Abstract
Whether a certain level of impact needs to be exceeded for physical activity (PA) to benefit bone accrual is currently unclear. To examine this question, we performed a cross-sectional analysis between PA and hip BMD in 724 adolescents (292 boys, mean 17.7 years) from the Avon Longitudinal Study of Parents and Children (ALSPAC), partitioning outputs from a Newtest accelerometer into six different impact bands. Counts within 2.1 to 3.1g, 3.1 to 4.2g, 4.2 to 5.1g, and >5.1g bands were positively related to femoral neck (FN) BMD, in boys and girls combined, in our minimally adjusted model including age, height, and sex (0.5–1.1g: beta = −0.007, p = 0.8; 1.1–2.1g: beta = 0.003, p = 0.9; 2.1–3.1g: beta = 0.042, p = 0.08; 3.1–4.2g: beta = 0.058, p = 0.009; 4.2–5.1g: beta = 0.070, p = 0.001; >5.1g: beta = 0.080, p < 0.001) (beta = SD change per doubling in activity). Similar positive relationships were observed between high-impact bands and BMD at other hip sites (ward's triangle, total hip), hip structure indices derived by hip structural analysis of dual-energy X-ray absorptiometry (DXA) scans (FN width, cross-sectional area, cortical thickness), and predicted strength (cross-sectional moment of inertia). In analyses where adjacent bands were combined and then adjusted for other impacts, high impacts (>4.2g) were positively related to FN BMD, whereas, if anything, moderate (2.1–4.2g) and low impacts (0.5–2.1g) were inversely related (low: beta = −0.052, p = 0.2; medium: beta = −0.058, p = 0.2; high: beta = 0.137, p < 0.001). Though slightly attenuated, the positive association between PA and FN BMD, confined to high impacts, was still observed after adjustment for fat mass, lean mass, and socioeconomic position (high: beta = 0.096, p = 0.016). These results suggest that PA associated with impacts >4.2g, such as jumping and running (which further studies suggested requires speeds >10 km/h) is positively related to hip BMD and structure in adolescents, whereas moderate impact activity (eg, jogging) is of little benefit. Hence, PA may only strengthen lower limb bones in adolescents, and possibly adults, if this comprises high-impact activity. © 2012 American Society for Bone and Mineral Research.
Highlights
Mechanical strain is an important determinant of skeletal growth and modeling
Animal studies have demonstrated that bone strain stimulates bone formation in proportion to its rate and magnitude.[1,2] Bone strain is directly related to the strength of applied force, which for the lower limbs comprises ground reaction forces generated by the musculature during locomotion, with body mass serving as resistance.[3,4] There has been considerable interest in the effect of physical activity (PA) on bone development in childhood, and in particular whether increased weight-bearing activities during this time results in a higher peak bone mass and strength of the lower limb, thereby reducing the risk of osteoporotic hip fracture in later life
An Actigraph device has been employed in the Avon Longitudinal Study of Parents and Children (ALSPAC), physical activity being defined as light, moderate, or vigorous, based on thresholds of 3600 and 6200 counts per minute, reflecting the transition from normal to brisk walking, and brisk walking to jogging, respectively.[8]. Based on this approach, we found that vigorous day-to-day PA is associated with increased cortical bone mass in adolescents, through a combination of increased periosteal growth and reduced endocortical resorption, whereas light or moderate PA has no detectable association.[9]. These findings suggest that a threshold of strain needs to be exceeded to affect bone development
Summary
Mechanical strain is an important determinant of skeletal growth and modeling. For example, animal studies have demonstrated that bone strain (ie, deformation relative to bone length) stimulates bone formation in proportion to its rate and magnitude.[1,2] Bone strain is directly related to the strength of applied force, which for the lower limbs comprises ground reaction forces generated by the musculature during locomotion, with body mass serving as resistance.[3,4] There has been considerable interest in the effect of physical activity (PA) on bone development in childhood, and in particular whether increased weight-bearing activities during this time results in a higher peak bone mass and strength of the lower limb, thereby reducing the risk of osteoporotic hip fracture in later life. An Actigraph device has been employed in the Avon Longitudinal Study of Parents and Children (ALSPAC), physical activity being defined as light, moderate, or vigorous, based on thresholds of 3600 and 6200 counts per minute, reflecting the transition from normal to brisk walking, and brisk walking to jogging, respectively.[8] Based on this approach, we found that vigorous day-to-day PA is associated with increased cortical bone mass in adolescents, through a combination of increased periosteal growth and reduced endocortical resorption, whereas light or moderate PA has no detectable association.[9] These findings suggest that a threshold of strain needs to be exceeded to affect bone development. The upper range of acceleration that can be detected by the Actigraph is 2.5g,(10) implying this device is unable to distinguish higher-impact activities such as jumping, in which accelerations generally exceed 5g.(11)
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