Heart rate variability
ECTOPIC BEATS
On the day of the assessment, participants were asked to lie supine for 10 min or more to attain complete relaxation. Their skin was prepared by using a razor to remove the hair from the intended site of the HR sensor (in males only), cleaning the skin and drying it with gauze. They were asked to sit in a chair with armrests. The participants were instructed to wear an HR sensor on an elastic chest strap around the thorax in direct contact with the skin. RR intervals (the time elapsed between two successive R-waves of the QRS signal on the electrocardiogram) were recorded at a sampling frequency of 1000 Hz using the HR sensor (Polar H7, Polar Electro Oy, Kempele, Finland), which was connected via Bluetooth to an Android smartphone application (Elite HRV, Asheville, NC, USA). A digital electrocardiogram (ECG) was recorded for 5 min, and blood pressure was assessed in a sitting positionTime-domain analysis of HRV offers insights into the heart rate's temporal characteristics and the autonomic nervous system's influence on cardiac regulation. The time-domain measures of HRV were derived from a series of RR intervals. Various time-domain HRV indices were examined, including the mean of the normal-to-normal RR interval (MeanRR), the root mean square of successive differences between adjacent RR intervals (RMSSD), and the standard deviation of normal-to-normal RR intervals (SDNN). Additionally, frequency-domain indices were explored to analyze the distribution of power spectra across different frequency bands, revealing sympathetic-parasympathetic balance and autonomic heart rate control. These indices included total power (TP), reflecting overall signal variability (0.003–0.4 Hz); low-frequency spectral power (LF), associated with both sympathetic and parasympathetic activity (0.04 and 0.15 Hz); high-frequency spectral power (HF), mainly linked to parasympathetic activity (0.15 and 0.4 Hz); normalized unit of low frequency (nuLF), representing the percentage of total power in the LF band and indicating the sympathetic-parasympathetic balance; normalized unit of high frequency (nuHF), expressing the percentage of total power in the HF band and signifying parasympathetic activity; and the ratio of low frequency to high frequency (LF/HF), frequently employed as an indicator of sympathetic-parasympathetic balance. A higher LF/HF ratio implies greater sympathetic activity, while a lower ratio indicates higher parasympathetic activity. Collectively, these HRV variables provide comprehensive insights into the relative contributions of the sympathetic and parasympathetic branches of the autonomic nervous system to heart regulation. All HR recordings underwent meticulous visual inspection to ensure stationarity and were corrected for artifacts and ectopic beats using Kubios' built-in piecewise cubic spline interpolation