Do F‐wave measurements detect changes in motor neuron excitability?

The use of F waves to assess motor neuron excitability in experimental paradigms has never been validated. Our objective was to determine whether F-wave area, amplitude, and persistence measurements change in response to manipulations known to alter the excitability of motor neurons. The effects of muscle vibration, contraction of a remote muscle, and high-intensity stimulation of ipsilateral or contralateral fingers were assessed in 12 healthy volunteers. F-wave area, amplitude, and persistence all declined with ipsilateral cutaneous stimulation. The other maneuvers facilitated some, but not all, of the F-wave measurements. Changes in F-wave area and amplitude were correlated, but neither correlated with changes in persistence. A sample size of 50-75 F waves was needed to approximate amplitude and area results from 100 F waves with an accuracy of +/- 25%. We conclude that changes in F waves are better at detecting inhibition than facilitation of motor neurons. F waves reflect motor neuron excitability in a general way but do not allow for accurate measures of short-term changes in excitability.