TIME-COURSE OF CHANGES IN EMG ACTIVITY OF FAST MUSCLES AFTER PARTIAL DENERVATION

After partial denervation, the remaining motor units (MUs) of adult fa st extensor digitorum longus muscle (EDL) expand their peripheral fiel d. The time course of this event was studied using tension measurement and recordings of electromyographic (EMG) activity. The results show that after section of the L4 spinal nerve, when only 5.3 +/- 0.63 of t he 40 MUs normally supplying EDL muscle remain, the force of individua l motor units starts to increase between the 1st and 2nd week after th e operation and continues to do so for a further week. The drastic red uction of the number of motoneurones supplying the fast EDL leads to a n increase in activity of the remaining MUs. In the 1st week after par tial denervation, there was a sharp increase in the EMG activity of re maining motor units. During the next 12 days, this increase became les s marked, but EMG activity remained nevertheless significantly higher than that of the unoperated EDL muscle. Many MUs became tonically acti ve during posture. The EMG activity pattern during locomotion was also altered, so that the burst duration was positively correlated with th e step cycle duration. Moreover, shortly after partial denervation, th e interlimb coordination was disturbed but returned to its original sy mmetrical use 1-2 weeks later.