| Summary: | Human cardiovascular control during exercise is regulated by central command, muscle mechanoreflex stimulation and muscle metaboreflex activation. The muscle mechanoreflex can be stimulated by passive muscle stretch, which causes cardiovascular responses. However, the influence of passive stretch-induced muscle mechanoreflex stimulation on the baroreflex is unknown. Therefore, this thesis investigated the effects of muscle mechanoreflex stimulation via passive calf muscle stretch on baroreflex function in humans. Firstly, spontaneous baroreflex sensitivity decreases progressively during isometric exercise of increasing intensity. A concomitant rightward resetting of the baroreflex occurs, which shifts further rightward as exercise intensity increases. Secondly, muscle mechanoreflex stimulation by passive calf muscle stretch decreases spontaneous baroreflex sensitivity at rest, and during graded levels of local metabolite accumulation following isometric exercise of increasing intensity. Thirdly, muscle mechanoreflex stimulation by passive calf muscle stretch during concurrent local metabolite accumulation decreases the maximal gain of the function curve for carotid baroreflex control of heart rate, but not blood pressure. Overall, these findings suggest that muscle mechanoreflex stimulation via passive muscle stretch decreases baroreflex sensitivity via cardiac vagal inhibition, likely by modulating inputs at central integration sites. Also, metabolite sensitisation of stretch-sensitive muscle mechanoreceptive afferents is implied, which augments this cardiac vagal inhibition.
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