F wave - Definition and Overview

In neuroscience, an F wave is the second of two voltage changes observed after electrical stimulation is applied to the skin surface above the distal region of a nerve. F waves are often used to measure nerve conduction velocity, and are particularly useful for evaluating conduction problems in the proximal region of nerves (i.e., portions of nerves near the spinal cord).

Overview

In a typical F wave study, a strong electrical stimulus is applied to the skin surface above the distal portion of a nerve so that the impulse travels both distally (towards the muscle fiber) and proximally (back to the motor neurons of the spinal cord). (These directions are also known as orthodromic and antidromic, respectively.) When the orthodromic stimulus reaches the muscle fiber, it elicits a strong M wave indicative of muscle contraction. When the antidromic stimulus reaches the motor neuron cell bodies, the impulse is reflected and travels back down the nerve towards the muscle. This reflected stimulus evokes the second, weaker F wave when it reaches the muscle.

Properties

F wave properties include:

• amplitude (μv) - F wave height
• duration (ms) - length of F wave
• latency (ms) - period between F wave and initial stimulation

Related topics

• H reflex
• Electromyograph (EMG)

References

• Weber, G.A. Nerve conduction studies and their clinical applications. Clin. Podiatr. Med. Surg. 1990;7(1):151-178.