O2uptake and blood pressure regulation at the onset of exercise: interaction of circadian rhythm and priming exercise

Abstract

Circadian rhythm has an influence on several physiological functions that contribute to athletic performance. We tested the hypothesis that circadian rhythm would affect blood pressure (BP) responses but not O2uptake (V̇o2) kinetics during the transitions to moderate and heavy cycling exercises. Nine male athletes (peak V̇o2: 60.5 ± 3.2 ml·kg−1·min−1) performed multiple rides of two different cycling protocols involving sequences of 6-min bouts at moderate or heavy intensities interspersed by a 20-W baseline in the morning (7 AM) and evening (5 PM). Breath-by-breath V̇o2and beat-by-beat BP estimated by finger cuff plethysmography were measured simultaneously throughout the protocols. Circadian rhythm did not affect V̇o2onset kinetics determined from the phase II time constant (τ2) during either moderate or heavy exercise bouts with no prior priming exercise (τ2moderate exercise: morning 22.5 ± 4.6 s vs. evening 22.2 ± 4.6 s and τ2heavy exercise: morning 26.0 ± 2.7 s vs. evening 26.2 ± 2.6 s, P > 0.05). Priming exercise induced the same robust acceleration in V̇o2kinetics during subsequent moderate and heavy exercise in the morning and evening. A novel finding was an overshoot in BP (estimated from finger cuff plethysmography) in the first minutes of each moderate and heavy exercise bout. After the initial overshoot, BP declined in association with increased skin blood flow between the third and sixth minute of the exercise bout. Priming exercise showed a greater effect in modulating the BP responses in the evening. These findings suggest that circadian rhythm interacts with priming exercise to lower BP during exercise after an initial overshoot with a greater influence in the evening associated with increased skin blood flow.