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1. F waves are a late response received from a round trip from the stimulus site, to the spinal motor neuron & back to the muscle. Normal: 32ms in the UL and 55ms in the LL.

2. F waves are heterogeneous, derived from different populations of motor neurons for each response.

3. Because of this variability, one must sample multiple F waves (typically 10 to 20).

4. Parameters measured: minimal latency (the shortest latency of multiple responses), mean latency (the average latency), maximal latency (the longest latency of multiple responses), chrono-dispersion (difference between the shortest and longest latencies), persistence/penetrance is measured as the percentage of stimulations that produce an F wave.

5. Absence of F waves is considered abnormal.

6. Rate of false-positive results is cumulative, hence only minimal/mean latency & absence of F-wave is useful.


1. Wilkinson, I. (2017). Oxford handbook of clinical medicine. Oxford: Oxford University Press.
2. Hannaman, R. A., Bullock, L., Hatchell, C. A., & Yoffe, M. (2016). Internal medicine review core curriculum, 2017-2018. CO Springs, CO: MedStudy.
3. Image: no reference available.

Ⓒ A. Manickam 2018

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