Bone Abstracts

Objectives: Conventional wisdom suggests that bone is most responsive to physical activity during the growing the years, especially the period just before puberty. Few studies have addressed the entire period of adolescence and even fewer have done so using bone imaging techniques to capture structural outcomes which contribute to bone strength. Using a well-defined cohort (The Iowa Bone Development Study, IBDS), this report examined the magnitude and consistency of the association between physical activity and structural bone strength measures from adolescence to young adulthood.

Methods: IBDS members with at least three bone scanning visits between age 11 and 19 year were studied (n=551, 280 females, 1844 total records). DXA scans of hip, used with the hip structural analysis program, and pQCT scans of tibia at 4% (trabecular) and 38% (cortical) sites were obtained. Outcomes included femoral neck (FN) section modulus (Z), FN cross-sectional area (CSA), tibia bone strength index (BSI), cortical thickness (CTh), and tibia torsion strength (pSSI). Physical activity was assessed using the self-report PAQ-A and Actigraph accelerometry (hip worn) analyzed with the Evenson equation. Age at peak height velocity (PHV age) was estimated from the Mirwald equation. Sex-specific bone trajectories were developed as two-level growth models with up to five repeated measurements. Models included height, weight, and PAQ-A as time-varying covariates. Models included cubic polynomial for time variable (biological age as years from peak height velocity age) with random effects for intercept and time at the individual level to describe growth over time. All models were repeated using ActiGraph-measured minutes per day of vigorous physical activity (VPA). Fewer records were available for the ActiGraph (n=431, 215 females, 1582 total records).

Results: PAQ-A was positively associated (P<0.05) with all bone strength measures for males and females with the exception of pSSI for females. VPA was positively associated (P<0.05) with BSI, Z, and CSA in males and BSI, pSSI, Z, and CSA in females.

Conclusion: Bone remains responsive to the mechanical loading effects of physical activity throughout adolescence and into young adulthood. Greater attention should be placed on promoting bone-strengthening physical activity after the pre-pubertal years when adult exercise patterns are more likely to be formed.

Supported in part by NIH grants R01-DE09551, R01-DE12101, M01-RR00059, and UL1-RR024979.

Disclosure: The authors declared no competing interests.