Boys usually have higher values of bone mineral density measured by dual-energy X-ray absorptiometry than girls, but contradictorily also have a greater incidence of fractures during growth. The purpose of this study was to investigate sex- and maturity-related differences in bone speed of sound (SoS) at the radius and tibia in a sample of 625 healthy children aged 10-14 y and to analyze the contributions of physical activity (PA) to possible dissimilarities. Radial and tibial SoS was evaluated by quantitative ultrasound, maturity was estimated as the years of distance from the peak height velocity age, and PA was assessed by accelerometry. Comparisons between sexes and maturity groups (low: below average [<-2.5 y], high: average or above [≥-2.5 y]) were made by two-sample t-tests with unequal variances. Girls in the high-maturity group had higher SoS at the radius and tibia compared with girls in the low-maturity group (p < 0.001). There were no SoS differences at the radius or tibia between the high- and low-maturity groups in boys. Within high-maturity children, girls had higher SoS than boys at the radius and tibia (p < 0.001). There were no differences at the radius and tibia between girls and boys with low maturity. The results were not modified after controlling for PA. Regardless of PA, the results provide insight into sex- and maturity-related differences in bone SoS at the distal radius and midshaft tibia from maturity less than 2.5 y from the peak height velocity age, with boys having lower SoS.

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BPAQ was a significant positive predictor for all BMD variables (p<0.05) except TR BMD in girls and FN BMDs in boys (>0.05).  At least one geometric variable was significant in the estimated models for the BMD ratios: in girls, the IAD was a positive predictor of TR:PF (p<0.001) and ALA was a negative predictor of FN:PF; in boys, the IAD was a positive predictor of FN:PF (p<0.01) and IM:SL (p<0.05); also in boys, ALA was a negative predictor of the IM:SL (p< 0.001).  The interaction of IAD*ALA predicted IM:SL positively in girls and negatively in boys (p<0.01).  The IAD and the ALA, as indicators of the main lever arms of the biomechanics of the hip, may play a role in the relative mineralization of the proximal femur in peripubertal boys and girls, as was theoretically expected..  However unlike total lean body mass and PA, the same geometric variables don't seem to influence the absolute BMD levels at the proximal femur neck and trochanter..  Further research is needed to better understand the effects of geometric variables on the relative mineralization of the proximal femur regions including the development of a specific biomechanical model to simulate the vector forces exerted on these regions.

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