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University Department of Neurology, Institute of Psychiatry, and King's College Hospital Medical School, de Crespigny Park, London SE5 8AF

1. The strategy of accurate movement of the human thumb has been studied in nine subjects. An open-loop hypothesis, which states that a new final position is defined by re-setting the agonist/antagonist spring constants, was tested

2. Subjects were trained to flex the top joint of the thumb rapidly through 20 deg in about a third of a second from a fixed starting position against a load. Occasionally, and unpredictably, the viscous friction of the load was altered prior to it's being moved. The spring hypothesis predicts that such a change in load should have no effect on final position accuracy.

3. Under normal conditions no final position error developed when the viscous friction was increased. A small overshoot occurred when the viscous friction was decreased.

4. The electromyogram recorded from surface electrodes over the belly of flexor pollicis longus in the forearm revealed an increase in activity in response to an increase in viscous friction and a decrease in activity when the viscous friction was reduced.

5. When the joint and cutaneous afferents from the thumb were anaesthetized, the e.m.g. response to a change in viscous friction was severely attenuated and consistent final position errors developed.

6. Even though the compensatory open-loop muscle properties went some way towards maintaining accuracy, the change in final position error that occurred as a result of thumb anaesthesia correlated well (r = 0·84) with the amount of muscle e.m.g. response that was lost.

7. The latency of the e.m.g. response to a change in viscous friction was compared to that of a voluntary response by asking the subject to push down or let go upon perception of the load change. Approximately the first 100 ms of the e.m.g. response was unaffected by the voluntary intervention of the subject.

8. We conclude that the spring hypothesis does not explain human thumb movement. It is argued that the long-latency stretch reflex machinery is responsible for some automatic compensation for unexpected interference with movement.

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