F-waves are evoked by strong electrical stimuli (supramaximal) applied to the skin surface above the distal portion of a nerve.
[2][3][4][5] This antidromic 'backfiring' elicits an orthodromic impulse that follows back down the alpha motor neuron, towards innervated muscle fibers.
[15] However, these same axonal segments remains excitable or relatively depolarized for a sufficient period of time, allowing for rapid antidromic backfiring, and thus the continuation of the orthodromic impulse towards innervated muscle fibers.
The shape and size of F-waves, along with the probability of their presence is small, as a high degree of variability exists in motor unit (MU) activation for any given stimulation.
[4] Thus, the generation of CMAP's which elicit F-waves is subject to the variability in activation of motor units in a given pool over successive stimuli.