Darcy Ummels

34 | Chapter 2 Figure 2.6 Bland ‐ Altman plot of Digi ‐ Walker CW ‐ 700 and the gold standard. Discussion Principal findings The results of this study showed that none of 9 selected commercially available activity trackers was valid for measuring steps while individuals engage in activities of daily living among a diverse group of patients with various chronic diseases receiving physiotherapy in the Netherlands. All activity trackers in this study had an average underestimation in step count except the Activ8, which overestimated step count. The Digi ‐ Walker CW ‐ 700 and Lumoback consistently underestimated step count in every participant, while the other activity trackers had a combination of under ‐ and overestimation. For all trackers, the correlations between step count measured by the activity trackers and the observed steps were low. On group level, the Fitbit One seemed to be the best activity tracker due to its low mean difference; however, on individual basis the scatter and Bland ‐ Altman plots showed a large under ‐ and overestimation in step count. Several studies have shown that a low walking speed decreases the validity of activity trackers. 12,16,50,51 For an activity tracker to measure the number of steps correct, a walking speed of 0.8 m/s is required. All of our participants walked faster than 0.8 m/s