Seventy-one participants (24F/47M) with wide range of body mass indices (19.5–40.7 kg/m2, mean 25.7 ± 3.8 kg/m2) between the age groups of 18-55 years (< 20, 20–29, 30–39, 40–49, > 50 years; 13, 69, 8, 4, and 6%, respectively) completed the study. Total video steps vs. total Apple Watch steps (mean ± SD) were 2965 ± 144 vs. 2964 ± 145 steps; P < 0.001. Lin’s concordance correlation coefficient showed a strong correlation (r = 0.96; P < 0.001) between the two measurements. There was a total error of 0.034% (1.07 steps) for the Apple Watch across all stages and across all participants.
shows the mean ± SD and minimum-maximum range of step counts for the video and the Apple Watch with the reported relative percent error for Apple Watch steps at different speeds. Apple Watch consistently overestimated the step counts for the slow, moderate, and brisk paces (relative error − 2.6, − 0.9, and − 1.6%) and underestimated (relative error 3.4%) for the jogging pace of the treadmill protocol. The least amount of relative error (− 0.9%) was observed for the moderate intensity pace (3.0 mph). There were no statistically significant differences in step counts between genders, across age groups, and with different BMI categories.
reports a Bland-Altman plot comparing the video steps and the Apple Watch steps. Comparison of the plots suggests good absolute mean-level differences in video steps and Apple Watch step estimates (mean difference = 1.07 steps, 95% CI − 82.62 to 84.76; P = 0.93). There was an overall negligible underestimation (1.07steps) of step count by Apple Watch. There was no proportional bias (b = − 0.03; t = − 0.083; P = 0.93) between the two measurements.