The Mechanoreflex Response to Static and Dynamic Passive Limb Movement

Abstract

During exercise, the exercise pressor reflex (EPR) activates to increase efferent sympathetic nervous activity, increasing blood pressure (BP) and heart rate (HR), and thus increasing perfusion pressure. One component of the EPR, the mechanoreflex, has only recently been studied in terms of its central and peripheral cardiovascular effects due to the difficulty in separating it from the other component of the EPR, the metaboreflex. Two methods have been used to experimentally activate the mechanoreflex, static stretch (SS) and dynamic passive limb movement (PLM), though no study has yet to directly compare them. PURPOSE: The purpose of the present study was to compare the central and peripheral hemodynamic responses between these two methods (PLM and SS) of isolating the mechanoreflex. METHODS: Two minutes of PLM and SS were performed in 23 young healthy participants, in a counterbalanced fashion. Central hemodynamic responses (cardiac output [CO], stroke volume [SV], and HR), were measured using a Finometer, while peripheral hemodynamic responses were measured using a near-infrared spectrometer (NIRS) to assess microvascular responses via percent tissue oxygen saturation (StO2). RESULTS: No significant differences were observed between the central or peripheral baseline values obtained for PLM and SS (p>0.05). Both methods significantly increased HR and CO from baseline; however, PLM elicited significantly higher central cardiovascular than SS (ΔHR: 15.0±9.5 vs 8.6±8.1 bpm, ΔCO: 1.3± 0.8 vs. 0.8±0.7 L/min) (p<0.05). Peripherally, PLM elicited an increase in StO2 from baseline (p<0.05), while SS had an opposite effect with a significant decrease in saturation from baseline (ΔStO2: 2.80±2.49 % vs -1.31±1.1 %) (p<0.05). CONCLUSION: The results from this study call into question whether SS may be a clear isolation of the mechanoreflex, with the observed reduction in tissue oxygenation, and thus possible activation of the metaboreflex. Finally, static stretch and dynamic muscle movement appear to exhibit divergent responses, and this ought to be considered when designing or interpreting studies using these models of the mechanoreflex to explore health and disease.