ENDURANCE TRAINING ATTENUATES THE VESTIBULOSYMPATHETIC REFLEX IN RUNNERS

Abstract

A possible mechanism for exercise training-induced orthostatic intolerance is attenuation of the vestibulosympathetic reflex due to constant stimulation of the otolith organs during running. We tested the hypothesis that 8 weeks of endurance training will attenuate the vestibulosympathetic reflex in runners, but not stationary bikers. Arterial blood pressure, heart rate, calf blood flow, and muscle sympathetic nerve activity (MSNA) were recorded in healthy, sedentary, prone subjects (25±1 yr) at baseline and during head-down rotation (HDR) before and after 8 weeks of endurance training. Subjects were assigned to running (n=13), stationary biking (n=12), or time control (n=12) groups. HDR increased MSNA and decreased calf vascular conductance (CVC) significantly in all groups before training. In contrast, after training MSNA did not increase (p>0.05) during HDR in runners (Δ2±1 bursts/min from baseline), but did in bikers and controls (Δ5±1 and Δ4±2 bursts/min, respectively; p<0.05). Furthermore, CVC did not decrease from baseline with HDR in runners, but decreased in bikers and controls (p<0.001). These data suggest an attenuation of the vestibulosympathetic reflex in endurance-trained runners, but not stationary bikers. This is the first study to suggest that endurance running decreases the sensitivity of the otolith organs to changes in gravitational inputs. NIH DC006459, HL077670, and MO1RR10732