Influence of tonic neck reflexes on the upper limb stretch reflex in man.

This study was designed to test the influence of tonic neck reflexes (TNR) on the elbow extensor muscles. Surface electromyography (EMG) of triceps brachii (TB), anconeus (ANC) and biceps brachii (BB) muscles were recorded simultaneously with elbow acceleration. Stretch reflex responses from the elbow extensors were analysed in seven healthy subjects for three head positions, two positions of the eyes and two initial EMG levels. At the opposite of the short latency stretch component (M1), the occurrence of the long-latency stretch responses (M2-3) did not depend on the EMG level. The head position influenced the EMG activity only for the lowest initial EMG level: a rotated head position towards the stretched extensor muscles increased the tonic EMG level of these muscles and facilitated the M2-3 component. This influence was modulated by the position of the eyes and occurred in both anconeus and triceps brachii recordings. These results support the hypothesis that a low EMG activity is needed to express TNR. Changes in M2-3 with head position could result from the interaction of messages from limb afferents and neck afferents at a supraspinal level.