Abstract
The precise contributions of afferent feedback to cardiovascular and respiratory responses to exercise are still unclear. The aim of this crossover study was to assess whether and how autonomic cardiovascular and respiratory control differed in response to dynamic (DYN) and isometric contractions (ISO) at a similar, low heart rate (HR) level. Therefore, 22 healthy males (26.7 ± 3.6 yrs) performed two kinds of voluntary exercises at similar HR: ISO and DYN of the right quadriceps femoris muscle. Although HR was eqivalent (82 ± 8 bpm for DYN and ISO, respectively), rating of exertion, blood pressures, and rate pressure product were higher, whereas 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. Heart rate variability (HRV) indicators, SDNN, HF-Power and LF-Power, representing both vagal and sympathetic influences, were significantly higher during ISO. Sample entropy, a non-linear measure of HRV was also significantly affected by contraction mode. It can be concluded that, despite the same net effect on HR, the quality of cardiovascular control during low intensity exercise is significantly different between DYN and ISO. HRV analysis indicated a sympatho-vagal coactivation during ISO. Whether mechanoreceptor feedback alone, a change in central command, or the interaction of both mechanisms is the main contributor of the distinct autonomic responses to the different exercise modes remains to be elucidated.
Highlights
Several mechanisms are thought to be responsible for the modulation of the circulatory and respiratory responses during voluntary muscle contractions: influences from higher brain centers, called central command, and reflex activity primarily involving inputs from chemo, mechanoand baroreceptor afferents (Rowell and O’Leary, 1990)
At similar heart rate (HR) isometric handgrip elicited a stronger increase of systolic arterial blood pressure (SBP) and diastolic arterial blood pressure (DBP) compared to cycling (Lindquist et al, 1973)
In a previous experiment with matched HR between dynamic muscular actions (DYN) and isometric contractions (ISO), we found ISO to elicit a stronger blood pressure (BP) response
Summary
Several mechanisms are thought to be responsible for the modulation of the circulatory and respiratory responses during voluntary muscle contractions: influences from higher brain centers, called central command, and reflex activity primarily involving inputs from chemo-, mechanoand baroreceptor afferents (Rowell and O’Leary, 1990). The aim of this study was to assess circulatory and respiratory responses during ISO and DYN of the lower limb at the same HR level to elucidate the mechanisms of autonomic control during the different contraction modes.
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