Polar flow web service

Initially, the 12-lead ECG and Polar V800^™ HR monitor were started to simultaneously record R-R intervals at an acquisition rate of 1000 Hz. Cardiology XML files obtained from the ECG were imported into Kubios HRV Premium (ver. 3.2) (The Biomedical Signals Analysis and Medical Imaging Group, University of Kuopio, Finland) in order to export the R-R intervals detected automatically by its built-in QRS detection algorithm. Kubios HRV Premium (ver. 3.2) marked each R-wave with “+” sign that could be moved or removed to correct falsely detected R-waves. The detected R-R intervals were manually inspected on the ECG tachogram to ensure there were no false R-wave detections. If an R-wave was falsely determined, one of the authors (BTC) replaced it by moving “+” signs on the correct R-wave. The detected R-R intervals from ECG were then checked by another author (DAG) to ensure data accuracy and saved in a space delimited ASCII text file. In addition, the R-R intervals recorded by Polar V800^™ HR monitor were exported from the Polar Flow web service (ver. 2.3; Polar Electro Oy, Kempele, Finland) in a space delimited ASCII text file.

The interval exercise session consisted of 3-min bouts of exercise repeated 6 times at an intensity ≥ 80% of the participant’s second ventilation threshold (VT₂), and at ≥80% of the participant’s maximum heart rate recorded during the VO₂ max test. The rest interval was 2 min at 2.5 mph and 0 degrees of inclination. The exercise was conducted under the supervision of a certified exercise physiologist from the Canadian Society for Exercise Physiology at the CTEP lab. Each exercise session included a 5–10-min warmup, the interval workouts, and a 5-min cooldown. Continuous ECG, oxygen uptake, and ventilation were recorded with a metabolic analyzer (Vmax Encore 29, CareFusion, USA) and 12-lead ECG (CardioSoft, GE Healthcare, USA). Furthermore, the participants wore a heart rate monitor (V800, Polar Electro Oy, Finland), and the data were uploaded to the Polar Flow web service (Polar Electro Oy, Finland) for analysis. The exercise training session began around 17 h00–18 h00, when the exercise was, at most, 6 h before bedtime; this time of day has been shown to be popular for adults to exercise (US Census Bureau 2015).