Initiation of increase in muscle sympathetic nerve activity delay during maximal voluntary contraction.

To investigate the effects of maximal voluntary exercise on sympathetic nerve activity, contraction force and muscle sympathetic nerve activity (MSNA) were recorded during maximal (MVG) and submaximal voluntary isometric handgrip (SVG) for 2 min in eight healthy subjects. MSNA was determined by a microneurographic technique, and handgrip force, heart rate (HR) and arterial blood pressure (ABP) were measured by a non-invasive method during exercise. Grip force decayed rapidly to 58% of maximal grip force (MGF) at 10 s after commencement of exercise and was almost constant (approximately 30% of MGF) 40 s after exercise. MSNA increase was delayed by 20 s during MVG, followed by a gradual increase. HR was elevated immediately after onset of exercise, while mean ABP rise showed a 20 s lag from initiation of MVG exercise. During SVG increases in MSNA, HR and mean ABP were delayed by 50, 40 and 20 s, respectively, relative to commencement of exercise. Thereafter, these parameters increased time-dependently. These results suggested that the MSNA increase during MVG may be predominantly because of the metaboreflex.