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A model for the motor-unit action-potential train is developed, based on previously obtained empirical information. The auto and cross-correlation functions are calculated. The autocorrelation function is used to derive the mean rectified value, the variance and the root-mean-squared value of a motor-unit action-potential train. These parameters are solved by using two approximations for the motor-unit action-potential; a piece-wise approximation and a Dirac Delta function approximation. The Dirac Delta function approximation sufficiently simplifies the mathematics so that the model can be extended to myoelectric signals. The cross-correlation function contains information about the synchronization of motorunit action-potential trains that may be useful as an objective indicator of muscle fatigue.

The construction of a lightweight quadrifillar needle electrode offering four monopolar and six bipolar microelectrode combinations is described. The electrode has three useful features: 1) it can record distinct individual motor unit action potentials during strong muscle contractions, 2) the frequency response curve of the electrode can be altered to a simple functional relationship within the frequency range of myoelectric signals, and 3) the electrode minimizes the subject's discomfort.