Heart rate and variability
The electrical activity of the heart was recorded using electrocardiography (ECG) by two pre-gelled Ag/AgCl electrodes placed ~ 5 cm cranial and ~ 3 cm left-lateral from the distal end of the sternum and over the anterior to mid-axillary line at the fifth left rib. ECG signals were continuously recorded (sample rate 1000 Hz) and processed in real-time to calculate heart rate (HR [bpm]) and interbeat intervals (IBI [ms]). The IBI timeseries were checked for artifacts and erroneous intervals were excluded and replaced by polynomial interpolation (2