The dominating role of finite muscle and limb dimensions in surface electromyography

Theoretical volume conductor studies of the neuromuscular system often start by assuming an infinite length muscle or nerve fiber placed in an unbounded volume conductor. Although this approach may apply well to the potential field close to a long nerve fiber, it is expected to be no more than a rough approximation when studying surface EMG. Four different descriptions of an active muscle fiber in a volume conductor are compared in this study to illustrate the separate effects. It is shown that the extinction of a single muscle fiber action potential (SFAP) at the muscle tendon leads to a dominating positive peak in the SFAP at larger observation distances. Another interesting observation is that the amplitude of this peak is considerably enhanced after replacing an infinite volume conductor with a finite description of the volume conductor properties of the limb. The results were experimentally verified by isolation of motor unit activity from the surface EMG