Heart rate response to a standardized walking exercise in the Arctic circumpolar region in morning vs. evening during the polar night and midnight sun

Aim. Awareness of daytime and/or seasonal variation in performance and exercise efficiency can be important for athletes and coaches in order to suitably plan training sessions and avoid over-training. The study goal was to evaluate and compare walking-speed and time-related heart rate (HR) responses to a walking task at two times of day and year. Methods. Five healthy females (age 21-35 years) performed 9 km outdoor walking at 09:00h and 18:00h on 4 consecutive days in a period without sunlight (January) and with continuous daylight (May). Walking speed, average-heart rate (HR) and peak-HR (as %HR(max)) were compared across all exercise sessions. Results. In January the subjects achieved a higher peak-HR when they trained in the evening vs. morning, while average-HR and walking speed were not modified significantly. In May they achieved a higher peak-HR and kept a higher average-HR when they trained in the morning, under the same walking speed. In both training-times carried out in May, average-HR and peak-HR were lower compared with January, while the walking speed was unvaried. Conclusions. A lower exercise average and peak HR, that could potentially be a favourable condition for exercise conditioning, was observed at both daily test times in May vs. January, with overall lowest HR observed in May when exercise occurred at 18:00 h. These findings support the presence of both seasonal and time of day effects on HR responses to a standardized exercise.