From motion to metabolism: investigating the relationship between accelerometer and VO₂ metrics across five age groups for optimal calibration of physical activity intensity

The aim was to examine the relationship between accelerometer and oxygen consumption (VO2) metrics and to what extent the metrics are normalized across age and body size in a diverse sample, to allow a single calibration regression line for absolute physical activity intensity. Approach: Hip-mounted accelerometer data and VO2 measurements were collected from 51 participants across five age cohorts (4-5; 6-8; 10; 15 and 20 years) during resting, walking and running on a treadmill in laboratory setting. Linear regressions were used to determine four accelerometer metrics' (AG, 4 Hz FEM, 10 Hz FEM and ENMO) contribution to explained variance (adjusted R2) in six VO2 metrics (VO2, VO2/kg1, VO2/kg0.67, VO2/kg0.75, METmeasured and METfixed). Plots were generated for visual representations together with log-linear regression, finding the optimal scaling exponent for VO2. Main Result: 10Hz FEM explained the highest amount of explained variance when related to VO2/kg0.75, 92.4%, with minimal remaining between-group and inter-individual variance. The most used accelerometer metric, AG counts, together with traditionally used reference standard, METfixed, show substantially lower explained variance, 60.2%, with large between-group and inter-individual variance, insufficiently adjusting for inter-individual physiological and biomechanical variability across age groups. The best body weight scaling factor for VO2 was 0.77. Findings support the use of a single calibration regression line for absolute physical activity intensity across wide-ranging age-groups, accounting for biomechanical and physiological variance. Significance: This enables reliable and meaningful comparisons of physical activity intensity across age-groups, possibly also across childhood into adulthood, overcoming traditional limitations and enhancing research quality. .