The single fibre electromyography (SFEMG) electrode has similar external proportions to a CN EMG electrode, but instead of having the recording surface at the tip, it is on the side above the tip and its recording surface is much smaller. Because of the arrangement of muscle fibres in a normal motor unit, a SFEMG needle will record only 1–3 single muscle fibres from the same motor unit. The SFEMG parameter that reflects motor unit morphology is fibre density (FD), which is the mean number of muscle fibres belonging to an individual motor unit per detection site. To measure FD, recordings from 20 different detection sites within the examined muscle are necessary and the number of component potentials to each motor unit recorded and averaged. The normal fibre density for the anal sphincter is less than 2.0.
Due to its technical characteristics a SFEMG electrode is able to record changes that occur in motor units due to reinnervation, but is less suitable to detect changes due to denervation itself (i.e. abnormal insertion and spontaneous activity). The SFEMG electrode is also suitable for recording instability of motor unit potentials, the ‘jitter’. This parameter has not been much used in PFM. SFEMG has been used in research but is not widely used, even for diagnostics in general clinical neurophysiological laboratories. The recording needles are very expensive, and disposable needles are not available.