H10 chest strap heart rate monitor

Both the HIIT exercise and rest condition in this study were conducted on a Wattbike stationary exercise bike (Wattbike, Geelong, Australia). Heart rate was measured using a Polar H10 chest strap heart rate monitor, and was tracked using the Polar Beat mobile app (Polar Electro, Finland).
A 64-channel Ag/AgCl electrode cap (ActiCHamp Plus, Brain Products, Germany) with a 10–20 system montage was used to record continuous EEG data at a 500 Hz sampling rate (low cut-off: DC; high cut-off: 140 Hz), using an ActiCHamp amplifier and BrainVision Recorder software (Brain Products, Germany). Electrode impedances were kept below 10kΩ^{29 (link)}. All EEG recordings were conducted with an online reference at FCz, and later re-referenced to the average of all channels during pre-processing.

The main instrument used for the pilot study was the Polar Ignite smartwatch with integrated Global Positioning System (GPS) and wrist-based pulse rate [10 ]. It is a small (dimensions: 43 × 43 × 8.5 mm) and lightweight (weight: 35 g with wristband) device with GPS, Bluetooth, and heart rate monitoring capabilities. The device is worn on the wrist of the non-dominant hand, with the device fitting snugly but comfortably to the wrist. For data collection purposes, the “Other Indoor” mode on the device was used, which constantly tracked the participants’ heart rate (1 s averages) and total sleep length with interruptions. Previous research has demonstrated that heart rate data from the Polar Ignite smartwatch showed strong correlation values (r > 0.90) when compared to the Polar H10 chest strap heart rate monitor during exercise [11 (link)]. Furthermore, a similar device by Polar, the A370 fitness watch [12 ], showed no significant difference when compared to polysomnography in terms of measuring the time between sleep onset and offset (i.e., falling asleep and waking up) in both young and older age groups (p = 0.10 and 0.46) [13 (link)]. The minimum wear time criterion for inclusion in the study was 4 days of 24 h.