It became recently known that there are two types of characteristic spikes in EMG at voluntary contruction of skeletal muscle; the one is of low voltage and long duration, and the other, of high voltage and short duration. In this study a detailed observation was done for analysis of these spikes. 1) They would not be changed in duration by alternation of the recording electrode position. 2) In the histograms of the voltage and duration of the spikes recorded separately from flexor digitorum profudus muscle and soleus muscle, it was revealed that the one from the first muscle was of 50-550 μV in voltage range with two peaks at 100-200 μV and 300-450 μV and of 0.5-5.5 ms in duration range with also two peaks at 1.0-1.5 ms and 2.0-3.0 ms. The one from the latter muscle was ranged in similar manner as in the first, but the peak was found solely at 100-150 μV in voltage and 2.5-3.5 ms in duration. 3) In another figure drawn both voltage and duration, it is interesting to know that the first group was subgrouped into two; the one is of short duration and high voltage and the other is of long duration and low voltage. In contrast to it, the latter was of low voltage and long duration. 4) In the r-s curve based on average discharge interval and standard deviation of its irregular variation of these spikes, the one of high voltage and short duration manifested a sharp increment and the other of low voltage and long duration, a slow increment. 5) Two types of such characteristic diphasic spikes were also obtained in M wave of provocation EMG. The latency appeared to be short in the spike of high voltage and short duration compared with the other. 6) The reflactory periods, both in absolute and relative, were also found to be short in the spike of high voltage and short duration incomparison with the other. 7) It is to be noted that the intermediate spike between these two types was found, although relatively in few incidence, both at voluntary contruction and in M wave at provocation. It seem to be concluded that the spike of high voltage and short duration corresponds with the one of kinetic NMU and the other, with the one of tonic NMU.