Chapter 9 Late responses (F- and A-waves): methodology and techniques

Abstract

This chapter describes late muscle responses— that is, the F- and A-waves that occur over a variable period of time after the direct or the orthodromic M-response when a mixed peripheral nerve is stimulated. They include F-waves and the H-reflex at comparable delays after the M-response. Waveforms with latencies intermediate among the M-response and the F-wave and H-response include motor axon reflexes, indirect double discharges, peripheral late waves, myoaxonal ephaptic responses, and motor axon loop responses. The term “A-wave” has been used to denote just motor axon reflexes, just indirect double discharges or peripheral late waves, all of them and, more recently, also constant and stable waveforms within the F-wave or with more prolonged latencies than the F-waves. F-waves are considered late recurrent muscle responses generated by groups of motor neurons stimulated antidromically, originally with supra-maximal stimuli, though they can also be obtained with submaximal stimuli. Considerable progress has been made in refining the methodology and techniques and in defining the clinical applications of F-waves.