JMIR Publications

ABSTRACT

Background:

Step counting is comparable among many research-grade and consumer-grade accelerometers in laboratory settings, but few studies have compared step count measurement among devices outside of the laboratory, in a community setting.

Objective:

The purpose of this study was to compare agreement between Actical and Apple Watch step-counting in a community setting.

Methods:

Among Third Generation Framingham Heart Study participants (n=3486), we examined agreement of step-counting between those who wore a consumer-grade (Apple Watch Series 0) and research-grade accelerometer (Actical) on the same day(s). Secondarily, we examined agreement during each hour when both devices were worn to account for differences in wear time between devices.

Results:

We studied 523 participants (n=3223 person-days, mean age 51.7 years, 57% women). Between devices, we observed modest correlation (intraclass correlation [ICC]=0.56, 95% confidence interval [CI]=0.54, 0.59), poor continuous agreement (29.7% of days having steps counts with ≤15% difference), a mean difference of 499 steps/day higher count by Actical, and wide limits of agreement, roughly +/-9000 steps/day. However, devices showed stronger agreement in identifying who meets various step/day threshold (e.g. at 8000 steps/day, kappa coefficient=0.49), for which devices were concordant for 74.8% of participants. In secondary analyses, of hours during which both devices were worn (456 participants, 18760 person-hours), the correlation was much stronger (ICC=0.86, 95% CI=0.85, 0.86), but continuous agreement remained poor (27.3% of hours having step counts with ≤15% difference) between devices and was slightly worse for those with mobility limitations or obesity.

Conclusions:

Our investigation suggests poor overall agreement between steps counted by the Actical and Apple Watch devices, with stronger agreement in discriminating who meets certain step thresholds. The impact of these challenges may be minimized if accelerometers are used by individuals to determine whether they are meeting physical activity guidelines or tracking step counts. It is also possible that some of the limitations of these older accelerometers may be improved in newer devices.