GoKawiilStudies
Between October 2020 and March 2024, we conducted five independent, prospective laboratory studies and a prospective free-living study to obtain datasets to develop and validate PHRM. All study protocols were approved by an institutional review board (Quorum, now known as Advarra, and WCG). We obtained informed consent from all participants, and the study was performed in accordance with the principles of the Declaration of Helsinki.
In the laboratory validation studies, we objectively measured skin tone from each participant using a RM200QC spectrocolorimeter (Pantone) to image the skin of the cheeks and forehead. For the free-living study, because it was entirely remote with no in-person component, we provided participants with a visual representation of the MST to self-assess their skin tone51.
Reference measurements
To validate HR measurements of PHRM in laboratory settings, we used ECG recorded by the BIOPAC MP160 system as the reference ground truth. We used a custom LabVIEW (National Instruments) application to record three-lead ECG signals from electrodes placed on study participants’ upper chests (or upper arms) and lower abdomens.
For validating HR measurements of PHRM in real-world, free-living conditions, we used the Polar H10 ECG chest strap. The Polar H10 has been validated as providing accurate HR measurements during physical activity52,53. Participants were instructed to put the chest strap on every morning and to wear it for at least seven hours each day, except during showers or sleep.
Because aggregating multiple watch HR measurements provides more consistent RHR values, compared with spot measurements in a supine or sitting position10, we chose to use the daily RHR from the Fitbit Charge 6 (Google) as our primary reference for RHR. The daily RHR produced by Fitbit devices is computed by combining multiple HR measurements across ‘at rest’ periods throughout the day, in which the on-device accelerometer has determined that the person is at rest, and has not been moving recently. If available, sleeping HR is also used to improve the daily RHR estimate. The Fitbit daily RHR has been shown to be closest to RHR measurements that are taken when people are lying down, immediately after waking up12. In addition, participants were instructed to perform two conventional RHR measurements first thing in the morning, before eating, drinking, exercising or showering. After putting on the ECG chest strap, they lay in a supine position for 6 min. Next, they sat still for another 6 min. Supine and sitting RHR measurements were computed as the minimum HR from the ECG recordings and served as secondary references. HR has been found to stabilize in most individuals after four minutes of inactivity54.
Time synchronization
To synchronize the clocks across all of the study devices during the free-living study, participants performed a daily routine comprising a series of three jumps. We instructed participants to stand still with their hands placed in front of the chest. They held their smartphone enrolled in the study in their dominant hand; the wearable HR tracker was placed on the off-hand. To perform the series of jumps, we asked participants to start a timer, and complete the following sequence: standing still for one minute, three jumps spaced by 10 s, followed by standing still for another 10 s. We aligned the timestamps of the smartphone and the ECG chest strap by maximizing the cross-correlation between their respective accelerometer signals after resampling the signals to 60 Hz.
PHRM-HR module
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