[F-waves].

F-waves result from the discharge of the motoneurons following their antidromic activation. The F-wave appears, as an indirect (the F-wave latency decreases when the stimulation site moves away from the muscular detection) and late response (occurring after the M response). In practice, the most useful parameter is the F-wave minimal latency, provided that at least seven distinct F-waves are evoked. When the analysis is relative either to the controlateral side, or to a former examination, this parameter is one of most sensitive in electroneuromyography. F-wave evocation implies conduction along the entire peripheral nervous system, and particularly its proximal part, which is not investigated by nervous trunks conduction velocity studies. Thus, F wave study is the most useful in plexopathies and polyradiculonevritis. In the early phase of Guillain-Barré syndrome, their absence may be the unique sign indicative of proximal conduction blocks.