Non-stationary analysis of electromyographic activity prior to a ballistic voluntary contraction

The electromyogram (EMG) in a ballistic voluntary action of muscle after a slight sustained contraction shows a short period of decrease or disappearance of the activity prior to the onset of phasic discharge. The EMG of elbow extensor (triceps brachii) prior to phasic discharge was analyzed as a non-stationary stochastic process. The distribution of the EMG amplitude began to change from 100 ms before the phasic discharge. The statistical test showed that the EMG activity became non-Gaussian from 30 to 50 ms before the phasic discharge. The standard deviation of the EMG showed a decreasing tendency from 80 to 100 ms prior to the phasic discharge. The statistical evidence for the EMG depression was also confirmed in fourth and sixth order coefficients of the orthogonal expansion of the probability density function. These results suggested that the EMG process profoundly changed its statistical characteristics just before the phasic discharge.