Intensity-dependent modulation of sympathetic neurocirculatory control during incremental rhythmic handgrip exercise in humans

The exercise intensity at which muscle sympathetic nerve activity (MSNA) recorded in the inactive limb begins to rise during rhythmic handgrip (RHG) exercise remains unknown. Forearm blood flow during RHG exhibits an attenuated rate of increase at ∼50% maximal voluntary contraction force (MVC), although whether this occurs in parallel with sympathetic activation remains unknown. Therefore, we tested the hypotheses that MSNA increases at intensities <50% MVC and coincides with an attenuation in forearm vascular conductance (FVC) during incremental RHG. Twenty-nine young adults (11 males; 26 [4] yrs; mean [SD]) underwent assessments of blood pressure (photoplethysmography), cardiac output and total peripheral resistance (TPR; Modelflow), brachial artery blood flow of the exercising arm (doppler ultrasound), and MSNA (microneurography) during incremental RHG exercise to fatigue. An operational onset threshold (deviation beyond baseline variability) for MSNA burst frequency, amplitude, and total activity occurred between 36-41% MVC. Cardiac output began to rise at 37% MVC, whereas mean arterial pressure began to rise at 45% MVC. TPR demonstrated a linear decline. FVC and brachial artery diameter increased rapidly at low intensities, with attenuation in the rate of rise at ∼20% MVC, whereas forearm blood flow and shear rate increased linearly. Collectively, these data challenge the prior view that during RHG, MSNA increases only at intensities above 50% MVC, and indicate that sympathetic recruitment occurs during light-to-moderate stages of incremental exercise. Moreover, the dissociation between MSNA and FVC indicates that resting limb MSNA does not predict direct efferent sympathetic vasoconstrictor signalling within contracting skeletal muscle.