Influence of motoneuron firing synchronization on SEMG characteristics in dependence of electrode position

The frequency content of the surface electromyography (SEMG) signal, expres sed as median frequency (MF), is often assumed to reflect the decline of mu scle fiber conduction velocity in fatigue. MF also decreases when motor uni t firings synchronize, and we hypothesized that this effect can explain the electrode-dependent pattern in our previous recordings from the trapezius muscle. An existing motoneuron (MN) model describes the afterhyperpolarizat ion following a spike as an exponential function on which membrane noise is superimposed. Splitting the noise into a common and an individual componen t extended the model to a MN pool with a tunable level of firing synchrony. An analytical volume conduction model was used to generate motor unit acti on potentials to simulate SEMG. A realistic level of synchrony decreased th e MF of the simulated bipolar SEMG by similar to 30% midway between endplat e position and tendon but not above the endplate. This is in accordance wit h experimental data from the biceps brachii muscle. It was concluded that t he pattern of decrease of MF during sustained contractions indeed reflects MN synchronization.