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

<i>Objective.</i>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, to allow a single calibration regression line for absolute physical activity (PA) intensity.<i>Approach.</i>Hip-mounted accelerometer data and VO₂measurements 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 frequency extended method (FEM), 10 Hz FEM and Euclidean norm minus one) contribution to explained variance (adjusted<i>R</i>²) in six VO₂metrics (VO₂, VO₂<b>/</b>kg¹, VO₂<b>/</b>kg^0.67, VO₂<b>/</b>kg^0.75, MET_measuredand MET_fixed). Plots were generated for visual representations together with log-linear regression, finding the optimal scaling exponent for VO₂.<i>Main result.</i>10 Hz FEM explained the highest amount of explained variance when related to VO₂<b>/</b>kg^0.75, 92.4%, with minimal remaining between-group and inter-individual variance. The relationship demonstrated a linear shape. The most used accelerometer metric, AG counts, together with traditionally used reference standard, MET_fixed, show substantially lower explained variance, 60.2%, with large between-group and inter-individual variance, insufficiently adjusting for physiological and biomechanical variability. The best body weight scaling factor for VO₂was 0.77. Findings support the use of a single linear calibration regression line for absolute PA intensity across wide-ranging age-groups, accounting for biomechanical and physiological variance.<i>Significance.</i>This enables reliable and meaningful comparisons of PA intensity across age-groups, possibly also across childhood into adulthood, overcoming traditional limitations and enhancing research quality.