MUSCLE SYMPATHETIC-NERVE RESPONSES TO GRADED LEG CYCLING

The aim of this study was to clarify the relationship between sympathe tic outflow to skeletal muscle and oxygen uptake during dynamic exerci se. Muscle sympathetic nerve activity (MSNA) was recorded from the rig ht median nerve microneurographically in eight healthy volunteers duri ng leg cycling at four different intensities in a seated position for a 16-min bout. Work loads selected were 20, 40, 60, and 75% of maximal oxygen uptake (VO2max). Heart rate and blood pressure were measured d uring each exercise test. MSNA burst frequency was suppressed by 28% d uring cycling at 20% VO2max (23 vs. 33 bursts/min for control). Therea fter, it increased in a linear fashion with increasing work rate, with a significantly higher burst frequency during 60% VO2max than the con trol value. Both heart rate and mean blood pressure rose significantly during 20% VO2max from the control value and increased linearly with increased exercise intensity. During light exercise, MSNA was suppress ed by arterial and cardiopulmonary baroreceptors as a result of the he modynamic changes associated with leg muscle pumping. The baroreflex i nhibition may overcome the muscle metaboreflex excitation to induce MS NA suppression during light exercise. These results suggest that durin g light exercise MSNA is inhibited, perhaps due to loading of the card iopulmonary and arterial baroreflexes, and that during heavier exercis e the increase in MSNA occurs as muscle metaboreflexes are activated.