Influence of force on muscle and skin sympathetic nerve activity during sustained isometric contractions in humans.

1. Our purpose was to test the hypothesis that efferent sympathetic nerve activity to non-active skeletal muscle (MSNA) and skin (SSNA) is independent of the level of force during sustained submaximal isometric contractions in humans. 2. In twelve healthy subjects, arterial blood pressure, heart rate, and MSNA (n = 6) or SSNA (n = 6) (peroneal microneurography) were recorded before and during isometric handgrip contractions sustained to exhaustion at 20, 40 and 60% of maximal force. Responses were examined at similar percentages of endurance time at each level of force. 3. Contraction duration decreased progressively with increasing force (495 +/- 54, 140 +/- 13, 73 +/- 8 s, respectively), but peak ratings of perceived effort were similar for the three force levels. 4. The peak increases in systolic pressure were not different among the three levels of force. The increases in diastolic and mean pressure were similar at 40 and 60% of maximal force, but were smaller at the end of 20% of maximal force. The contraction-induced rise in heart rate was directly related to the level of force. 5. The contraction-evoked stimulation of both MSNA and SSNA was similar during handgrip at 40 and 60% of maximal force, but was much less during handgrip at 20% of maximal force. The increases in SSNA were associated with increases in both skin blood flow and skin electrical conductance suggesting primarily sudomotor fibre activation. 6. These findings indicate that there is a minimum force necessary to elicit peak levels of MSNA and SSNA during sustained isometric contractions in humans. When normalized to endurance time, however, the regulation of these sympathetic outflows appears to be independent of force above this minimum level. The results also indicate that during this type of muscle activity the relationship between force and heart rate is different to that between force and peripheral sympathetic discharge.