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1. Blood flow in resting forearm and calf were measured plethysmographically in healthy young men during isometric contraction performed both as a handgrip and as a dorsiflexion of the foot. The isometric contraction was maintained for 2 min at one third maximal voluntary contraction for the handgrip and half maximal voluntary contraction for the dorsiflexion of the foot. In some experiments the possible influence on blood flow of inadvertent muscle activation in the resting limb was checked by recording the e.m.g.

2. Both handgrip and dorsiflexion of the foot produced substantial increases in heart rate and mean arterial pressure, the pressure rise being almost linear with time throughout the contraction.

3. Isometric contraction produced a rapid increase in forearm blood flow which reached a maximum on average two and a half times the resting value after 1 min and then declined slightly. Similar increases in forearm blood flow were produced by handgrip and dorsiflexion of the foot.

4. Unlike in the resting forearm, the blood flow in the resting calf increased only slightly during the first minute of contraction and then decreased again to the resting level.

5. The flow increase in the resting limbs could not be ascribed to inadvertent muscle activation as judged from the e.m.g. recordings.

6. It is concluded that isometric muscle contraction produces a rapid increase in blood flow in the resting forearm, but only a very slight flow increase in the calf. Since the flow increased faster than the arterial pressure it must to a certain extent be induced by active neurogenic vasodilatation. Similarly, the relative flow decrease during the latter half of the contraction concomitantly with a progressively rising arterial pressure suggests that the neurogenic effect on the resistance vessels changes character, becoming more vasoconstrictive, and this might be related to increased effort in sustaining the contraction.

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