Reflex stimulation of sympathetic outflow during rhythmic exercise in humans

The aim of this study was to determine whether a chemically generated reflex arising in nonischemic skeletal muscle normally increases sympathetic outflow during rhythmic exercise. To accomplish this aim, we recorded muscle sympathetic nerve activity (MSNA, peroneal nerve) in conscious humans during exercise interventions designed to alter the relationship between muscle blood flow and metabolic demand. Under normal conditions, MSNA increased during moderate but not during mild levels of rhythmic handgrip (0.67 Hz) and one-arm cycling (0.83 Hz). MSNA remained elevated when the moderate level of rhythmic handgrip was followed by forearm vascular occlusion, a maneuver that sustains muscle chemoreflex stimulation. Complete arm vascular occlusion was needed to increase MSNA during mild arm cycling, whereas even partial vascular occlusion greatly amplified the stimulation of MSNA normally produced by the moderate level of arm cycling. We conclude that a chemically generated reflex arising in nonischemic working muscle plays an important role in the normal activation of sympathetic discharge to nonexercising leg muscles during moderate but not during mild levels of rhythmic arm exercise.