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 (%VO(2)R) 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 %VO(2)R 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 %VO(2)R 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 %VO(2)R 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 %VO(2)R 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 %VO(2)R. 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).