Task Dependent Synchronization of Motor Units of the Medial Gastrocnemius Muscle Revealed in EMG-Currents

Abstract

During sport activities the gastrocnemius muscle must be activated in an orderly way to fulfill the tasks of force development, control of stability. The fascicles do not span the whole muscle and there are multiple motor units that have to be coordinated. Failure to do so results in excessive or uncoordinated forces in the joints. Thus it is important to be able to observe how synchronization of motor units changes under load. The present work is a first introduction to the possibilities of measuring the correlation of raw EMG-currents of four adjacent surface electrodes attached to the proximal region of the medial gastrocnemius muscle. In summary, the results showed and confirmed the hypotheses that the EMG-signals were correlated, that correlation was present when standing on the forefoot (tiptoe position) but increased during a repetitive calf-rising movement. The EMG-signals of adjacent electrodes could be decomposed into a reverse- and an in-phase component. The median frequencies decreased with increasing synchronization and were larger in the reverse-phase component than in the in-phase component. The results can be explained by a task dependent synchronization of motor units. Thus the method offers the possibility to study the effect of synchronization at higher levels of muscle activity.