The F-wave is an action potential evoked intermittently from a muscle by a supramaximal electrical stimulus to the nerve due to an antidromic activation of motor neurons. When compared with the maximal amplitude of the compound muscle action potential (CMAP) also called “M wave“, it is smaller (1–5 % of the M wave) and has a variable configuration. Its latency is longer and more variable than the M wave. Since 1970s, some authors studied F-wave several parameters in carpal tunnel syndrome (CTS) population, and the values of ulnar and median F-wave minimal latencies (FWML) were more recently compared in the diagnosis of CTS because in healthy subjects the median FWML occurs earlier than the ulnar FWML, a pattern that should reverse in patients with CTS due to the decrease in the median nerve conduction velocity of the digit-wrist segment. This abnormal pattern is called “F-wave inversion“.