Evaluation of activity monitors in controlled and free-living environments

Numerous studies have established the usefulness of pedometers and accelerometers as objective activity monitors. Under laboratory conditions, some of these devices have been shown to provide accurate and reliable measures of steps. However, limited data exist on the performance of these devices under free-living conditions.

This study aimed 1) to compare the effects of speed and body mass index (BMI) on the step count accuracy of five different accelerometer-based activity monitors and a pedometer during treadmill walking, 2) to compare the performance of these devices in a free-living environment, and 3) to compare the step counts of three generations of a single device (ActiGraph) against a criterion method.

Fifty-six individuals wore six activity monitors while performing treadmill walking (40, 54, 67, 80, and 94 m·min⁻¹) and during 1 d of free-living activity. The criterion measure of steps during treadmill walking was investigator-determined steps, whereas the criterion measure of steps during the free-living condition was the StepWatch.

BMI had no effect on step count accuracy during treadmill walking. The StepWatch, activPAL™, and the AG7164 were the most accurate across all speeds, whereas the remaining devices were only accurate at 67 m·min⁻¹ and faster. In the free-living environment, the AG7164 recorded 99.5% ± 27% (mean ± SD) of StepWatch-determined steps.

We demonstrated that BMI does not affect the step output of commonly used activity monitors during walking. In addition, 67 m·min⁻¹ seems to be the minimum speed required for accurate step counting, at least for most waist-mounted activity monitors. Finally, the StepWatch, AG7164, and activPAL™ were the most accurate devices on the TM, but only the AG7164 yielded comparable step counts to the StepWatch in the free-living environment.