CONTROL OF BLOOD-FLOW THROUGH HUMAN FOREARM MUSCLES FOLLOWING BRIEF ISOMETRIC CONTRACTIONS

Abstract

Blood flow through the forearm was measured 2 s after single, brief isometric hand-grip contractions. The tension and duration of those contractions varied from 10-100% of the maximal voluntary contraction (MVC) and from 2-12 s, respectively. Blood flow increased linearly with tension up to .apprx. 60% MVC but further increases in tension, up to 100% MVC, did not elicit higher blood flow rates than were found at 60% MVC. The same relationship between tension and the resultant blood flow held for all durations of contractions, from 2-12 s. Blood flow immediately after (2 s) contractions at a given tension increased linearly with contraction duration from 2-12 s. Maximal exercise blood flow was approached only in response to the longest contractions (12 s) at tensions of 60% MVC or higher. Brief alterations (2-5 s) of transmural pressure across blood vessels did not result in a significant change of blood flow in the resting forearm or when the vessels were dilated by brief, isometric contractions. When tension was applied or released rapidly or gradually (ramp contractions) there was no correlation between the rate of change of stretch on arterial vessels and the resultant blood flow. There was a direct relationship between a force-time integral (duration of contraction .times. peak tension) and blood flow. Changes in blood flow in the forearm elicited by brief isometric contractions are not the result of a myogenic reflex but are metabolically induced. Successive contractions exerted at 60% MVC for 4 s induced a blood flow of 21.2 .+-. 1.6 ml/min .cntdot. 100 ml. when a rest interval of 8 s was allowed between contractions. Blood flow remained constant at this submaximal level, even when muscular fatigue was induced, and when there was an accompanying large increase in blood pressure. Isometric muscular activity by the contralateral arm which resulted in fatigue and was associated with a large increase in mean blood pressure, did not alter the vasodilatation that was induced by brief, isometric contractions in the test arm. Vasodilatation in response to intermittent isometric contractions may be the result of metabolic vasodilatation of distal segments and continued sympathetic vasoconstriction of the proximal segments of the forearm vascular bed.