Validity of iPhone health application step count in semi free-living conditions

The iPhone's validity for measuring steps has been mainly investigated under laboratory conditions, while studies that include real-world conditions are still scarce. We examined the validity of iPhones in measuring steps in real-world walking conditions, while using direct observation with video as reference. A sample of 100 adults who owned an iPhone 5S or higher was included and participants were randomly allocated to one of two protocols. Limits of Agreement (LoA), Mean Absolute Percentage Error (MAPE), linear-regression and Bland-Altman analyses were carried out. In Protocol-1, which includes straight-line and zigzag conditions, we observed a low MAPE lower than 4%. Bland-Altman analyses assessed a high accordance of approximately 6 steps under both conditions. Differences between the iPhone and direct observation were only noticeable in straight-line condition (p = 0.002). Likewise, protocol-2 (three 50-step conditions: an upward 5 % slope, a flat surface, and a downward -5% slope) showed low MAPE values (3.78 %) for the upward slope, 2.41 % for the downward slope, and 2.37 % in absence of slope), and differences between the iPhone and direct observation were only observed for the downward slope (p = 0.017), with the iPhone overestimating. Our findings revealed that iPhone might be a reliable tool for monitoring walking in real-life conditions, however, downward slope seems to generate overestimation, which deserves future investigation.