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 (BE) muscles w ere recorded simultaneously with elbow acceleration. Stretch reflex re sponses from the elbow extensors were analysed in seven healthy subjec ts for three head positions, two positions of the eyes and two initial EMG levels. At the opposite of the short latency stretch component (M 1), the occurrence of the long-latency stretch responses (M2-3) did no t depend on the EMG level. The head position influenced the EMG activi ty only for the lowest initial EMG level: a rotated head position towa rds the stretched extensor muscles increased the tonic EMG level of th ese muscles and facilitated the M2-3 component. This influence was mod ulated by the position of the eyes and occurred in both anconeus and t riceps 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 affer ents and neck afferents at a supraspinal level. Copyright (C) 1996 Els evier Science Ltd.