Synchronization of human motor units: possible roles of exercise and supraspinal reflexes

Some normal human subjects show definite synchronization of the motor units in hand muscles (i.e., the impulses from two or more motor units coincide in time more frequently than expected for independent random processes). Subjects who show synchronization tend to use their hands to exert large, brief forces, either in their work (e.g., manual laborers) or recreational activities (e.g., weightlifters). In this study all seven weightlifters examined showed a significant degree of synchronization. Furthermore, after 6 weeks of using the first dorsal interosseus muscle of the hand to exert maximal, voluntary contractions, the level of synchronization increased substantially in four control subjects, and the average level became significantly different from zero. Weightlifters also showed greater late reflex responses than control subjects, but no significant difference in earlier spinal reflexes. Two late reflex waves are described which probably involve fast pathways to and from motor cortes. We suggest that supraspinal connections from motor cortex directly to spinal motoneurons may be enhanced as a result of training to the point where they produce a significant synchronization of motor units during steady, voluntary contractions.