Bone strength and size in lower limbs were positively affected by exercise with higher vertical impacts in a study of day-to-day weight-bearing physical activity in older women. The study investigated vertical acceleration peaks as measured by 7-day accelerometer recordings, according to an article published Dec. 13 in Osteoporosis International.
Observations in younger individuals have shown exposure to higher exercise impacts to confer greater lower limb bone strength. The Hannam et al. study asked whether the observed benefit of habitual weight-bearing physical activity on bone strength in lower limbs in older women is also attributable to higher impacts. To address that question, 408 women (mean age 76.8 years) from the Cohort for Skeletal Health in Bristol and Avon (U.K.) consented to 7-day physical activity monitoring with the GCDC X15-1c triaxial accelerometer. Mid and distal peripheral quantitative computed tomography scans of the tibia and radius were performed, as were hip and lumbar spine Dual Xray Absorptiometry (DXA) scans.
The monitoring device, which measured vertical acceleration peaks according to g level, had shown in earlier research that impacts exceeding 3.9 g confer bone size and strength benefits in premenopausal women, and impacts of 4.2 g confer benefit in adolescents. Allowing that osteogenic thresholds in younger populations may not be applicable to older individuals with weaker skeletons, investigators measured exposure to vertical impacts >1.5 g in their cohort of older women. Specifically, Hannam et al. asked if habitual exposure to such higher impacts (despite the relatively low 1.5 g threshold and the relative rarity of such impacts), as opposed to medium or low level impacts, explain the benefits of weight-bearing physical activity.
High impacts of 1.5 g or greater exceed that those associated with walking (typically 0.5–1.0 g), but are achieved in the majority of aerobics class exercises undertaken by older individuals, particularly those with a jumping component, Hannam et al. state.
In the measurement week, analysis revealed 8809 low impacts, 345 medium impacts and 42 higher impacts. Bone strength of lower limbs, fully adjusted for numerous potentially confounding factors, was positively associated with higher vertical impacts as reflected by cross-sectional moment of inertia (CSMI) of the tibia [0.042 (0.012, 0.072) p = 0.01] and hip [0.067 (0.001, 0.133) p = 0.045] (beta coefficients show standard deviations change per doubling in impacts, with 95% confidence interval). Higher impacts were positively associated with tibial periosteal circumference (PC) [0.015 (0.003, 0.027) p = 0.02], but unrelated to hip bone mineral density. Low or medium impacts were not associated with equivalent positive changes.
“We found that higher, but not medium or low, vertical impacts,” Hannam et al. concluded, “were positively related to estimated bone strength as reflected by hip and tibial cross-sectional moment of inertia (CSMI), and tibial strength strain index.”
The authors underscored that the bone strength in lower limbs appeared to reflect changes in overall bone size, with little relationship to bone mineral density. Because hip bone mineral density increases are more strongly associated with reduced hip fracture risk than are CSMI increases, fracture reductions are likely to be limited. Higher impacts on the order of 4-g, Hannam et al. suggested, may be necessary (and are achievable), to reduce fracture risk.
K. Hannam, K. C. Deere, A. Hartley, et al. “Habitual levels of higher, but not medium or low, impact physical activity are positively related to lower limb bone strength in older women: findings from a population-based study using accelerometers to classify impact magnitude,” Osteoporosis International. Dec. 13, 2016. DOI: 10.1007/s00198-016-3863-5.