2.1. Devices

Following a comprehensive literature and online search, 45 manufacturers of wrist-worn devices were identified. Criteria for inclusion included: wrist-worn watch or band; continuous measurement of HR; stated battery life >24 h; commercially available direct to consumer at the time of the study; one device per manufacturer. Eight devices met the criteria; Apple Watch; Basis Peak; ePulse2; Fitbit Surge; Microsoft Band; MIO Alpha 2; PulseOn; and Samsung Gear S2. Multiple ePulse2 devices had technical problems during pre-testing and were therefore excluded. All devices were bought commercially and handled according to the manufacturer’s instructions. Data were extracted according to standard procedures described below.

Devices were tested in two phases. The first phase included the Apple Watch, Basis Peak, Fitbit Surge and Microsoft Band. The second phase included the MIO Alpha 2, PulseOn and Samsung Gear S2.

Healthy adult volunteers (age ≥18) were recruited for the study through advertisements within Stanford University and local amateur sports clubs. From these interested volunteers, study participants were selected to maximize demographic diversity as measured by age, height, weight, body mass index (BMI), wrist circumference, and fitness level. In total, 60 participants (29 men and 31 women) performed 80 tests (40 with each batch of devices, 20 men and 20 women). Participant characteristics are presented in Table 1

2 max) was measured with the Quark CPET (COSMED, Rome, Italy) by incremental tests in running ( n = 32) or cycling ( n = 6) to volitional exhaustion, or estimated from the submaximal cycling stages ( n = 22) using the Åstrand nomogram [ Skin tone at the wrist was rated independently by two investigators using the Von Luschan Chromatic scale (1–36), and the average rating was then transformed to the Fitzpatrick skin tone scale (1–6) [ 11 ]. Maximal oxygen uptake (VOmax) was measured with the Quark CPET (COSMED, Rome, Italy) by incremental tests in running (= 32) or cycling (= 6) to volitional exhaustion, or estimated from the submaximal cycling stages (= 22) using the Åstrand nomogram [ 12 ]. In the running test, the subject began the test running at 5.0 mph, 1% incline. Each minute, the speed was increased by 0.5 mph and the subject was asked to assess their exertion level using the Borg Rating of Perceived Exertion (RPE) scale [ 13 ]. In order to complete the test within a 10-minute period, the incline was increased by 0.5% each minute beginning at minute 7 until the subject reached volitional exhaustion. For subjects who performed the cycling test, initial resistance was set at 125 W and increased by 25 W each minute until volitional exhaustion. As with the running test, subjects rated their perceived exertion on the Borg RPE scale at the end of each minute.

The study was conducted in accordance with the principles outlined in the Declaration of Helsinki and approved by the Institutional Review Board of Stanford University (protocol ID 34651, Euan Ashley). All participants provided informed consent prior to the initiation of the study.