2.3. Heart Rate Variability (HRV)
A Zephyr BioHarness strap and BioModule sensor (Medtronic, Boulder, CO) were used to continuously monitor electrographic (ECG) and respiratory data at a sampling frequency of 1000 Hz. All participants were supine with free breathing for a stationary assessment of heart rate for 5 min. From this, the RR intervals were quantified, and HRV was determined (Kubios HRV, Version 3.4.1, Kuopio, Finland).HRV was analyzed in time and frequency domain measures [