Circannual variations in physiological response during unsteady-workload exercise

Purpose: We aimed to identify circannual variations in physiological responses during unsteady-workload exercise. Methods: The study included 18 healthy men who provided written informed consent before participation. The study was performed in summer and winter. Heart rate (HR), blood pressure (BP), oral temperature, and cardiac autonomic nervous system modulation expressed as the natural logarithm of the high-frequency band of HR variability (lnHF) were measured at rest. All subjects performed a two-part cycling exercise (with steady and unsteady workload) for a total of 32 min, followed by recovery in the supine position for 10 min. The steady-state exercise test consisted of three 4-min bouts of exercise at 20, 60, and 40% of maximal oxygen intake (VO2max). The unsteady-workload exercise test consisted of five 4-min bouts of exercise with gradual increase and decrease in workload between 20 and 60% of VO2max. HR, systolic BP, diastolic BP, and rating of perceived exertion (RPE) were measured under both conditions. The HR maximal values, minimal values, amplitude, and phase lag (response to maximal/minimal workload) during unsteady-workload exercise were calculated. Results: Resting systolic and diastolic BP, as well as lnHF were significantly lower in summer than in winter, whereas no significant differences in HR or oral temperature were noted. During exercise, summer was associated with significantly higher HR, RPE, and HR amplitude, but significantly lower diastolic BP, and significantly longer HR phase lag during minimum and maximum workload. Post-exercise HR was significantly lower in winter than in the summer. There was no significant difference in systolic BP between the two seasons, but diastolic BP and lnHF were significantly higher in winter. Conclusion: Exercise prescriptions should take into consideration the human circannual rhythm to avoid adverse cardiac and cerebrovascular events during exercise.