The human pressor response during and following voluntary and evoked isometric contraction with occluded local blood supply.

1. Changes in heart rate and blood pressure were observed, in nine healthy subjects, during and after voluntary and electrically evoked isometric contractions of the triceps surae under conditions of local circulatory arrest. 2. The progressive increases in heart rate and blood pressure seen during 2 min voluntary and evoked contractions at 30% of maximal voluntary contraction were not significantly different in the two conditions. On cessation of contraction but with circulatory arrest maintained, heart rate fell to control levels while blood pressure fell to a similar though still significantly elevated level in both conditions. Elevated blood pressure was maintained for 2 min until the circulatory occlusion was removed; however it was maintained at a significantly higher level for the last 60-90 s of occlusion following electrically evoked contractions compared to voluntary contraction. 3. Comparison of the responses to voluntary and involuntary electrically evoked contractions suggest that 'central command' is not necessary for the initial increases in heart rate and blood pressure to occur. In addition the removal of central command on cessation of contraction need not account for the return of heart rate to control levels or the drop in blood pressure at that time. Maintained blood pressure elevation during circulatory occlusion would seem to be due to the trapping of chemical substances within the muscle interstitium.