Cardiac Autonomic Control and Muscle Sympathetic Nerve Activity during Dynamic Exercise

We compared sympathetic outflow to the heart and skeletal muscle (MSNA) during dynamic exercise to test whether their mechanisms of control were the same. The sympathetic component to the heart was evaluated by heart rate variability analysis of the power spectrum. MSNA was recorded from the median nerve during graded leg cycling lasting 16min at loads of 20, 40, 60, and 75% of maximal oxygen uptake (V_O2max) in the sitting position. The R-R interval and heart rate variability as well as low (P₁) and high (P_h) power frequency decreased with increasing exercise intensity while no significant change was observed in total power (P₁). The indicator of the cardiac sympathetic component, P₁/P_h, and the parasympathetic component, P_h/P₁, increased and decreased relative to exercise intensities, respectively. MSNA, represented as burst frequency (BF), was suppressed by 21.4% at 20% V_O2max, and thereafter BF increased with the exercise intensity by 23.5% and by 79.4% at 60 and 75% V_O2max, respectively, compared to the baseline level. There was a close positive and negative correlation between changes in BF and those in P₁/P_h, (r = 0.593,p<0.002) and P_h/P₁ (r=-0.681,p<0.0001), respectively. These results indicate acceleration of the sympathetic component of heart rate and increase in sympathetic outflow to the skeletal muscle during graded exercise. However, the exact control mechanisms of these sympathetic responses to graded exercise in two different organs remain unclear.