Synchronization of human leg motor units during co-contraction in man.

Cross-correlograms were constructed from the spike trains of pairs of individual motor units recorded from the tibialis anterior (TA) and soleus (Sol) muscles. In most correlograms the synchronization consisted of a central peak with a short duration. When the motor units were recorded from the same muscle this synchronization was seen in almost 100% of the cases (TA 92%, Sol 89%) and the peak of synchronization was large (TA 0.060 extra sample spikes per trigger spike, Sol 0.030). In contrast, clear peaks of short duration synchronization was seen in only 4 out of 30 antagonistic TA and Sol motor unit pairs (0.025 extra sample spikes per trigger spike). In another 4 motor unit pairs central troughs were seen. In approximately 10% of all combinations of motor unit pairs a low-amplitude long-duration synchronization was seen. This type of synchronization was seen either as the only type of synchronization or at the same time as the short-duration synchronization. Finally, secondary peaks on both sides of the central peak were seen in a few cases. The distance between the sites of recording of TA motor unit pairs was systematically varied. The latency of the short-duration synchronization became longer and the amount of synchronization smaller the longer the distance between the recording sites. The amount of synchronization of two motor units recorded from the same muscle was shown to be larger when the muscle was activated in co-contraction rather than in an isolated agonist movement. The results are discussed in relation to the organization of the descending command eliciting co-contraction.