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

We tested the hypothesis that sympathetic vasoconstrictor nerve activity to nonactive skeletal muscle (MSNA) decreases immediately before and remains suppressed during initiation of conventional large muscle upright dynamic exercise in humans. In 11 healthy young subjects, adequate recordings of MSNA from the radial nerve in the arm were obtained during upright seated rest (control) and throughout 1 min of leg-cycling exercise at one or more submaximal workloads (range 33-266 W; similar to 10-80% of peak power output). MSNA was analyzed during four consecutive time intervals: control, preparation for cycling (end of control to onset of pedal movement), initiation of cycling (onset of pedal movement to attainment of target power output), and the initial 60 s of cycling at target power output. MSNA decreased (P < 0.05) abruptly and markedly in all subjects [to 19 +/- 4% (SE) of control levels] during the preparation period before the 33-W load and remained suppressed throughout the period of initiation of cycling in 8 of 11 subjects; MSNA increased during the initiation period in three subjects in whom diastolic arterial pressure fell below control levels. This general pattern was observed at all loads. MSNA remained at or below control levels 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 cycling 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 inhibited during a period of preparation for leg-cycling exercise, remains suppressed during the initiation of exercise in most subjects, and increases above control levels only during the initial minute of cycling at moderate-to-heavy submaximal exercise intensities.