Validity of simplified, calibration-less exercise intensity measurement using resting heart rate during sleep: A method-comparison study with respiratory gas analysis

Background: The recent development of wearable devices has enabled easy and continuous measurement of heart rate (HR). Exercise intensity can be calculated from HR with indices such as percent HR reserve (%HRR); however, this requires an accurate measurement of resting HR, which can be time-consuming. The use of HR during sleep may be a substitute that considers the calibration-less measurement of %HRR. This study examined the validity of %HRR on resting HR during sleep in comparison to percent oxygen consumption reserve (%VO₂R) as a gold standard. Additionally, a 24/7%HRR measurement using this method is demonstrated. Methods: Twelve healthy adults aged 29 ± 5 years underwent treadmill testing using the Bruce protocol and a 6-min walk test (6MWT). The %VO₂R during each test was calculated according to a standard protocol. The %HRR during each exercise test was calculated either from resting HR in a sitting position (%HRR_sitting), when lying awake (%HRR_lying), or during sleep (%HRR_sleeping). Differences between %VO₂R and %HRR values were examined using Bland-Altman plots. A 180-day, 24/7%HRR measurement with three healthy adults was also conducted. The %HRR values during working days and holidays were compared. Results: In the treadmill testing, the mean difference between %VO₂R and %HRR_sleeping was 1.7% (95% confidence interval [CI], - 0.2 to 3.6%). The %HRR_sitting and %HRR_lying values were 10.8% (95% CI, 8.8 to 12.7%) and 7.7% (95% CI, 5.4 to 9.9%), respectively. In the 6MWT, mean differences between %VO₂R and %HRR_sitting, %HRR_lying and %HRR_sleeping were 12.7% (95% CI, 10.0 to 15.5%), 7.0% (95% CI, 4.0 to 10.0%) and - 2.9% (95% CI, - 5.0% to - 0.7%), respectively. The 180-day, 24/7%HRR measurement presented significant differences in %HRR patterns between working days and holidays in all three participants. Conclusions: The results suggest %HRR_sleeping is valid in comparison to %VO₂R. The results may encourage a calibration-less, 24/7 measurement model of exercise intensity using wearable devices. Trial registration: UMIN000034967. Registered 21 November 2018 (retrospectively registered).