A model of the surface-recorded motor-unit action potential

A model has been developed of the surface-recorded motor-unit action potential (MUAP) in terms of the gross anatomical properties of the motor unit. The MUAP is modeled as arising from six current sources (two tripoles). To account for the distribution of fibers and endplates, each current source is diffused throughout a volume defined by the motor unit´s cross section and the length of the endplate zone. To account for the MUAP´s initiation and termination, the space and time domains are treated separately: each source remains stationary at the endplate until its departure time, then moves along the muscle axis until it reaches the tendon, and then remains stationary there. The results of the model agree qualitatively with actual surface MUAPs, and quantitatively with results from a more complex model in which each MVAP is modeled individually. The results suggest that surface-recorded MUAPs can be used to estimate anatomical motor-unit properties such as size and depth.<>