THE INFLUENCE OF TONIC NECK REFLEXES ON VOLUNTARY FATIGUING ELBOW MOVEMENTS IN HUMANS

Tonic neck reflexes (TNR) are often assumed to be included in the buil ding of voluntary motor programmes. The present study was designed to test the importance of TNR in the performance by healthy human adults of high level dynamic exercise. The subjects were placed in an experim ental situation similar to the original one used by previous authors, but in a fixed restrained posture. They repeatedly lifted weights by e xtending the elbow, with the head rotated either towards the side of t he active upper limb or towards the opposite side. The rate of movemen t was 0.5 Hz, and the initial amplitude 60-degrees. Successive series of 15 movements separated by an equivalent period of rest were made by each subject until exhaustion. The mechanical work performed during e ach series of movements was calculated. Surface electromyograms (EMG) from triceps brachii, anconeus, and biceps brachii muscles were record ed simultaneously with the elbow rotation displacement and then integr ated. The EMG from the sterno-cleido-mastoidei muscles were also recor ded. With successive series of lifting, movement amplitude decreased p rogressively as a result of fatigue. Thus there was a decrease in the work performed. In agreement with the experimental data of the previou s authors as well as with the expected effect of TNR, the decrease in work was less marked when the subjects kept their heads turned towards their moving limb. However, in contrast with the results from other a uthors, the facilitation was very low. From these results, we conclude d that TNR can slightly influence the amount of work that a subject ca n produce. Nonetheless direct facilitation exerted by TNR on the prime mover muscles is lower than the one exerted through facilitating post ures.