Muscle Metaboreflex Modulation of Heart Rate Kinetics During the Transition to Low-Moderate and High-Moderate Exercise Intensities

Abstract

Our classical understanding of heart rate kinetics is that a sympathovagal balance favouring parasympathetic tone at rest or during low level exercise contributes to a rapid heart rate adaptation (i.e. fast kinetics) during low level exercise transitions. Conversely, higher-level exercise transitions elicit slower heart rate kinetics, due in part to a sympathovagal shift to greater sympathetic tone. PURPOSE: To test the hypothesis that increasing sympathetic activity to the heart via muscle metaboreflex activation would slow heart rate kinetics during lower vs higher level exercise transitions. METHODS: Twelve healthy, active, and untrained adults (m/f = 6/6; age = 23 ± 3 yrs; peak VO2 = 40 ± 3 ml/kg/min) performed square-wave moderate intensity exercise transitions in two equal step changes between 20W to 90% ventilatory threshold (VT). The first transition was in the low moderate domain (LMD: 20W to 55 ± 13 W) and the second transition in the high moderate domain (HMD: 55 ± 13 W to 90 ± 27 W). These transitions were performed without (control; LMD-CTL and HMD-CTL) and with muscle metaboreflex activation induced by 2 min of handgrip at 40% maximal voluntary contraction during the pre-transition baseline followed by 5 min of brachial occlusion (OCC) during either the LMD (LMD-OCC) or HMD (HMD-OCC) transition. Trials were randomized and three repetitions of each condition were completed, time-aligned, and averaged by condition. Heart rate kinetics were determined using mono-exponential curve fitting. Data was analysed with non-parametric tests. P < 0.05 was significant. RESULTS: Heart rate amplitude was lower during LMD-OCC vs LMD-CTL (15 ± 5 vs 17 ± 6 beats/min; P = 0.012) and the heart rate time constant, tau, was faster during LMD-OCC vs LMD-CTL (18 ± 9 vs 26 ± 11 s; P = 0.01). Heart rate amplitude was not different between HMD-OCC vs HMD-CTL (18 ± 6 vs 19 ± 8 beats/min) and heart rate tau was not different between HMD-OCC vs HMD-CTL (49 ± 28 vs 45 ± 24 s; all P > 0.05). CONCLUSIONS: Contrary to our hypothesis, an increase in sympathetic activity induced by the muscle metaboreflex speeded heart rate kinetics during LMD but not HMD exercise transitions. Our data suggests that elevating sympathetic activity can speed heart rate kinetics to a certain point, beyond which heart rate kinetics via sympathetic activity may already be maximized.