ARM MUSCLE SYMPATHETIC-NERVE ACTIVITY DURING PREPARATION FOR AND INITIATION OF LEG-CYCLING EXERCISE IN HUMANS

We tested the hypothesis that sympathetic vasoconstrictor nerve activi ty to nonactive skeletal muscle (MSNA) decreases immediately before an d remains suppressed during initiation of conventional large muscle up right dynamic exercise in humans. In 11 healthy young subjects, adequa te recordings of MSNA from the radial nerve in the arm were obtained d uring upright seated rest (control) and throughout 1 min of leg-cyclin g exercise at one or more submaximal workloads (range 33-266 W; simila r to 10-80% of peak power output). MSNA was analyzed during four conse cutive time intervals: control, preparation for cycling (end of contro l to onset of pedal movement), initiation of cycling (onset of pedal m ovement to attainment of target power output), and the initial 60 s of cycling at target power output. MSNA decreased (P < 0.05) abruptly an d markedly in all subjects [to 19 +/- 4% (SE) of control levels] durin g the preparation period before the 33-W load and remained suppressed throughout the period of initiation of cycling in 8 of 11 subjects; MS NA increased during the initiation period in three subjects in whom di astolic arterial pressure fell below control levels. This general patt ern was observed at all loads. MSNA remained at or below control level s throughout the 1 min of cycling exercise at 33-166 W. MSNA increased above control levels during the latter portion of the 1 min of cyclin g only at loads greater than or equal to 60% of peak power output. Our results indicate that MSNA in a relaxed arm is almost completely inhi bited during a period of preparation for leg-cycling exercise, remains suppressed during the initiation of exercise in most subjects, and in creases above control levels only during the initial minute of cycling at moderate-to-heavy submaximal exercise intensities.