Influence of the surface EMG electrode on the compound muscle action potential.

The recording characteristics of surface EMG electrodes were investigated. Compound muscle action potential (CMAP) and surface recorded motor unit action potentials were recorded from different muscles, using different surface electrode shapes and sizes. The CMAP was smaller for larger surface electrodes. This was more pronounced in smaller muscles. The CMAP was minimally affected by the geometry of the recording surface. With larger surface electrodes, shunting contributes to the reduction in MUAP amplitude. This is offset by a larger uptake area which gives a much smaller reduction in the CMAP amplitude for the larger muscles.