![]() ![]() F waves therefore are recorded routinely only from muscles of the hand, foot, and leg, with standard stimulation sites at the wrist, ankle, and knee respectively. Recording from proximal muscles, however, is difficult because the low-amplitude F waves are superimposed on the associated M wave. This is sometimes helpful clinically, but the contraction will alter F-wave parameters such as amplitude and will increase the possibility of contamination by H reflexes.į waves are ubiquitous in distribution. Slight voluntary contraction will enhance F waves. The active electrode is placed as for H reflexes.į waves are recorded routinely with the muscle relaxed. For calf F responses, muscle belly recordings often are preferred because the effects of extraneous muscle activity are decreased when recording the low-amplitude F waves. Muscle belly–tendon recordings are standard for hand and foot muscles, with the recording cathode placed over the motor point. When there is low persistence, more stimuli than usual may be needed to obtain adequate data. Accurate determination of the number of individual waves that repeat may require more than 90 F waves. Even two F waves may define an abnormal chronodispersion if the two latencies are dispersed sufficiently, but determination of a truly representative value requires 45 to 55 F waves (50 to 60 stimuli). F waves usually are recorded from antigravity muscles with high persistences, so that the use of 20 stimuli is recommended for routine purposes. The same number of F waves is adequate for measuring F/M ratios and the percentage of responses in a series of F waves that are the same (i.e., repeater waves). ![]() Accurate F-wave latency measurement requires 16 to 20 F waves and is expressed best as a mean rather than a minimal value. 54, 59, 60 Data from 8 to 10 identifiable, sequential F waves can provide a reasonable estimate of persistence. To be clearly identifiable, F waves should be at least 20 μV in peak-to-peak amplitude.Īs discussed earlier, the number of F waves required to permit measurement of different F-wave parameters has been examined. As such, different recording parameters usually are required to evaluate fully the associated larger M wave. This therefore should be used until it is established that submaximal stimulation can produce meaningful, reproducible information for all F-wave parameters, and the number of stimuli required for such values has been determined.Īdequate display of F waves usually requires an amplifier gain of 200 or 500 μV per division and a sweep of 5 or 10 msec per division. To date, all normative F-wave data are based on supramaximal stimulation. Submaximal stimulation does not affect F-wave latency or duration, but does affect all other F-wave parameters and would require more stimuli for accurate data. There has been interest in eliciting F waves using low-intensity stimulation since this could lessen patient discomfort. This has the advantage of increased patient comfort and may provide adequate information in situations in which the only question is whether F-wave latencies are markedly prolonged. F waves are present at submaximal stimulation. This provides a consistent physiologic environment for eliciting F waves. The conventional stimulus intensity is 25 percent above maximal for eliciting a direct response. High stimulus intensities also block axon reflexes that may be confused with F waves, as well as limiting contamination by H reflexes. In contrast to H reflexes, F waves are enhanced by stimulation at high intensity because of increased amplitude and persistence. The author stimulates at rates less than 0.5 Hz to avoid the effects of an earlier stimulus on a subsequent response. Unlike for H reflexes, a long stimulus duration is not required because there is no reason to activate preferentially the large afferent fibers. ![]() Although the importance of anodal block has been questioned, 70 the standard methodology is for the stimulating cathode to be proximal to the anode. Fisher, in Aminoff's Electrodiagnosis in Clinical Neurology (Sixth Edition), 2012 Technique of Recording F Wavesį waves are recorded in a manner similar to that used for direct motor responses. ![]()
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