Dependence of autogenic and heterogenic stretch reflexes on pre-load activity in the human arm.

1. Subjects held their right arm in a horizontal plane. The angle of the elbow was 90 deg. They exerted forces in several directions in the plane of the arm, varying independently the pre-load torques about shoulder and elbow. We measured electromyographic (EMG) activity in several arm muscles in response to force perturbations which extended the shoulder, without changing the elbow angle. 2. The EMG activity in flexors of both shoulder and elbow showed reflex responses at short latency (approximately 25 ms). In all muscles the reflex activity increased with the pre-load activity of that muscle. 3. The short-latency reflex activity of m. brachialis, which was not stretched by the perturbations, was independent of the pre-load activity of the muscles acting over the shoulder. 4. From these results we conclude that the force resulting from the short-latency reflex, assessed from the EMGs, does not counteract the perturbations exactly. Having found that the short-latency reflex is dependent on the pre-load direction, we argue that this dependence makes the short-latency reflex suitable for correcting fast movements for misjudgements of load. 5. At longer latencies (greater than 50 ms) the direction of the force resulting from the reflex, assessed from the EMGs, was almost independent of the direction of the pre-load. In our experiment the force resulting from the long-latency reflex counteracted the perturbations quite well.