Decomposition of the compound muscle action potential into source and weighting functions

The compound muscle action potential (CMAP) can be modeled as the convolution of a source function related to the intracellular action potential (IAP) and a weighting function related to the muscle anatomy and the conduction velocity. The authors show that since the muscle IAP consists of a narrow spike and a long negative after potential, it can be accurately characterized by three scalar parameters: the monopole, dipole, and quadrupole moments of the spike. The authors present a method for estimating the spike moments and the weighting function from the CMAP waveform. This makes it possible to dissociate effects related to the weighting function from effects related to the source function. To illustrate, the authors show that the method can be used to estimate the velocity recovery function associated with double-pulse stimulation in spite of concomitant changes in the negative afterpotential. Results from seven normal subjects agree with results from the literature.