Accuracy of wrist-worn wearable devices for determining exercise intensity

As an indicator of exercise intensity, heart rate can be measured in a timely manner using wrist-worn devices. No study has attempted to estimate a target exercise intensity using wearable devices. The objective of the study was to evaluate the validity of prescribing exercise intensity using wrist-worn devices. Heart rate was analyzed at the lower and upper limits of each exercise intensity (HR40, HR60, and HR89). No study has attempted to estimate a target exercise intensity using wearable devices. Therefore, the primary objective of this study was to evaluate the accuracy of exercise intensity measurement based on the HRmax measured by wearable devices. To minimize the risk of complications, the included individuals comprised healthy subjects aged between 20 and 40 years who answered “no” to all questions in the 2019 Physical Activity Readiness Questionnaire. Excluded individuals included those with resting blood pressure > 140/90 mmHg, body mass index > 30 kg/m, resting HR >100 beats per min, those who received medications that affect HR, were incapable of performing exercises of vigorous intensity (e.g. running, climbing, cycling fast, and playing basketball, football, and other competitive sports), and pregnant women. The subjects’ HR data were simultaneously recorded by the Apple Watch at 5-s intervals, the Garmin Forerunner at 1-s intervals, and ECG at different time intervals based on different respiratory rates throughout the CPET. The formulas for determining the target HR range recommended by the ACSM for medium-intensity and vigorous-intensity exercise in healthy adults, which were adopted in this study, are The target HR calculated from ECG was used as a reference for prescribing exercise intensity.

The target HRs from the two wrist-worn wearable devices were also calculated using the lower and upper limits of HRs obtained during moderate and vigorous-intensity exercises. Mean absolute error and mean absolute percentage error were calculated for both devices, formulas, and ECG for different exercise intensities, with upper and lower limits. The concordance correlation coefficient showed a substantial correlation between Apple Watch and 12-lead ECG, from moderate to vigorous-intensity exercises, with a concordance correlation coefficient of 0.977–0.979. Apple Watch showed very low error, with a mean absolute percentage error of < 1% in moderate to vigorous-exercise intensities. The mean absolute error and mean absolute percentage error of Garmin were slightly higher than those of Apple Watch in moderate and vigorous-intensity exercises. The plots show a high correlation without systematic error, from moderate to vigorous-intensity exercises. Fox and Tanaka’s equations overestimated the subjects’ HRmax, and the error rate was more pronounced as exercise intensity increased.

The study validates the accuracy of wrist-worn devices for establishing exercise intensity through their HR assessment function. We found that the exercise intensity prescribed by the two wrist-worn devices was highly consistent with that prescribed by 12-lead ECG, from moderate to vigorous-intensity exercise, in healthy adults. Previous studies focused on the accuracy of HR monitoring in different exercise intensities. In this study, the exercise intensity prescribed by Apple Watch seemed more precise and was associated with a lower error rate than that prescribed by Garmin Forerunner at each intensity. However, the overall error for Garmin remained very low (1.16–1.39%), from moderate to vigorous-intensity exercises. The error in the determination of HR using the formulas increased with increasing exercise intensity. This is the first study to analyze the validity of exercise intensity prescription using wrist-worn devices. Consequently, healthy adults may only require wearable devices and exercise equipment to establish the exercise intensity required for their training, and whether their HR falls within the training target throughout the exercise could be monitored synchronously. In conclusion, we found that the exercise intensities prescribed by commercial wearable devices, including Apple Watch and Garmin Forerunner, were highly consistent with those prescribed by ECG in healthy adults.