The influence of tonic neck reflexes on voluntary fatiguing elbow movements in humans

Summary

Tonic neck reflexes (TNR) are often assumed to be included in the building 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 experimental situation similar to the original one used by previous authors, but in a fixed restrained posture. They repeatedly lifted weights by extending the elbow, with the head rotated either towards the side of the active upper limb or towards the opposite side. The rate of movement was 0.5 Hz, and the initial amplitude 60°. Successive series of 15 movements separated by an equivalent period of rest were made by each subject until exhaustion. The mechanical work performed during each series of movements was calculated. Surface electromyograms (EMG) from triceps brachii, anconeus, and biceps brachii muscles were recorded simultaneously with the elbow rotation displacement and then integrated. The EMG from the sterno-cleïdo-mastoidei muscles were also recorded. With successive series of lifting, movement amplitude decreased progressively as a result of fatigue. Thus there was a decrease in the work performed. In agreement with the experimental data of the previous 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 authors, the facilitation was very low. From these results, we concluded that TNR can slightly influence the amount of work that a subject can produce. Nonetheless direct facilitation exerted by TNR on the prime mover muscles is lower than the one exerted through facilitating postures.