Accelerometer-derived daily impact score is positively associated with bone strength accrual during adolescence

Bone adapts to its loading environment throughout the lifespan, with rapid gains during adolescence. Accelerometers are commonly used to measure physical activity (PA) and examine its associations with bone outcomes; however, common approaches reduce high-resolution data to a few metrics (e.g., minutes per day (min/d) in various activity intensities). In this observational cohort study, we investigated whether using new accelerometry metrics to synthesize count-based accelerometry data would explain relationships between PA and estimated bone strength accrual. Four years of longitudinal data from over 300 children and adolescents in the Healthy Bones Study III were used to examine associations between the accelerometer-derived intensity gradient (IG) and daily impact score (DIS) with estimated bone strength (failure load, N; 8 % site of the distal tibia) as measured with high-resolution peripheral quantitative computed tomography. We also explored whether associations were independent of a traditional metric of min/d of vigorous physical activity (VPA). In linear mixed effects models, IG was not associated with bone strength independent of VPA (β_IG = -515.2 (-1302.1, 270.4), p = 0.20; β_VPA= 25.5 (14.0, 37.0), p < 0.001), but DIS was positively associated with bone strength independent of VPA (β_DIS = 25.2 (7.0, 43.6), p = 0.007; β_VPA= 3.2 (-6.1, 1.4), p = 0.67). Our findings suggest that the DIS may be a more appropriate metric for evaluating accelerometry data in relation to adolescent bone strength accrual compared with traditional duration (minutes of PA per day) metrics.