Changes in time and frequency related aspects of motor unit action potentials during fatigue

During fatigue the shape of motor unit action potentials (MUAPs) change. Characteristics of the MUAPs described before concern several time related aspects. No attention has been given to the frequency spectrum changes of MUAPS. The median frequency of MUAPS has now been determined for motor units in the extensor digitorum longus muscle of the rat. Some examples of the changes are shown during a fatiguing stimulation pattern (250 s of 40 Hz stimulation during one third of each second). In general the median frequency tends to change more quickly than the time characteristics of the MUAPs. The results do not allow to draw yet conclusions about the time and frequency characteristics during fatigue for mechanically identified types of motor unit