Strong bones are essential to overall health and quality of life. Optimizing peak bone mass during growth is a key strategy in preventing fragility fractures in later life. Understanding the biological process of bone growth and its regulators assumes the greatest importance in realizing this strategy. This study aimed to investigate bone growth in tenns of bone size, bone mineral content (BMC) and volumetric mineral density (vBMD) and its relationship with sex hormones, physical activity (PA), body composition and muscle strength in pubertal girls. A total of 258 healthy girls aged 10-13 years were included in the 2-years observational study at baseline, The PA level was assessed using a qtuestionnaire, Body composition and BMC were assessed by dual-energy x-ray absorptiometry, Bone geometry and vBMD at the distal radius (DR) iand tibial shaft (TS) were measured by peripheral quantitative computed tomography. Muscle strength of elbow flexors and knee extensors was determined by a dynamometer. Sex hormones were assessed by fluoroimmunoassay. The results showed that the timing of peak growth velocity of BMC lags behind that of bone size by more than 6 months in pubertal girls. This growth asynchrony resulted in vBMD of DR (the diaphysical trabecular bone site) decreasing sliglitiy from prepuberty up to 1 year before menarche. The cross-sectional area (CSA) of marrow cavity at TS (the diaphyseal cortical bone site) did not change prior to rnenarche but decreased thereafter. 17-Beta-Estradiol (E2) was not correlated with total bone CSA, but was positively correlated with cortical CSA and cortical thickness (Cth) at TS. On the other hand testosterone (T) was positively correlated with total bone CSA, but not with cortical CSA and Cth. PA level was positively associated with BMC in early pubertal girls. Total body FM correlated sigraficantly higher with leg BMC than with arm BMC. BMC of arm and leg was highly correlated with muscle strength of elbow flexors and knee extensors, respectively, However, BMC per unit of muscle strength was significantty higher in leg than in arm. The observational study suggests that in pubertal girls, the growth of bone size precedes that of bone mass, and this phenomenon may responsible for the elevated fracture risk during pubertal growth spurt. EZ might be important to counteract this transitory elevated fracture risk, because it makes bone stronger by inhibiting bone resorption and even promoting bone formation at the endocortical surface. E2 may also interact with mechanical loading to facilitate bone mass accretion during pubertal growth. In addition, muscle strength together with body composition may play an important role in determining bone mass.
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