What the study looks at
A routine motor nerve conduction recording measures the direct response, sometimes called the M wave, between the stimulus and a nearby muscle. The F-wave appears later. The stimulus travels up the motor nerve to the spinal cord. A few of the motor nerve cells there send a signal back down the same nerve, and the muscle twitches again.
Only a small share of those nerve cells respond on any one stimulus, so the technologist gives a series of stimuli and looks at the responses that return. The measurement used most often is the shortest F-wave latency. A prolonged latency, or no F-wave when the direct motor response is present, draws attention to conduction along the proximal nerve as well as the distal nerve.
F-waves are not the same as H-reflexes. An H-reflex is a true reflex arc and is a separate recording. This page is only about F-waves.
How the visit is performed
F-waves are recorded with the same surface electrodes used for a motor nerve conduction study. The patient feels the same brief stimulus, repeated so several late responses can be captured. They are usually added to a nerve conduction visit rather than scheduled as a separate trip.
Atlas brings the equipment and a technician to the physician’s office. If you want F-waves included, say so when you request the study and name the nerves you have in mind.
Why a practice orders it
Physicians add F-waves when the question involves the proximal part of a motor nerve. That includes some radiculopathies and acute neuropathies in which the distal nerve can still look relatively preserved. They are one part of the study, not a complete answer.
A normal F-wave does not rule out a focal root or nerve problem. An abnormal F-wave is read with the rest of the nerve conduction study, the EMG if one was done, and the exam.