Physical Activity and Composite Indices of Femoral Neck Strength in 9-10 Yrs Children

Abstract

2019 Examining the relationship between habitual physical activity (PA) and bone resistance to fracture are limited by the use of self-reported PA. PURPOSE: To analyze the relationship between objectively measured intensity and duration of weight bearing PA and composite indices of femoral neck strength, a measure that integrates bone density, bone and body size variables. METHODS: PA was assessed with a uni-axial accelerometer in 143 girls and 150 boys (9.7 ± 0.3 yrs). Assessment of femoral neck bone mineral density (BMD), femoral neck width (FNW), and hip axis length (HAL) was performed with DXA. Compressive (BMD*FNW/weight) and bending strength {BMD* (FNW)2/HAL*weight} express the forces that femoral neck has to withstand in weight bearing, while impact strength {BMD*FNW*HAL/(height*weight)} expresses the energy that femoral neck has to absorb in an impact from standing height (Karlamangla et al., 2002). RESULTS: ANCOVA (adjusted for fat free mass and age) showed significant differences between boys (B) and girls (G) of about 9% for compressive (B 5.96 ± 0.07 vs. G 5.48 ± 0.07 g/kgm, p≤ 0.001), 10% for bending (B 1.95 ± 0.03 vs. G 1.77 ± 0.03 g/kgm, p≤ 0.001), and 9% for impact strength (B 0.37 ± 0.01 vs. G 0.34 ± 0.01 g/kgm, p≤ 0.001). Stepwise regression analysis using time spent at sedentary, light, moderate and vigorous intensity of PA as predictors, revealed that vigorous intensity PA explained 5–9% (p≤ 0.01) of the variance of femoral neck strength variables in both genders, except for femoral bending strength in boys. Vigorous intensity PA was then used to categorize boys and girls into quartiles. Pairwise comparison indicated that boys in the third and fourth quartile (accumulation of > 26 min/d) demonstrated higher values of compressive (11–12%, p≤ 0.01), bending (10%, p≤ 0.05) and impact strength (14%, p≤ 0.01) than boys in the first quartile. In girls, pairwise comparison revealed a significant difference between the fourth (accumulation of > 25 min/d) and the first quartile for bending strength (11%, p≤ 0.05). CONCLUSIONS: Femoral neck strength is higher in boys than girls. Vigorous intensity PA emerged as the main predictor of composite indices of femoral neck strength. Daily vigorous intensity PA for at least ∼25 min seems to be related to improved femoral neck bone health in children.