Figure 6. Single motor unit twitch and unfused tetanic contractions.

An example of a single fast motor unit (contraction time, 31 ms), sampled from a normal muscle (A) showing a typical ‘sag’ response during an unfused tetanic contraction (B). An example of a single fast contracting motor unit (contraction time, 28 ms), sampled from a reinnervated muscle after N-M suture (C) showing an atypical ‘no sag’ response (D). These atypical fast contracting motor units cannot be classified as fast or slow on the basis of ‘sag’ of unfused tetanic contraction and speed of contraction. They are therefore considered ‘unclassifiable’.