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Prince of Wales Medical Research Institute, School of Physiology and Pharmacology, University of New South Wales, Kensington, Sydney, Australia.

1. Subjects' ability to detect movements imposed at the elbow during active flexion was measured. Movements of three different angular velocities (0.04, 0.4 and 4.4 deg/s) were applied to the arm while subjects maintained one of two force levels of active flexion. The threshold magnitudes for detection of the direction of imposed movement were found. 2. All thresholds were very small. At the lowest velocity of movement the average threshold was 0.13 deg and no subject had a threshold of greater than 0.3 deg. This contrasts with thresholds of over 2 deg measured in a previous study when the muscles about the joint were relaxed. Thresholds decreased further with increasing velocity of movement. 3. No difference was found between the two levels of contraction of the elbow flexors. However, extension (stretch of the contracting muscle) and flexion thresholds were calculated separately, and smaller extensions than flexions could be detected. 4. These findings indicate that conscious detection of imposed movements is greatly enhanced during active muscle contraction. Movements which cause unloading of the contracting agonist, as well as movements which result in stretch, are more easily detected than when the muscle is contracting. The discussion focuses on possible mechanisms for this enhancement.

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