Absolute and relative intensity physical activity of children with healthy and low levels of cardiorespiratory fitness

Little is known about how the relative intensity of children's physical activity (PA) volume and intensity distribution relates to health or fitness. To address this, we examined associations between accelerometer-derived absolute and relative PA volume and intensity distribution with cardiorespiratory fitness (CRF), and differences in these PA outcomes for children stratified by CRF level. In 9-10-year-old children (N = 235) PA was assessed using wrist accelerometers and CRF estimated from the 20-m multistage shuttle run test (20mSRT). Children were classified as Healthy or Low CRF. Absolute PA outcomes were PA volume (average acceleration; AvAcc_abs) and intensity distribution (intensity gradient; IG_abs). Equivalent relative PA outcomes were generated (AvAcc_rel and IG_rel). Regression models found that absolute, but not relative PA outcomes were associated with CRF (AvAcc_abs Std<i>β</i> = 0.21, <i>p</i> = 0.02; IG_abs Std<i>β</i> = 0.21, <i>p</i> = 0.03). Absolute PA outcomes were higher among Healthy CRF children (AvAcc_abs Std<i>β</i> = 0.40, <i>p</i> = 0.007; IG_abs Std<i>β</i> = 0.46, <i>p</i> = 0.008), but there were no differences for relative PA outcomes. Children were similarly active relative to their physiological capacity, despite the Healthy CRF group being more active in absolute terms. Future studies should seek to better understand the influence of relative PA on CRF among child populations who differ on a range of physical, physiological and demographic characteristics.