F waves

The F wave is visible as a low amplitude late potential following the M response. External stimulation depolarizes peripheral nerves, triggering electrical potentials that propagate both proximally and distally. With motor nerves, electrical activity moving distally results in a direct motor response, the compound muscle action potential, also known as the CMAP or the M response. Electrical activity propagating proximally activates a small percentage of motor neurons (they "backfire"), which then generate the F wave. The F wave latency varies depending upon the conductivity of the subpopulation of motor neurons activated.