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. Author manuscript; available in PMC: 2025 Sep 1.
Published in final edited form as: Muscle Nerve. 2024 Jul 4;70(3):395–401. doi: 10.1002/mus.28197

F Wave Analysis Based on the Compound Muscle Action Potential Scan

Xiaoyan Li a,b, Maoqi Chen c, Paul E Barkhaus a, Sanjeev D Nandedkar a,d, Brian Schmit e, Ping Zhou c
PMCID: PMC11324398  NIHMSID: NIHMS2003961  PMID: 38963007

Abstract

Introduction/Aims:

Conventional F wave analysis involves a relatively uniform physiological environment induced by supramaximal stimulations. The F wave characteristics in a dynamic physiological condition, however, are rarely investigated. This study aimed to improve understanding of F wave properties in the more dynamic process by introducing a novel method to analyze F waves based on the compound muscle action potential (CMAP) scan technique.

Methods:

Twenty-four healthy subjects participated in the study. The CMAP scan was applied to record muscle responses in the abductor pollicis brevis (APB) and abductor digiti minimi (ADM) muscles, respectively. F wave characteristics including mean F wave amplitude and latency (F-M latency), persistence and activation threshold were quantified.

Results:

An average of 200 F waves per muscle were obtained from the CMAP scan recording. Weak to moderate correlations between F wave amplitude and stimulating intensity were observed in most of the APB (19 muscles; r= 0.33±0.14, all p<0.05) and ADM (23 muscles, r= 0.46±0.17, all p<0.05) muscles.. Significantly longer mean F latency and lower activating F-threshold were found in the ADM muscles (F-M latency: APB: 25.43±2.39 ms, ADM: 26.15±2.32 ms, p<0.05; F-threshold: APB: 7.68±8.96% CMAP, ADM: 2.35±2.42% CMAP, p<0.05), indicating innervating differences in nerves.

Discussion:

This study introduces new features of F waves using the CMAP scan technique and identifies differences of F wave characteristics between the hand muscles. The CMAP scan based F waves analysis can be combined with the motor unit number estimation to assess functional alterations in motor neurons in neurological disorders.

Keywords: F wave analysis, Compound muscle action potential (CMAP) scan, motor neuron (), electrophysiology, hand muscles

Summary

A new F wave analytical technique is presented based on the CMAP scan recordings in the hand muscles. This study introduces a new feature in F wave analysis, the F wave activating threshold. We observed lower F activating threshold in the ADM compared with the APB muscles. It would appear that the F wave activating threshold may be associated with motor axon excitability. The normative data of F waves in this study may be used for investigation of split hand in ALS. This technique can be combined with CMAP scan based MUNE to improve understanding of motor neuron function and degeneration in neurological disorders.

Introduction

F waves provide valuable information on motor nerve function and signal transmission. 1,2. Conventional studies examine F waves in a relatively uniform physiological environment using repetitive electrical supramaximal stimulations at a constant intensity 3,4. There are limited data on F wave characteristics in the dynamic conditions. The CMAP scan is a technique of motor unit number estimation (MUNE) 5–8, which applies finely graded declining electrical stimuli to progressively deactivate the motor neuron (MN) pool. As the stimulus intensity varies from supramaximal strength to the lowest motor unit’s subthreshold, a more dynamic environment to study F waves is possible.

In this study, we investigated the F wave properties in hand muscles using the CMAP scan technique 9,10. First, we examined the relationship between F wave parameters and stimulus intensity. F waves at different stimulation intensities have been assessed in previous studies 3,11–13. However, the levels of stimulus intensity or sample size were limited. The CMAP scan involves hundreds of stimuli at different intensities and an average of 200 F waves per muscle are obtained as in this study. Given the quantity of F waves and the range of stimulus intensity, this study provides a more comprehensive assessment of the relationship between F wave parameters and stimulus intensity. We hypothesize that changes of F wave parameters may be correlated with stimulus intensity.

The second aim of the study was to examine differences of F characteristics in hand muscles. Split-hand is a well-described phenomenon in amyotrophic lateral sclerosis (ALS) 14,15 with unclear pathophysiology. Understanding the nerve differences in the abductor pollicis brevis (APB) and abductor digit minimi (ADM) muscles can help discriminate the pathological changes and develop an appropriate index to evaluate the phenomenon 16. Currently, there are limited studies assessing nerve differences based on F wave characteristics 17. We further hypothesize that there are significant differences of F wave parameters between these two hand muscles.

In this study, we introduce an approach to analyze F waves based on CMAP scan recordings. This approach may expand the application of CMAP scans in clinical studies as both F waves and MUNE information can be simultaneously extracted, thus reducing test time and subject discomfort. Information from these CMAP scans can also be used to assess motor axon conduction and MN excitability in neurological disorders.

Methods

Subject Information

Healthy subjects were recruited from the local community. None of them had known symptoms of radiculopathy, carpal tunnel syndrome, or ulnar neuropathy, and none had known muscle atrophy. Nerve conduction studies were performed on the median and ulnar nerves prior to experiments to exclude any subclinical neuropathy. All subjects provided signed informed consent, and the study was approved by the Institutional Review Board of the Medical College of Wisconsin.

Experimental setup and procedure

CMAP scan test: Subjects were seated comfortably in an adjustable chair. They maintained an upright posture with the tested arm in a rested position during the test. Limb temperature was maintained at a minimum of 30° C throughout the tests. A Synergy electromyography system (Natus Medical Inc, Middleton, WI) was used to record the muscle responses to electrical stimuli from the APB and ADM muscles on the dominant side. Active (E1), reference (E2) and ground electrodes (E0) were placed in a montage similar to a previous study 18. A standard bar electrode was positioned and fixed over the median or ulnar nerve to deliver electrical stimuli.

The CMAP scan procedure consisted of two steps: determining the stimulating thresholds and carrying out the actual scan. The first step involved an automated search of the upper and lower thresholds of the scan. The lower threshold of the scan was defined as the intensity evoking the motor unit with the lowest threshold. The upper threshold of the scan was defined as the lowest intensity to evoke the maximal CMAP.

After these two thresholds were identified, the program provided an interface for the experimenter to manually modify the two thresholds. Given the inherent variability in axonal thresholds, a larger stimulating range was applied in the study by extending the lower and upper thresholds 2–4 mA from the original range. Use of a larger stimulating range guaranteed recording of the progressive muscle responses from full muscle activation (upper threshold) to no muscle response (lower threshold).

The scan was performed after confirmation of these two thresholds. Repetitive stimuli were delivered automatically with the intensity declining linearly from the upper threshold to the lower one in 500 steps. The pulse width of the stimulus was 0.1 ms and the stimulating frequency was 2 Hz. This protocol can optimize the CMAP scan recording by reducing subject discomfort, artefact, and time of recording 19,20. The scan was completed after the lower threshold was reached. All muscle responses were saved in 100 ms epochs at a sampling frequency of 19,200 Hz.

Strength measurement: Subjects maintained an upright posture with the shoulder adducted and neutrally rotated, the elbow flexed between 80° to 100° and the forearm in the neutral position. A portable Jamar Plus digital hand dynamometer (Jamar, Los Angeles, CA, US) and a pinch gauge (B&L Engineering, Santa Fe Springs, CA, US) were used to measure the grip and pinch strength. The key pinch force was measured as the thumb pad against the lateral middle phalanx of the index finger 21. Subjects were instructed to squeeze the dynamometer or the pinch gauge with their maximum strength. Measurement of the grip and pinch strength were repeated at least twice, and the highest value was selected. A brief rest was given between trials to avoid fatigue.

Data Analysis.

CMAP scan data were exported to Matlab (MathWorks Inc, Natick, MA, USA) for automatic analysis. All CMAP and F wave parameters were measured and calculated in Matlab. CMAP amplitude was measured at each stimulating intensity and the pairs of data were plotted in an S-R curve (S= stimulus intensity, R= muscle response). The slope of the curve was calculated between 25–75% of maximal CMAP using regression analysis (Fig. 1b). The maximal CMAP amplitude and the slope of the S-R curve were measured for all muscles.

Fig. 1.

Fig. 1.

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CMAP scan recordings from a subject’s APB muscle. (a) CMAP waveforms in response to 500 electrical stimuli. Signals were truncated to 40 ms in duration for display. Darker lines show examples of a few F waves. (b) Response-Stimulus curve. Each dot in the curve represents a CMAP amplitude (response) plotted against stimulus intensity. Darker dots indicate responses containing F waves. The slope of the S-R curve is estimated between 25–75% of maximal CMAP amplitude (dotted line).

F waves are usually superimposed on the afterpotential of the CMAP waveforms (Fig. 1a), which may shift the baseline of F waves and cause bias in F wave analysis. To eliminate the displacement of the baseline, we applied linear regression22 to estimate the segment of baseline drift and subtracted it from the raw signal to obtain the cleaned F waves.

An F wave was considered valid if the difference between the F wave negative and positive peaks was ≥ 40 µV23,1. For each valid F wave, we measured the amplitude, latency and stimulating intensity. F amplitude (FAmp) was defined as the differences between the negative peak and the baseline. F latency (FLat, or F-M latency) was calculated as the interval between the onset of the F wave and distal motor latency 17,4.

In this study, we collected an average of 200 F waves per muscle from each CMAP scan. The distributions of F wave amplitude and latency were quantified for each muscle. In addition, F wave persistence and activating threshold were calculated for each muscle. F wave persistence was defined as the number of F waves recorded between the range of 25–75% of the maximal CMAP divided by the number of stimuli in the range. We used ‘Persit50’ to define the persistence in the study. For example, in Fig. 1b, there were 88 electrical stimuli delivered between 25%−75% of CMAP amplitude which evoked a total of 55 F waves. Persit50 was calculated as 63.22% (55/87×100%). Use of 25–75% of the CMAP to estimate persistence ensured consistence in calculation and comparison between muscle groups.

The F activating threshold was defined as the intensity evoking the first valid F wave in the study (marked in Fig 1b). The absolute intensity of muscle responses is generally influenced by stimulating site, subcutaneous thickness, axonal threshold, etc. To reduce the above effects, we used a relative activating threshold (F-threshold), defined as the CMAP amplitude evoked at the F activating threshold normalized by the maximal CMAP (CMAPF1 / maximal CMAP ×100%). For example, the intensity of the F-threshold in Fig. 1b was 6.89 mA which elicited the CMAP of 0.305 mV. Given the maximal CMAP amplitude of 12.35 mV, the F-threshold was estimated as the intensity evoking 2.47% (0.305/12.35×100%) of maximal CMAP. Using this relative value facilitated comparison of the F-threshold across different muscles.

Statistical analysis

Statistics analysis was performed using the Statistics and Machine Learning Toolbox in Matlab. The analysis was carried out on two levels: at the individual muscle level and between two muscle groups (APB and ADM groups). For each muscle, we applied the Kolmogorov-Smirnov test (for sample size > 100) to examine the normality of F amplitude and latency distributions. The null hypothesis was rejected in the two distributions for all muscles. On the other hand, the Central Limit Theorem suggests that the sampling distribution of the mean tends to follow a normal distribution24,25 as long as the sample size is large. Given the quantity of F waves (an average of 200 F waves) per muscle, we used the mean and standard deviation (std) to quantify the amplitude and latency distributions for each muscle in the study. In addition, a Pearson correlation was applied to examine the relationship between F parameters (amplitude or latency) and stimulus intensity in each muscle.

For each muscle group (APB or ADM), the Shapiro-Wilk test was applied to assess the normality of the variables including the maximal CMAP amplitude, S-R slope, mean of amplitude and latency distributions, Persit50, and F-threshold. The normality of distribution of the variables was confirmed in both muscle groups. A paired t-test was applied to compare the differences of the above variables between the two muscle groups. Pearson correlation coefficients were calculated to examine the relationship between the CMAP amplitude and age or hand strength for each muscle group. In addition, the Pearson correlation was applied to examine the relationship between the pairs of variables including S-R slope, F-threshold, mean F amplitude, mean F latency and Persit50 in each muscle group. Statistical significance was defined as p < 0.05. Results are reported in the mean ± std format.

Results

Data from a representative subject

Figures. 1–3 show the CMAP scan and F wave data from a representative subject’s APB muscle. Fig.1a illustrates an example of the raw CMAP scan recordings. The waveforms represent muscle responses at different intensities. The S-R curve, based on the paired of CMAP amplitude and stimulating intensity, is plotted in Fig. 1b. The slope of the curve was estimated as 2.71 mV/mA (r2=0.98, p<0.001) between 25–75% CMAP.

Fig. 3.

Fig. 3.

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Correlation analysis in the same muscle as in Fig. 1. (a) F wave amplitude correlates positively with stimulus intensity. (b) F wave latency correlates negatively with stimulus intensity. The dashed lines indicate linear relation.

In total, there were 227 F waves detected from the CMAP scan recording. The F-threshold was observed at the stimulus intensity evoking 2.47% of the CMAP and the F wave persistence was 63.22% between 25–75% of maximal CMAP amplitude. F wave amplitude and latency distributions are displayed in Fig. 2. The F wave amplitude varied between 42.93 and 506.73 µV with a mean amplitude of 182.49 ± 103.68 µV (Fig. 2a). The F wave latency ranged between 20 and 26.56 ms with the mean value of 22.24 ± 0.77 ms (Fig. 2b). Changes of F wave amplitude and latency with stimulus intensity were assessed and the results were illustrated in Fig. 3. Weak but significant correlations were observed between F wave amplitude and stimulus intensity as well as between F wave latency and intensity in the APB muscle.

Fig. 2.

Fig. 2.

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Examples of F wave (a) amplitude and (b) latency distributions from the same muscle as in Fig. 1. The heavy weighted dotted lines indicate normal distributions.

Group analysis

Details of all subjects’ information are presented in Table 1. Correlation analysis between the maximal CMAP amplitude and age showed a negative relationship in the ADM muscle (ADM: r=−0.46, p<0.05). No significant correlations were observed between the maximal CMAP amplitude and strength (grip or pinch). Paired t-test indicated a significant larger slope in the S-R curve in the APB muscle (p<0.001).

Table 1.

Subject demographic information and electrophysiological measurement.

Age (year) Sex Pinch (kg) Grip (kg)
Range 27~65 13 F 5.3–12.3 24.6–60.2
Mean ± std 40 ± 12 11 M 6.67±2.18 32.52±11.02
APB Max_CMAP (mV) Slope (mV/mA) F-threshold (%CMAP) Persit50(%)
Range 8.15–17.43 1.07–10.77 0.56–43.25 14.29–77.69
Mean ± std 12.31±2.06 4.58±2.59 7.68±8.96 53.91±18.61
ADM Max_CMAP (mV) Slope (mV/mA) F-threshold (%CMAP) Persit50(%)
Range 9.13–15.84 0.5–6.97 0.39–8.95 14.76–84.38
Mean ± std 12.26±1.93 2.06±1.56 * 2.35±2.42 * 52.3±20.02
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Max_CMAP: maximal CMAP amplitude; Slope: S-R slope. F-threshold: normalized F wave activating threshold; Persit50: persistence calculated between 25–75% CMAP.

*

: significance between APB and ADM muscles.

Correlations between F wave parameters and stimulus intensity

A total of 9613 F waves were collected in the study with 4925 F waves in the APB and 4688 F waves in the ADM muscles. This represents an average of 205 F waves per APB muscle and 195 F waves per ADM muscle. The quantity of F waves per muscle allowed us to perform correlation analysis between F wave parameters (amplitude or latency) and stimulus intensity in each muscle. Significant positive correlations were observed between F amplitude and stimulating intensity in 19 APB and 23 ADM muscles (87.5% of all muscles). The coefficients of the above muscles were averaged and the results are shown in Table 2. Note that the coefficients of muscles not showing any significance in correlation analysis were not included in the table.

Table 2.

Correlation analysis between F wave amplitude/latency and stimulating intensity.

APB ADM
Amp Lat Amp Lat
Number of positive correlations 19 3 23 2
Correlation coefficient 0.33±0.14 0.2±0.05 0.46±0.16 0.25±0.06
Number of negative correlations 0 5 0 10
Correlation coefficient NA −0.28±0.09 NA −0.28±0.1
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Amp: significant correlations between F wave amplitude and stimulus intensity; Lat: significant correlations between F wave latency (F-M latency) and stimulus intensity.

Nerve difference and correlation analysis within F parameters

The distributions of F wave amplitude and latency from other subjects demonstrated similar patterns to those seen in Fig. 2. Paired t test indicated that the mean latency of the ADM was significantly longer than that of the APB group (F-M latency: APB: 25.43±2.39 ms, ADM: 26.15±2.32 ms, p<0.05). No significant changes were observed in the mean F wave amplitude between the two muscles.

The average F wave threshold in the APB muscle was significantly higher than that in the ADM (Table 1). In contrast, no significant difference of F wave persistence (Persit50) was found between the two muscles. The relationship between pairs of variables including F wave mean amplitude, mean latency, F-threshold, Persist50 and S-R slope was examined within the APB or ADM muscle group. A negative linear relationship was found between F-threshold and Persit50 in both muscles but only APB showed significance (APB: r=−0.55, p<0.01). No significant correlations were observed in other pairs of variables.

Discussion

This study disclosed significant correlations between F wave amplitude and stimulus intensity in the hand muscles. The longer mean F wave latency and lower activating F-threshold in the ADM muscle indicated differences between the median and ulnar nerves.

Influence of stimulating intensity

F wave amplitude and latency data in this study are within the normal range of F waves in the literature 17,26,27. Insignificant correlations between F wave latency and stimulus strength were observed in our study. These results conform to previous studies that examined the relationship of stimulating intensity and F wave characteristics at levels of 10–20% 11, 25%, 50%, 75% 3 and 100% of maximal CMAP amplitude.

F wave amplitude was found to increase linearly with stimulus intensities in this and previous studies 3. As the F wave represents a small portion of MNs in the entire pool 28,29, the increased F amplitude at high intensity may be associated with activation of more motor units and/or motor units with larger amplitude. It is suggested that the larger motor units are preferentially evoked as Renshaw cells inhibit the smaller motor units more effectively at higher intensity 11,29. F wave amplitude is also influenced by factors such as the location of the motor units, the distance to the recording electrodes, etc.30

Nerve difference on F characteristics

Similar to previous studies 31,23,32, F amplitude and latency were not normally distributed in the hand muscles. Despite that, we quantified the F amplitude and latency distributions in APB and ADM muscles. The mean F-M latency in the ADM muscle was significantly longer than that in the APB, which is consistent with findings from the standard F wave technique 17.

In contrast to a previous study 4, we did not find significant changes of persistence (Persit50) between the APB and ADM muscles. These differences in persistence can be explained as follows. The first is the technical difference: in our study the F waves were obtained from the CMAP scan technique as opposed to use of a constant stimulating intensity. Second, persistence was calculated at relatively lower stimulating intensities (between 25–75% CMAP) in our study in contrast to supramaximal stimulation 4. Lower stimulating intensities may have yielded lower average persistence in our study. F wave persistence is a sensitive index to assess split hand and differentiate ALS from intact muscles 33. Our study provides normative F wave data that may be used in investigation of the split hand index.

In this study, the chances of recording an H reflex are low based on the following considerations. The H reflex is generally evoked by stimuli of 0.5–1 ms in duration and ≤ 0.2 Hz in stimulus frequency to activate large sensory fibers and avoid the effects from the prior stimulus 34. In this study, F waves were evoked by stimuli of 0.1 ms in duration and 2 Hz frequency in the study. The chance of evoking an H reflex at low intensity with short stimulus duration is low in normal hand muscles. Second, H reflex amplitude is generally larger than the associated submaximal CMAP 29. We carefully inspected the amplitude of all late responses and did not find any of them larger than the associated submaximal CMAP. Third, if the H reflex was evoked at the low intensity, given that the stimulating intensity changes in 0.2% per step, we should observe such waveforms appearing after consecutive stimuli with waveform amplitudes first increasing and then decreasing with declining stimulus intensity. No such pattern was observed in this study. As illustrated in Fig. 3, F Amplitude and latency are discretely distributed at low intensities and their values vary from trial to trial at low intensities.

Limitations

In conventional F wave studies, the stimulating frequency is usually set at ≤1 Hz. In this study, F waves were evoked at a stimulus frequency of 2 Hz. Previous studies report inconsistent results of the effect of stimulus frequency of 2 Hz on F wave characteristics 35–37,3. Hence, it is unclear whether these differences in stimulus frequency and intensity would affect F wave parameters. As subject height was not measured in this study, we could not assess its influence. Similarly, as F waves were evoked only once at a given intensity, the shortest latency F wave could not be determined.

Acknowledgement:

This study was supported in part by NIH grant under 7 R21 NS113716-02, Advancing a Healthier Wisconsin Endowment (AHW), and NIDILRR RERC under 90REMM0001-01-00, and in part by Shandong Nature Science Foundation under grant ZR2020KF012.

Abbreviations:

ALS

amyotrophic lateral sclerosis

CMAP

compound muscle action potential

APB

abductor pollicis brevis

ADM

abductor digiti minimi

MUNE

motor unit number estimation

MN

motoneuron

Footnotes

Ethical Publication Statement: We confirm that we have read the Journal’s position on issues involved in ethical publication and affirm that this report is consistent with those guidelines.

Disclosure of Conflicts of Interest: Dr. Nandedkar is an employee of Natus Medical Inc and has no conflicts to report. None of the other authors has any conflict of interest to disclose.

Financial Disclosure Statement: There are no financial conflicts of interest to disclose.

Data Availability Statement:

The data that support the findings of this study are available upon request.

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Associated Data

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Data Availability Statement

The data that support the findings of this study are available upon request.

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