Introduction Estimating free-living physical activity (PA) with continuous heart rate (HR) monitoring is challenging due to individual variation in the relationship between HR and energy expenditure. This variation can be captured through individual calibration with graded exercise tests, but structured tests with prescribed load requires medical screening and are not always feasible in population settings. We present and evaluate an individual calibration method using HR response to a less demanding self-paced walk test.
Methods 643 participants from the Fenland Study (Cambridgeshire, UK) completed a 200-meter self-paced walk test, a treadmill test, and one week of continuous HR and accelerometry monitoring. Mixed effects regression was used to derive a walk test calibration model from HR response to the walk using treadmill-based parameters as criterion. Free-living PA estimates from the calibration model were compared with treadmill-calibrated as well as non-exercise calibrated estimates.
Results The walk calibration model captured 57% of the variance in the HR-energy expenditure relationship determined by the treadmill test. Applying the walk calibration method to data from free-living yielded similar PA estimates to those using treadmill calibration (52.7 vs 52.0 kJ·kg-1·day-1; mean difference: 0.7 kJ·kg-1·day-1, 95% CI: −0.0, 1.5) and high correlation (r=0.89). Individual differences were observed (RMSE: 10.0 kJ·kg-1·day-1; 95% limits of agreement: −20.6, 19.1 kJ·kg-1·day-1). Compared to using a non-exercise group calibration model (RMSE: 14.0 kJ·kg-1·day-1; 95% limits of agreement: −30.4, 24.5 kJ·kg-1·day-1), the walk calibration improved precision by 29%.
Conclusions A 200-meter self-paced walk test captures a significant proportion of the between-individual variation in the HR-energy expenditure relationship and facilitates estimation of free-living PA in population settings.