Polar h10 chest strap

We measured basic demographic data: sex, age, height, weight, body mass index (BMI), and exercise performance. We obtained weight with the usage of a TANITA device (TANITA Corporation, Arlington Heights, IL, USA) and height with the usage of a SECA stadiometer (SECA GmbH & Co., Hamburg, Germany). Both weight and height were measured in the morning before breakfast. HR was measured with the Polar H10 chest strap (Polar Electro Oy, Kempele, Finland), continuously synchronized with the Cortex B3 Metamax. VE, VCO₂, oxygen uptake, respiratory rate, and tidal volume were measured by the Cortex B3 Metamax using the breath-by-breath method (Hans Rudolph V2 Mask, Hans Rudolph, Inc., Shawnee, KS, USA). Variables were averaged in 15-s intervals. All measurement devices were calibrated for each usage in line with the producer’s instructions.
The v-slope method has been previously used to find VT1 [28 (link)]. VT1 was identified in all endurance athletes enrolled in this study. VE/VCO₂-slope was defined as the linear relationship between VE and VCO₂ from the start to VT1, excluding the first minute of the protocol, where noise values emerged. VE/VCO₂-Nadir was defined as the lowest continuous 30 s average. VE/VCO₂-Total was calculated across the whole CPET protocol without the first minute of the protocol.

Each participant was fitted with a Polar H10 heart rate sensor chest strap to assess HRV. The Polar H10 chest strap collected and processed HRV measurements by detecting the electrical signals of the heart. This method has been validated against electrocardiogram recordings (Gilgen-Ammann et al., 2019 (link)). Polar H10 chest straps were connected to the EliteHRV^© app (via Bluetooth 4.0 signal) that was downloaded on each participant's phone—as previously described (Speer et al., 2020 (link)). For each measurement, participants were required to lay resting, in supine, and collect a 5 min recording of their heart rate using the app. The raw R-R interval data of the 5 min recording was exported as a text file to Kubios HRV software (version 3.1.0, Biosignal Analysis and Medical Imaging Group, Kuopio, Finland) for analysis of HRV parameters within the time domain (root mean square of successive differences between normal heartbeats, RMSSD). This measure was then log transformed for analysis (Kobayashi et al., 2012 (link)). The RMSSD is a valid measure of HRV and has been shown to be responsive to sleep deprivation (Bourdillon et al., 2021 (link)).