31 female players were measured yearly using dual X-ray absorptiometry. From these values, asymmetry magnitudes were calculated and expressed as a percentage. Maturity offset was estimated using anthropometric measurements. Linear mixed effect models examined the development of BMD, BMC, and LM asymmetry magnitudes according to players` maturity offset, sex, and training volume. Results Adjusted for sex and training volume, a 1-year increment in maturity offset was associated with a significant increase in BMD (1.3% [2.2%]; P < .001) and BMC (0.6% [2.4%]; P = .011) asymmetry magnitudes. Male players displayed significantly higher LM asymmetry magnitudes (?3.2% [8.4%]; P = .002) compared with their female counterparts. Training volume was not significantly associated with asymmetry magnitude development. Conclusion In contrast to LM, male and female youth tennis players` upper-extremity bones are still responsive to mechanical loading with a significant increase in BMD and BMC asymmetry magnitudes according to maturity offset.
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