Co-Activation vs. Reciprocal Antagonism

Abstract

The model of a two-dimensional autonomic space, where equivalent heart rates (HR) can be considered as an end product achieved by different autonomic modes, such as reciprocal behavior or sympathovagal co-activation, has been introduced in the early 1990s (1). Findings of exercise physiologists question the traditional view of the interplay between both autonomic branches as a reciprocal antagonism, with a reduction of vagal efferent activity at the beginning or during low intensity exercise to a minimum at HRs around 100 bpm and a subsequent rise in sympathetic activity during higher exercise intensities (2-4). PURPOSE: To elucidate the mechanisms of autonomic cardiac control during low intensity isometric (ISO) vs. dynamic exercise (DYN). METHODS: Twenty healthy males performed two kinds of voluntary exercises at similar HR: ISO and DYN of the right quadriceps femoris muscle (randomized, cross-over design). RESULTS: Although HR was equivalent (82 ± 8 bpm for DYN and ISO, respectively, p = 0.489), heart rate variability (HRV) indicators representing dual autonomic and sympathetic influences were significantly higher during ISO. Rating of perceived exertion and blood pressure were higher, while breathing frequency, minute ventilation, oxygen uptake and carbon dioxide output were significantly lower during ISO. Tidal volume, end-tidal partial pressures of O2 and CO2, respiratory exchange ratio and capillary blood lactate concentration were comparable between both contraction modes. CONCLUSION: Low intensity ISO seems to contribute to a sympathovagal co-activation. Results support the model of a two-dimensional autonomic space and help to explain mechanisms of autonomic cardiovascular control under low intensity exercise. REFERENCES 1. Berntson GG, Cacioppo JT, Quigley KS. Cardiac psychophysiology and autonomic space in humans: empirical perspectives and conceptual implications. Psychol Bull. 1993;114(2):296-322. 2. Fisher JP. Autonomic control of the heart during exercise in humans: role of skeletal muscle afferents. Experimental Physiology. 2014;99(2):300-5. 3. Weippert M, Behrens K, Rieger A, Stoll R, Kreuzfeld S. Heart Rate Variability and Blood Pressure during Dynamic and Static Exercise at Similar Heart Rate Levels. PLoS One. 2013;8(12). 4. White DW, Raven PB. Autonomic neural control of heart rate during dynamic exercise: revisited. J Physiol. 2014;592(Pt 12):2491-500.