Mean F-wave Research Articles

Utility of minimum F‐wave latencies compared with F‐estimates and absolute reference values in S1 radiculopathies: Are they still needed?

The utility of F-waves in assessing radiculopathies is debated. The aim of this study is to determine the frequency of abnormal minimum tibial F-wave latencies compared to an F-estimate and an absolute reference value in patients with electromyography (EMG) confirmed S1 radiculopathies. A retrospective review of F-waves in patients with an EMG-confirmed isolated S1 radiculopathy was performed. The minimum and mean latencies of 8 tibial F-waves were compared with the calculated F-estimate and to an absolute reference value, and the frequencies of abnormal responses were determined. Of the 50 patients with an S1 radiculopathy, 4% had prolongation of the minimum reproducible F-wave latency, and 8% had prolongation of the mean latency relative to the calculated F-estimate. The minimum and mean F-wave latencies are infrequently abnormal when compared with an estimated F-wave latency in S1 radiculopathies and are insensitive in the assessment of S1 nerve root injury.

P 31. Effects of transcutaneous spinal Direct Current Stimulation (tsDCS) on the conduction along the human corticospinal system

Objective There are to date few studies investigating the effects of spinal direct current stimulation (tsDCS) on spinal ascending tract and reflexes. Whether tsDCS interferes with conduction along pyramidal tract at a spinal level is unknown. Methods Eleven healthy subjects underwent anodal and cathodal tsDCS of the thoracic spinal cord, at the T10–T12 level (2.5 mA, 20 min). The two conditions were tested in random order and at least 1 week elapsed between sessions. The subjects were blinded about the polarity of conditioning stimulation. MEPs elicited by transcranial magnetic stimulation (TMS) were recorded from abductor hallucis (AH) and abductor digiti minimi (ADM) before tsDCS and after the current offset. We also evaluated AH and ADM F-waves and soleus H-reflex. Results Baseline neurophysiological variables (MEPs threshold, area and latency; H-reflex threshold, latency, amplitude and Hmax/Mmax ratio; F-wave mean latency, minimal latency, amplitude and chronodispersion) from upper and lower limb were comparable in anodal and cathodal sessions. After tsDCS, whereas cathodal tsDCS left MEP threshold unchanged ( p > 0.1), anodal tsDCS increased significantly MEP threshold (two-way ANOVA as “time” and “stimulation” as factors: F (2,20) = 5.01, p = 0.017). Similarly, lower limb MEPs area significantly decreased after anodal tsDCS, increasing however after cathodal stimulation (two-way ANOVA as “time” and “stimulation” as factors: F (2,20) = 10.2, p = 0.0009). Both anodal and cathodal tsDCS failed to affect lower limb MEP latency, upper limb MEPs threshold, area and latency, H-reflex and F-wave. Discussion tsDCS modulates the function of human corticospinal pathways. The effects occur selectively in muscles controlled by corticospinal axons running below the tsDCS electrode over the lower thoracic spinal cord without concomitant effect on both upper limb muscles and motorneuronal excitability. Overall these findings suggest a direct effect of tsDCS on the corticospinal system below the stimulating electrode: among the possible explanations, anodal tsDCS could block impulse conduction along corticospinal axons. A further possibility is that changes of corticospinal function in the brain after tsDCS could be indirectly elicited by its effect on tonic resting activity of ascending spinal pathways-an important input to motor areas–ultimately influencing the excitability of corticospinal neurons at cortical level. Conclusions tsDCS over the dorsal spinal cord can modulate the corticospinal input to sacral motoneurons. The possibility of modulating corticospinal input to motorneurons has potential clinical implications.

Effect of low-frequency rTMS on motor neuron excitability after stroke

The purpose of this study was to test the effects of low-frequency repetitive transcranial magnetic stimulation (rTMS) over the non-lesional hemisphere on motor neuron excitability of the paretic upper limb in post-stroke patients by electrophysiological examination. Thirteen post-stroke patients with spastic upper limb hemiparesis were studied (age, 57.5 ± 11.1 years; time after stroke, 55.2 ± 51.4 months). Low-frequency rTMS of 1 Hz was applied for 20 min to the motor cortex of the non-lesional hemisphere. The M-response amplitude and F-wave parameters were recorded in the abductor pollicis brevis muscle following stimulation of the median nerve in both the affected and unaffected upper limbs. The F-wave frequency, F-max/M ratio (ratio of maximum F-wave amplitude to M-response amplitude), and F-mean/M ratio (the ratio of mean F-wave amplitude to the M-response amplitude) were measured before and after the 20-min rTMS, analyzed for both limbs. Application of low-frequency rTMS did not result in significant changes in the frequency of F-wave and F-max/M ratio in both upper limbs, but significantly decreased F-mean/M ratio in the affected upper limb (P < 0.005), but not in the unaffected limb. Low-frequency rTMS applied to the non-lesional hemisphere might be potentially useful therapeutically for post-stroke patients with spastic upper limb hemiparesis.

The effect of local corticosteroid injection on F-wave conduction velocity and sympathetic skin response in carpal tunnel syndrome.

The aim of this study was to evaluate the efficacy of steroid injection for the treatment of the carpal tunnel syndrome (CTS), with F-wave parameters and sympathetic skin response (SSR). Seventeen hands of 10 women patients were treated with local steroid injection with 2-month follow-up. All patients underwent single injection into the carpal tunnel. Response to injection was measured nerve conduction studies (NCSs), median nerve F waves, and SSR before and after treatment. To determine the normal values, 42 hands of 21 healthy women were also studied. There was a significant improvement of sensory and motor nerve conduction values when compared to baseline values (P < 0.01). At the end of follow-up period, the median sensory distal latency and the sensory latency differences between the median and the ulnar nerve were improved 35 and 65%, respectively. The maximum, mean F-wave amplitudes and chronodispersion showed a slight improvement with respect to baseline values and controls, but statistical significance was not achieved after treatment. Although no statistically significant improvements were observed in SSR parameters, slightly decreased amplitudes and increased habituation of SSR were noted at the end of the treatment. The present study shows that the local steroid injection results in improvement in NCSs values, but the F-wave parameters were not effectual in short-term outcome of CTS treatment. These findings suggest that the sensory latency differences between the median and the ulnar wrist-to-digit 4 are better parameters in the median nerve recovery after treatment than the median sensory distal latency. Furthermore, the SSR does not seem to be a sensitive method in follow-up of CTS treatment.

Reproducibility in nerve conduction studies and F-wave analysis

Objective Reproducibility testing of nerve conduction studies and F-wave latencies in a group of healthy volunteers. Methods A total of 32 healthy volunteers underwent sensory and motor nerve conduction studies of the ulnar and tibial nerves, including F-waves, elicited by 32 stimuli, repeated in two different days by the same examiner. Results The F-wave latencies showed less variability than the other parameters of conduction studies, and, among them, the F-wave mean latency of the tibial nerve has been the one with the higher reproducibility. Conclusions F-wave latencies were the parameters with best reproducibility, followed by conduction velocities and amplitudes. Significance Our data showed that the F-wave mean latency is a parameter with a high reliability, considering the reproducibility, for sequential studies.

F-wave conduction velocity, persistence, and amplitude for the tibial nerve in clinically normal cats

To establish a method of F-wave evaluation and to determine normative values of F-wave parameters, including F-wave conduction velocity, persistence, and amplitude for the tibial nerve in cats. 30 clinically normal cats. F-waves elicited in the interosseous muscles by stimulation of the tibial nerve were recorded, and linear regression analyses of the shortest latency versus the length of the tibial nerve and the limb length were performed. F-wave persistence was calculated by dividing the number of recorded F-waves by the number of stimuli. The correlation coefficient between F-wave latency and nerve length was 0.92, and that between F-wave latency and limb length was 0.58. Mean +/- SD F-wave conduction velocity of the tibial nerve was calculated to be 97.1 +/- 5.0 m/s. Linear regression analysis yielded the regression equation as follows: F-wave latency (milliseconds) = 2.60 + (0.02 x nerve length [mm]). Mean F-wave persistence and amplitude were 98.7 +/- 2.3% and 1.01 +/- 0.62 mV, respectively. Results indicated that nerve length should be used for nerve conduction studies of F-waves in felids. The regression equation for F-wave latency, conduction velocity, persistence, and amplitude may contribute to the diagnosis of nervous system diseases or injury in cats, such as trauma to the spinal cord or diabetic neuropathy.

TREATMENT WITH STATINS AND INVOLVEMENT OF THE PERIPHERAL NERVOUS SYSTEM: RESULTS OF A PROSPECTIVE CLINICAL AND NEUROPHYSIOLOGICAL FOLLOW-UP

To study the pathological changes in neurophysiological examination of lower-limb peripheral nerves in patients with long-term statin treatment. Forty-two patients (23 males, 19 females, mean age 51.9 and 52.3 years) with a definitive diagnosis of combined hyperlipidemia were studied. Other metabolic disorders or chronic ethanol abuse were excluded. Initial examinations included laboratory and neurophysiological measures (peroneal and tibial nerves: MNCV, CMAP, F-wave mean latency; superficial peroneal and sural nerve: SNCV, SNAP). Subsequently, treatment with simvastatin 20mg daily was initiated. Patients were followed for 24 months with examinations at 1, 6, 12 and 24 months after statin treatment initiation. None of the patients reported subjective symptoms typical for polyneuropathy. In laboratory findings, there was no elevation of muscle enzymes. Nevertheless, electrophysiological examination of lower-limb peripheral nerves demonstrated statistically significant prolongation of F-wave mean latency on peroneal and tibial nerves (p < 0.0001, paired t-test). A control group of 50 patients with combined hyperlipidemia but no statin treatment showed no changes over the same time interval. The study demonstrated that long-term The study demonstrated that long-term treatment with statins might cause a clinically silent but still electrophysiologically definite damage to peripheral nerves.

Evaluation of Parameters of Serially Monitored F-wave in Acute Cervical Spinal Cord Injury

In this study, we aimed to determine how physicians can evaluate the severity of acute traumatic spinal cord injury (SCI) and predict the prognosis of this injury using the relationships of changes in clinical features and electrophysiological examination results. Serial recordings of F-waves were performed on 20 consecutive cervical SCI patients. In 12 of the patients, changes in several parameters of F-waves which were elicited by median and ulnar nerve stimulations were examined by analyzing their relationships to clinical symptoms. The maximum amplitude of the F-waves (F-max) elicited by median nerve stimulation was found to be the most reliable (statistically significant) parameter for distinguishing clinically improved patients from nonimproved patients for the prognosis in the early stages after trauma. Other parameters, including the incidence of F-waves and the mean F-wave amplitude both of which were elicited by median nerve stimulation, were somewhat helpful for predicting the prognosis. These parameters of F-waves evoked by ulnar nerve stimulation could be useful for several weeks post-trauma.

Repeatability of Nerve conduction Measurements using Automation

ABSTRACTObjectiveTo quantify nerve conduction study (NCS) reproducibility utilizing an automated NCS system (NC-stat®, NeuroMetrix, Inc.).MethodHealthy volunteers without neuropathic symptoms participated in the study. Their median, ulnar, peroneal, and tibial nerves were tested twice (7 days apart) by the same technician with an NC-stat® instrument. Pre-fabricated electrode arrays specific to each nerve were used. Both motor responses (compound motor action potential [CMAP] and F-waves –all nerves) and sensory responses (sensory nerve action potentials [SNAP] –median and ulnar nerves only) were recorded following supramaximal stimuli. Automated algorithms determined all NCS parameters: distal motor latency (DML), mean F-wave latency (FWL), distal sensory latency (DSL), CMAP amplitude, and SNAP amplitude. Latency was adjusted for skin temperature deviation from reference. Pearson correlation coefficient (CC), intraclass correlation coefficient (ICC), coefficient of variance (CoV), and relative intertrial variation (RIV) were calculated.ResultsFifteen subjects participated in either upper or lower extremity studies with nine participating in both. With the exception of CMAP amplitude, all parameters had CoV less than 0.06. Upper extremity amplitude parameters had CCs greater than 0.85. CCs for latencies were greater than 0.80 except for the median nerve FWL (CC = 0.69). For lower extremity nerves, ICCs were highest for mean FWL (>0.90), followed by DML (>0.82) and then CMAP (peroneal 0.33, tibial 0.73). The 10th to 90th RIV percentiles were bounded by ±7% for F-wave latencies; ±9% for all DSLs; and ±11% for DML (except peroneal at 15%).ConclusionsThe reproducibility of NCS parameters obtained with an automated NCS instrument compared favorably with traditional electromyography laboratories. F-wave latencies had the highest repeatability, followed by DML, DSL, SNAP and CMAP amplitude. Given their high reproducibility, automated NCS instrument may encourage wider utilization of NCS in clinical and research applications.

Propofol Impairs the Central but Not the Peripheral Part of the Motor System

Propofol provides some degree of muscle relaxation. Previous studies have investigated the effects of propofol on either the central or peripheral parts of the motor system. In this study, we simultaneously assessed both central (spinal) and peripheral effects. In 15 patients, general anesthesia was induced and maintained with fentanyl and midazolam. Neuromuscular blocking drugs were not administered. To investigate the central portion of the motor system, we monitored spinal F waves, an electrophysiologic variable of α-motoneuron excitability. Direct electrophysiologic muscle responses (M waves) and mechanomyography were studied to detect the peripheral effects of propofol on neuromuscular transmission or muscle contraction strength. After baseline recordings, 3 IV boluses of propofol (2 times 1 mg/kg followed by 2 mg/kg) were administered at 5-min intervals. Mean F-wave amplitudes were significantly reduced compared with baseline measurements (mean ± sd, 0.22 ± 0.13 mV) after the first (0.13 ± 0.08 mV;P < 0.05), second (0.08 ± 0.09 mV;P < 0.05), and third (0.03 ± 0.04 mV;P < 0.01) propofol injections. M-wave amplitudes and mechanomyography signals remained unchanged. Our data suggest that the central part, but not the peripheral part, of the motor system is impaired after bolus administration of propofol.

Detection of lumbosacral nerve root compression with a novel composite nerve conduction measurement.

Multivariate logistic regression techniques were used to develop a composite nerve conduction measurement that detects lumbosacral (L5, S1, or both) nerve root compression. To evaluate the diagnostic efficacy of a composite nerve conduction measurement for detection of lumbosacral nerve root compression. Nerve root involvement is characterized by clinical abnormalities and confirmed by radiologic and electrodiagnostic studies. Imaging studies visualize structural abnormalities; however, they are associated with high false-positive rates. Electrodiagnostic methods assess the physiologic integrity of the nerve roots. One form of electrodiagnostic testing, nerve conduction studies, is widely used for evaluation of musculoskeletal and neuromuscular complaints. Although similar clinical value is expected for the evaluation of nerve root compromise, prior applications of nerve conduction studies have yielded widely varying results. Two groups of subjects were compared. The L5-S1 compression group was composed of 25 patients with magnetic resonance imaging-confirmed lumbosacral (L5, S1, or both) nerve root compression and symptoms in the appropriate segmental distribution. The majority of subjects (22) had at least one of the following findings on physical examination: positive straight-leg raise test, diminished ankle reflexes, sensory loss, or weakness. The control group consisted of 35 asymptomatic individuals with no history of radiculopathy or potentially confounding neuropathology. The posterior tibial and deep peroneal nerves were evaluated bilaterally in all study subjects using standard nerve conduction procedures, which consisted of the measurement of distal motor latencies and F-wave latencies that assess nerve root pathophysiology. A composite nerve conduction measurement was determined using multivariate logistic regression analysis. The efficacy of the composite measurement was assessed by receiver operating characteristic curve analysis and by the diagnostic sensitivity and specificity. Five F-wave latency parameters (peroneal mean F-wave latency, odds ratio = 0.42; peroneal seventh F-wave latency decile, odds ratio = 2.71; tibial mean F-wave latency, odds ratio = 8.90; tibial first F-wave latency decile, odds ratio = 0.47; tibial maximum F-wave latency, odds ratio = 0.44) were found to be predictive of nerve root compression. A composite nerve conduction measurement, NC composite, constructed from these five parameters (NC composite = exp(phi)/(1 + exp(phi)), phi = -31.2 + 1.0 * Per7 Decile - 0.88 * PerMean + 2.2 * TibMean - 0.88 * Tib1 Decile - 0.83 * TibMax) yielded an area under the receiver operating characteristic curve of 0.91. At a threshold of 0.20, NC composite had a diagnostic specificity of 84.3% and a sensitivity of 83.3%. This preliminary study suggests that a novel composite nerve conduction measurement, based on F-wave latency parameters, may be highly effective at detecting magnetic resonance imaging-confirmed lumbosacral nerve root compression. Because these measurements provide objective evidence of functional nerve root compromise and are noninvasive, they may be of diagnostic value to clinicians evaluating patients presenting with low back and leg pain.

Analysis of F -Wave Parameters in Normal Infants and Children

F-wave is a late response evoked by supramaximal stimulation to the nerve and occurring after direct muscle response. F-wave parameters were investigated in 167 normal Iraqi children, who were all below the age of 12 years. F-waves which were recorded from the abductor digiti minimi by stimulation of the ulnar nerve at the wrist were evaluated to assess developmental changes in the proximal segment of the nerve. The mean F-wave minimal latency value remained constant in the various age groups during the first 6 months of life, then it was reduced in the next 6-<9 months age group. After that, it increased linearly in the successive age groups, while the mean F-wave maximal latency values remained without statistical changes in the age groups during the first year of life but increased with age afterwards. Two simultaneous processes: a rapid increase in conduction velocity due to maturation (increase in nerve fiber diameter and in myelination) and an increase in upper limb length due to growth were responsible for these latency changes. F-wave minimal latency for the ulnar nerve have lower mean values in Iraqi children than in Western children due to height differences. Differences in the pattern of F-minimal and F-maximal latency changes with age reflect different rates of maturation in the different nerve fibers. F-chronodispersion (the difference between the F-maximal and F-minimal latencies) was used as a sensitive method to assess the variability between the changes affecting the fastest and the slowest proximal nerve fibers contributing to F-wave formation. The mean F-chronodispersion values were found to increase during the first year of life and reduce afterwards. This shows that the fastest conducting nerve fibers have more rapid and prominent changes than the slowest ones during the first year of life, after which the slowest conducting nerve fibers seem to have more gradual maturational changes than the fastest ones.

Reproducibility of neurophysiological and myometric measurement in the ulnar nerve-abductor digiti minimi system.

Reliability in strength and neurophysiological measurements is important in evaluating progression of neuromuscular diseases. No data are available on the variability of neurophysiological parameters as compared with maximal voluntary isometric contraction (MVIC) in the same muscles, in healthy subjects. A control population of 26 healthy subjects was studied twice on different days. We evaluated the reliability of neurophysiological parameters obtained from bilateral ulnar nerve stimulation, recording the response over the abductor digiti minimi (ADM) muscle, including distal motor latency, compound muscle potential amplitude and area, F-wave frequency and mean F-wave latency, and the derived neurophysiological index that we have described previously. MVIC force was measured in each session in both ADM muscles. The variances between the grouped data obtained in the two recording sessions were identical, indicating a low intrinsic variability with this experimental methodology. Comparison of the mean values obtained in the two sessions revealed no statistically significant differences. The reliability of these neurophysiological and strength measurements in the same nerve/muscle system suggests they may be useful in comparing the dynamics of weakness and neurophysiological change in neuromuscular disease.

A normative study on human facial F waves

F waves from the nasalis muscle were obtained bilaterally after transcutaneous constant-current stimulation of the facial nerve in 37 of 42 (88%) healthy volunteers examined. F waves were of varying shape and latency. Standard parameters (latencies, amplitudes, ratios, chronodispersion) of F-wave analysis were assessed. Of these, minimum and mean F-wave latencies, the F ratio, and F-wave and peripheral conduction times (PCT) were approximately normally distributed, with a low standard deviation. Latencies and conduction times exhibited a significantly positive correlation with body height, as has been demonstrated for the F-wave latencies in limb muscles. Side and sex differences were present but without statistical significance. Chronodispersion and chronodispersion range, F-wave amplitudes, and F-wave frequencies were broadly scattered. It is concluded that F-wave latencies and conduction times are best suited to define a range of reference values. Moreover, facial F waves bear characteristics similar to those described for F waves recorded from limb muscles. © 2001 John Wiley & Sons, Inc. Muscle Nerve 24: 900–904, 2001

A normative study on human facial F waves.

F waves from the nasalis muscle were obtained bilaterally after transcutaneous constant-current stimulation of the facial nerve in 37 of 42 (88%) healthy volunteers examined. F waves were of varying shape and latency. Standard parameters (latencies, amplitudes, ratios, chronodispersion) of F-wave analysis were assessed. Of these, minimum and mean F-wave latencies, the F ratio, and F-wave and peripheral conduction times (PCT) were approximately normally distributed, with a low standard deviation. Latencies and conduction times exhibited a significantly positive correlation with body height, as has been demonstrated for the F-wave latencies in limb muscles. Side and sex differences were present but without statistical significance. Chronodispersion and chronodispersion range, F-wave amplitudes, and F-wave frequencies were broadly scattered. It is concluded that F-wave latencies and conduction times are best suited to define a range of reference values. Moreover, facial F waves bear characteristics similar to those described for F waves recorded from limb muscles.

Remote F-wave changes after local botulinum toxin application

Objective: Although the therapeutic effects of botulinum toxin A can be explained by its action at the neuromuscular junction, central or more proximal effects have also been discussed. Methods: Eleven patients with torticollis spasmodicus and 3 patients with writer's cramp were studied before and 1 and 5 weeks after the first treatment with botulinum toxin. We measured compound muscle action potentials (CMAPs), motor conduction velocities (MCVs), the shortest (SFL) and the mean F-wave latencies (MFL) and F-wave persistence (30 trials) of untreated muscles for each side (ulnar nerve-abductor digiti minimi muscle, peroneal nerve-tibialis anterior muscle). Results: CMAPs and MCVs showed no significant changes. For both nerves, however, SFL and MFL were prolonged slightly 1 week after treatment and returned to about baseline after 5 weeks ( t test). The F-wave persistence was reduced 1 week after treatment for the right ulnar and both peroneal nerves ( t test). Conclusions: These results are not likely due to an impairment of neuromuscular transmission. Instead, we propose a decreased excitability of α-motoneurons supplying non-treated muscles. A reduction of muscle spindle activity or changes of the recurrent inhibition are discussed as possible causes.

Radial nerve F-waves: normative values with surface recording from the extensor indicis muscle

Objectives: In the present study we set out to obtain normative values for radial nerve F-waves, with surface recording from the extensor indicis muscle. Methods: Forty-nine patients with unrelated complaints were tested. Surface recording electrodes were placed on the extensor indicis muscle. This was found by asking the patient to extend the second digit against resistance. The active surface recording electrode was placed over the most distal portion of the muscle, near the radial border of the ulnar bone near the wrist. Stimulation was performed near the lateral epicondyle between the radial and ulnar bones. Results: The mean F-wave minimum latency was found to be 20.55 ms, with an upper limit of 24.35 ms. The absolute interside minimum latency difference was found to have a mean of 0.55 ms, with a maximum of 1.7 ms. The mean amplitude of the F-waves was 145.61 μV and the mean mF/M ratio was 0.022. F-waves were unobtainable in 2/62 (3.2%) of limbs. Normative values for the radial nerve motor response were also obtained. Three cases are described to illustrate the usefulness of the above technique. Conclusions: It is technically feasible to record radial nerve F-waves from the extensor indicis muscle.

Tibial nerve F-waves recorded from the abductor hallucis.

The purpose of this study was to develop a large database of normal values for the tibial nerve F-wave. A total of 159 asymptomatic subjects without risk factors for neuropathy were recruited and had ten tibial F-waves performed on each leg. Data were collected for the shortest F-wave (Fmin), the mean F-wave (Fmean), and the range of F-waves (Frange). Age, gender, and height were associated with differences in the results. The mean Fmin was 50.8 +/- 5.3 ms. Mean Fmean was 53.0 +/- 5.6 ms and mean Frange was 4.5 +/- 2.4 ms. The mean side-to-side difference for Fmin was 0.6 +/- 2.3 ms and the mean side-to-side difference for Fmean was 0.4 +/- 2.5 ms.

Ulnar nerve F-wave latencies recorded from the abductor digiti minimi.

This study was performed to create a large database of normal values for the ulnar F-wave study and to investigate the effect of various demographic factors on F-wave results. The study was designed to incorporate a standard distance measure and temperature control, which are lacking in some previous studies. One hundred ninety-three asymptomatic volunteers without risk factors for neuropathy were recruited and had ten F-waves performed on each arm. Data were collected for the shortest F-wave (Fmin), mean F-wave (Fmean), the number of F-waves present out of ten stimuli (Fpersist), and the range of latencies (Frange). An analysis of variance statistical procedure was applied, and the Fmin and Fmean were found to correlate with age, gender, and height, although the gender effect was relatively weak. For all subjects taken together, the mean Fmin was 26.5 +/- 2.5 ms. The Fmean was 27.7 +/- 2.5 ms, and the mean Frange was 2.6 +/- 1.2 ms. Ninety-seven percent of subjects had an Fpersist of five or more. Mean side-to-side difference for Fmin was 0.2 +/- 1.1 ms.

Anodal block in F-wave studies

Objective: To determine whether F-wave results differ with the anode proximal or distal to the cathode, ie, if clinical anodal block exists. Design: Prospective study of 30 healthy volunteers undergoing nerve conduction and F-wave studies in one median nerve. A needle cathode electrode was used with a surface anode placed alternately proximal and distal to the cathode. The same electromyographer performed all studies with a Dantec Counterpoint machine. Results: F-wave latencies were essentially unaffected by distal versus proximal positioning. Minimum, maximum, and mean F-wave latencies correlated extremely highly ( r = .973 to .988). For both F-wave and M-response latencies and amplitudes, differences between mean values obtained using the two methods were extremely small and were neither clinically nor statistically significant. The frequency of elicitation of F-waves may ( p < .05) have been slightly (3.5%) lower when the anode was in the distal position. Conclusion: Anodal block is not seen in F-wave studies when using needle electrode stimulations. Reversing the stimulator does not seem to be required. Further study with surface stimulating electrodes is underway to confirm results.