Identical late motor responses in early Guillain-Barré syndrome: A-waves and repeater F-waves.

Electrodiagnostic (EDx) studies play a key role in the investigation of suspected Guillain-Barré syndrome (GBS), providing diagnostic and prognostic information. However, initial EDx findings may not fulfill the neurophysiological criteria for the disease. The aim of this study was to estimate the occurrence and characteristics of A-waves and repeaters F-waves (Freps), both late motor responses identical in latency and configuration, in early stages of GBS. We retrospectively analyzed the initial nerve conduction study (NCS) of 26 GBS patients performed within 10 days from symptom onset. The final subtype diagnosis was acute inflammatory demyelinating polyneuropathy (AIDP) in 16 patients (six met the criteria at the initial EDx study and 10 at follow-up) and acute motor axonal neuropathy (AMAN) in 10 patients (six initially). Identical late responses were commonly found in the majority of nerves (84%). A-waves were present in 59% and an increased frequency of Freps was calculated in 61% of the 105 studied nerves. A-waves morphology (single or complex) could not distinguish between AIDP and AMAN. Nerves with normal NCS had a significantly higher frequency of A-waves, either isolated or in combination with increased index total Freps, as compared to nerves with low compound muscle action potential (CMAP) amplitudes or conduction block. Our findings suggest that both late responses can be useful as early markers of conduction changes of various pathophysiology, being frequently present even prior to abnormalities of CMAP parameters.

Which combined nerve conduction study scores are best suited for polyneuropathy in diabetic patients?

INTRODUCTION/AIMS: Nerve conduction studies (NCS) are widely used in diagnosing diabetic polyneuropathy. Combining the Z scores of several measures (Z-compounds) may improve diagnostics by grading abnormality. We aimed to determine which combination of nerves and measures is best suited for studies of diabetic polyneuropathy. METHODS: Sixty-eight patients with type 1 diabetes and 35 controls were included in this study. NCS measurements were taken from commonly investigated nerves in one arm and both legs. Different Z-compounds were calculated and compared with reference material to assess abnormality. A sensitivity proxy, the accuracy index (AI), and Cohen's d were calculated. RESULTS: Z-compounds with the highest AI consisted of the tibial and peroneal motor, and the sural, superficial peroneal, and tibial medial plantar sensory nerves in one or two legs. All Z-compounds were able to discriminate between diabetic subjects and nondiabetic controls (mean Cohen's d = 1.42 [range, 1.03-1.63]). The association between AI and number of measures was best explained logarithmically (R2  = 0.401), with diminishing returns above approximately 14 or 15 measures. F-wave inclusion may increase the AI of the Z compounds. Although often clinically useful among the non-elderly, the additional inclusion of medial plantar NCS into Z-compounds in general did not improve AI. DISCUSSION: Performing unilateral NCS in several motor and sensory lower extremity nerves is suited for the evaluation of polyneuropathy in diabetic patients. The use of Z-compounds may improve diagnostic accuracy in diabetic polyneuropathy and may be particularly useful for follow-up research studies as single summary measures of NCS abnormality development over time.

Effects of lower limb segmental muscle vibration on primary motor cortex short-latency intracortical inhibition and spinal excitability in healthy humans.

We examined the effects of lower limb segmental muscle vibration (SMV) on intracortical and spinal excitability in 13 healthy participants (mean age: 34.9 ± 7.8 years, 12 males, 1 female). SMV at 30 Hz was applied to the hamstrings, gastrocnemius, and soleus muscles for 5 min. Paired-pulse transcranial magnetic stimulation protocols were used to investigate motor-evoked potential (MEP)  amplitude, short-interval intracortical inhibition (SICI) and short-interval intracortical facilitation (SICF) from the abductor hallucis muscle (AbdH). These assessments were compared to the results of a control experiment (i.e., non-vibration) in the same participants. F-waves were evaluated from the AbdH on the right (vibration side) and left (non-vibration side) sides, and we calculated the ratio of the F-wave amplitude to the M-response amplitude (F/M ratio). These assessments were obtained before, immediately after, and 10, 20, and 30 min after SMV. For SICI, there was no change immediately after SMV, but there was a decrease over time (before vs. 30 min after, p = 0.021; immediately after vs. 30 min after, p = 0.015). There were no changes in test MEP amplitude, SICF, or the F/M ratio. SMV causes a gradual decrease in SICI over time perhaps owing to long-term potentiation. The present results may have implications for the treatment of spasticity.

Immobility and F-waves: Impact on lower motor neuron excitability.

INTRODUCTION: Immobility of the upper limbs has been associated with reduction of F-wave frequency. However, there are no similar studies on lower limb (LL) F-waves. We investigated the impact of LL rest on F-wave and H-reflex parameters. METHODS: The LLs of 14 healthy participants were studied after 90 minutes rest. F-waves (frequency, latencies, chronodispersion, and mean amplitude) and H-reflexes (latency and recruitment curve) were investigated bilaterally. In seven participants the protocol was repeated, but the temperature of one limb was reduced. RESULTS: Immobility only changed F-wave latencies, which increased significantly (mean value of 2 ms, P < .01). Limb cooling did not influence results. DISCUSSION: Contrary to what occurred in cervical lower motor neurons (LMN), LL LMNs did not show a reduced F-wave response to immobility, but their latency increased significantly. This could have been due to reduced Renshaw inhibition of small LMNs, thus facilitating their response to antidromic stimulation and causing delayed late responses.

A waves in electroneurography: differential diagnosis with other late responses.

Neurographic studies are an extension of clinical examination and are performed for the functional assessment of peripheral nerves. The study of motor and sensory conduction velocity and the presence, amplitude, morphology and symmetry of the response to electrical stimulation are crucial for the diagnosis and management of peripheral neuromuscular disorders. Neurography also plays an important role in the search for so-called late responses comprising the F wave, H reflex, axonal response and A wave. By analysing the parameters of each late wave, this paper addresses the pathophysiological features and the most common conditions impairing the physiology of late responses, with a special focus on A waves.

Single-fiber F waves compared with conventional surface F waves, and their utility in detecting early diabetic neuropathy.

INTRODUCTION: The single-fiber F-wave (SFF-wave) technique assesses the entire length of single motor fibers using a concentric needle. Herein we investigated the utility of this approach in the detection of early diabetes-related neuropathy, and compared it with the use of conventional surface F waves (CF waves). METHODS: Sixteen patients with diabetes and either no neuropathy or mild neuropathy were assessed and compared with 16 age- and height-matched control participants. RESULTS: Both CF and SFF waves were abnormal in all 5 patients who had mild neuropathy. However, SFF waves demonstrated subclinical abnormalities in 7 of 11 patients (64%) with no neuropathy, whereas only 2 of these patients (18%) had prolonged CF waves. Minimum F-wave latency was comparable between techniques, but maximum SFF-wave latency was more frequently prolonged, as these delayed motor units were better isolated, rather than buried among summated CF-wave responses. DISCUSSION: SFF waves highlight the segmental involvement in diabetic neuropathy, and use of the SFF-wave technique detects more abnormalities than with CF waves. Muscle Nerve 58: 665-670, 2018.

Human motoneurone excitability is depressed by activation of serotonin 1A receptors with buspirone.

KEY POINTS: In the adult turtle spinal cord, action potential generation in motoneurones is inhibited by spillover of serotonin to extrasynaptic serotonin 1A (5-HT1A ) receptors at the axon initial segment. We explored whether ingestion of the 5-HT1A receptor partial agonist, buspirone, decreases motoneurone excitability in humans. Following ingestion of buspirone, two tests of motoneurone excitability showed decreases. F-wave areas and persistence in an intrinsic muscle of the hand were reduced, as was the area of cervicomedullary motor evoked potentials in biceps brachii. Our findings suggest that activation of 5-HT1A receptors depresses human motoneurone excitability. Such a depression could contribute to decreased motoneurone output during fatiguing exercise if there is high serotonergic drive to the motoneurones. ABSTRACT: Intense serotonergic drive in the turtle spinal cord results in serotonin spillover to the axon initial segment of the motoneurones where it activates serotonin 1A (5-HT1A ) receptors and inhibits generation of action potentials. We examined whether activation of 5-HT1A receptors decreases motoneurone excitability in humans by determining the effects of a 5-HT1A receptor partial agonist, buspirone, on F waves and cervicomedullary motor evoked potentials (CMEPs). In a placebo-controlled double-blind study, 10 participants were tested on two occasions where either placebo or 20 mg of buspirone was administered orally. The ulnar nerve was stimulated supramaximally to evoke F waves in abductor digiti minimi (ADM). CMEPs and the maximal M wave were elicited in biceps brachii by cervicomedullary stimulation and brachial plexus stimulation, respectively. Following buspirone intake, F-wave area and persistence, as well as CMEP area, were significantly decreased. The mean post-pill difference in normalized F-wave areas and persistence between buspirone and placebo days was -27% (-42, -12; 95% confidence interval) and -9% (-16, -2), respectively. The mean post-pill difference in normalized CMEP area between buspirone and placebo days showed greater variation and was -31% (-60, -2). In conclusion, buspirone reduces motoneurone excitability in humans probably via activation of 5-HT1A receptors at the axon initial segment. This has implications for motor output during high drive to the motoneurones when serotonin may spill over to these inhibitory receptors and consequently inhibit motoneurone output. Such a mechanism could potentially contribute to fatigue with exercise.

Neuroprotective effect of epidermal growth factor in experimental acrylamide neuropathy: an electrophysiological approach.

The neuroprotective effect of epidermal growth factor (EGF) has been documented in different contexts, but its potential benefits in peripheral neuropathies have been little studied. We investigated the neuroprotective action of EGF in experimental neuropathy induced by acrylamide (ACR). Mice and rats were treated chronically with acrylamide for 6 and 8 weeks, respectively. Concurrently they received EGF in daily doses of 1 and 5 mg/kg in mice and 3 mg/kg in rats, or saline (PBS). ACR severely affected the neurological score, the muscle strength, and the muscle potential M, in mice, as well as F-waves (F-Wii), sensory potentials (SPii), and apomorphine-induced penile erection, in rats. EGF reduced the ACR effects in both species. A dose-dependent effect of EGF was manifested in the proportion of diseased animals at the end of treatments, as well as in the reduction of M amplitude throughout the treatment. F-Wii parameters were less protected by EGF than SP. The results show a protective effect of EGF in acrylamide-induced neuropathy and support previous studies concerning the neuroprotective action of this peptide.

[Sample size for the estimation of F-wave parameters in healthy volunteers and amyotrophic lateral sclerosis patients].

Objective: The study aimed to investigate whether sample sizes of F-wave study differed according to different nerves, different F-wave parameters, and amyotrophic lateral sclerosis(ALS) patients or healthy subjects. Methods: The F-waves in the median, ulnar, tibial, and deep peroneal nerves of 55 amyotrophic lateral sclerosis (ALS) patients and 52 healthy subjects were studied to assess the effect of sample size on the accuracy of measurements of the following F-wave parameters: F-wave minimum latency, maximum latency, mean latency, F-wave persistence, F-wave chronodispersion, mean and maximum F-wave amplitude. A hundred stimuli were used in F-wave study. The values obtained from 100 stimuli were considered "true" values and were compared with the corresponding values from smaller samples of 20, 40, 60 and 80 stimuli. F-wave parameters obtained from different sample sizes were compared between the ALS patients and the normal controls. Results: Significant differences were not detected with samples above 60 stimuli for chronodispersion in all four nerves in normal participants. Significant differences were not detected with samples above 40 stimuli for maximum F-wave amplitude in median, ulnar and tibial nerves in normal participants. When comparing ALS patients and normal controls, significant differences were detected in the maximum (median nerve, Z=-3.560, P<0.01; ulnar nerve, t=5.019, P<0.01; tibial nerve, Z=-2.475, P<0.05; peroneal nerve, Z=-2.088, P<0.05)and mean F-wave latency (median nerve, Z=-3.243, P<0.01; ulnar nerve, t=3.876, P<0.01; tibial nerve, Z=-2.206, P<0.05; peroneal nerve, Z=-2.205, P<0.05)in all four nerves, F-wave chronodispersion (Z=-3.152, P<0.01)in the ulnar nerve, F-wave persistence in the median (Z=6.139, P<0.01)and ulnar nerves(Z=5.350, P<0.01), mean F-wave amplitude in the tibial nerve(t=2.981, P<0.01), maximum F-wave amplitude in the ulnar (Z=-2.134, P<0.05)and tibial nerves (t=2.746, P<0.01)with 20 stimuli; for chronodispersion in tibial nerve (t=2.551, P<0.05)100 stimuli, for chronodispersion in peroneal nerve (Z=-2.086, P<0.05)80 stimuli, for F-wave persistence in tibial nerve (Z=2.119, P<0.05) 60 stimuli, for mean F-wave amplitude in ulnar (Z=-2.552, P<0.05)and peroneal nerve (Z=-2.228, P<0.05)40 stimuli, for maximum F-wave amplitude in peroneal nerve (t=2.693, P<0.01)60 stimuli were necessary to detect differences. Conclusions: Sample sizes of F-wave study differed according to different nerves, different F-wave parameters , and ALS patients or healthy subjects.

Effect of motor imagery on excitability of spinal neural function and its impact on the accuracy of movement-considering the point at which subjects subjectively determine the 50%MVC point.

[Purpose] This study aimed to examine the effect of motor imagery on the accuracy of motion and the excitability of spinal neural function. [Subjects and Methods] Thirty healthy volunteers (males, 15; females, 15; mean age, 20.3 ± 1.0 years) were recruited. F-waves was recorded at rest, while holding a sensor, and while using motor imagery. Next, subjects learned 50% maximum voluntary contraction. The pinch force was measured without visual feedback before and after motor imagery. F-waves were analyzed with respect to persistence and the F/M amplitude ratio. Correction time and coefficient of variation were calculated from the pinch force. [Results] Persistence and F/M amplitude ratio ware significantly higher in the holding sensor and motor imagery conditions than in the resting condition. In addition, persistence under motor imagery was significantly higher than that in the holding sensor condition. No significant differences were observed in relative values of correction time and coefficient of variation between the two pinch action conditions. The pinch force in task 2 approximated a more authentic 50%MVC than that in task 1. [Conclusion] Motor imagery increases the excitability of spinal neural function, suggesting that it also affects accurate control of muscle force.

Cavotricuspid-Dependent Atrial Flutter in Patients With Prior Atriotomy: 12-Lead ECG Interpretation and Electroanatomical Characteristics.

BACKGROUND: The aim of this study was to describe the electrocardiogram (ECG) morphology and electroanatomical characteristics of counterclockwise tricuspid annular atrial flutter (CCWTA-AFL) in the patients with prior atriotomy. METHODS AND RESULTS: This retrospective study included 34 patients with CCWTA-AFL after cardiac surgery and 20 patients with typical CCWTA-AFL without prior surgery. For patients in the postsurgical group, 19 had single-loop CCWTA-AFL and 15 had a double loop CCWTA-AFL. For single loop CCWTA-AFL the F-wave in lead I was very flat in l7 of 19 patients. The F-wave morphology in the inferior leads and V1 were found to be similar to typical AFL in 14 of 19 patients. For double loop CCWTA-AFL, F-waves were positive in lead I in 13 out of 15 patients. A long isoelectric activation was measured between F-waves in most of the patients. The presence of isoelectric segments between F-waves suggested a double loop AFL, which had a sensitivity of 86.7% and a specificity of 100%. Moreover, flat F-waves in lead I suggested a single loop AFL, which had a sensitivity of 89.5% and specificity of 86.7%. All patients in the group without prior cardiac surgery had typical ECGs features of CCWTA-AFL. None of the patients showed isoelectric segments between F-waves in the inferior leads. The amplitudes of most of the leads were higher than those for single loop CCWTA-AFL in patients with prior surgery.

Repeater F-waves are signs of motor unit pathology in polio survivors.

INTRODUCTION: The purpose of this study was to determine whether F-waves reveal electrophysiological features of anterior horn cells in polio survivors. METHODS: Forty-three polio survivors and 20 healthy controls underwent motor nerve conduction studies of the median and tibial nerves bilaterally, including sampling of F-waves elicited by 100 stimuli and the determination of motor unit number estimation (MUNE). RESULTS: A significant increase in abnormally stereotyped ("repeater") F-waves and a reduction of F-wave persistence were observed in both nerves in the polio group as compared with the control group. Repeater F-waves had a negative correlation with MUNE. CONCLUSIONS: These trends in F-wave persistence and repeater F-waves after motor unit loss are characteristic findings in polio survivors. Repeater F-waves are a sign of motor unit pathology.

Tibial nerve F-wave recordings.

INTRODUCTION: Tibial F-wave recordings are remarkable for their complexity and persistence. We postulate that the signal recorded by the E2 (reference) electrode causes this pattern. METHODS: Tibial F-wave recordings were made from the abductor hallucis (AH) muscle using the standard montage in 10 subjects. Additional far-field simultaneous F-wave recordings were made from the AH, the base of the large toe, and the base of the small toe with the E2 placed on the contralateral foot. RESULTS: F-wave recordings made in the standard manner and from the base of the large or small toes showed complex waveforms and similar latencies. Recordings made from the AH-contralateral foot had simple waveforms in most subjects; in 2 subjects the latencies were longer, and 1 showed reduced persistence. CONCLUSIONS: The tibial F-waves are composed primarily of volume conducted recordings of the tibial-innervated foot muscles from the E2 electrode.

F-wave and motor-evoked potentials during motor imagery and observation in apraxia of Parkinson disease.

INTRODUCTION: The amplitudes of F-waves and motor-evoked potentials (MEPs) increase during imagination or active motor performance. The aim of this study was to investigate F-wave and MEP facilitation during assessment of apraxia. METHODS: Eight Parkinson disease (PD) patients with apraxia, 11 patients without apraxia, and 8 healthy volunteers were enrolled. F-waves and MEPs were recorded during 4 states (resting, imagination, observation, and active movement). RESULTS: The mean amplitude of the F-waves increased significantly during imagination and active movement as compared with at rest in healthy individuals (P = 0.028) and in the nonapraxia group (P = 0.005). PD patients with apraxia did not have similar facilitation. The mean amplitude of the MEPs also showed a similar loss of facilitation in PD with apraxia. CONCLUSIONS: Loss of facilitation during the preparation for movement is closely related to the "gold standard" clinical praxis battery. This study provides additional support and a potential electrophysiological assessment method for apraxia in PD.

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

INTRODUCTION: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

The frequency of atrial fibrillatory waves is modulated by the spatiotemporal pattern of acetylcholine release: a 3D computational study.

In atrial fibrillation (AF), the ECG P-wave, which represents atrial depolarization, is replaced with chaotic and irregular fibrillation waves (f waves). The f-wave frequency, F f, shows significant variations over time. Cardiorespiratory interactions regulated by the autonomic nervous system have been suggested to play a role in such variations. We conducted a simulation study to test whether the spatiotemporal release pattern of the parasympathetic neurotransmitter acetylcholine (ACh) modulates the frequency of atrial reentrant circuits. Understanding parasympathetic involvement in AF may guide more effective treatment approaches and could help to design autonomic markers alternative to heart rate variability (HRV), which is not available in AF patients. 2D tissue and 3D whole-atria models of human atrial electrophysiology in persistent AF were built. Different ACh release percentages (8% and 30%) and spatial ACh release patterns, including spatially random release and release from ganglionated plexi (GPs) and associated nerves, were considered. The temporal pattern of ACh release, ACh(t), was simulated following a sinusoidal waveform of frequency 0.125 Hz to represent the respiratory frequency. Different mean concentrations (ACh¯) and peak-to-peak ranges of ACh (ΔACh) were tested. We found that temporal variations in F f, F f(t), followed the simulated temporal ACh(t) pattern in all cases. The temporal mean of F f(t), F¯f, depended on the fibrillatory pattern (number and location of rotors), the percentage of ACh release nodes and ACh¯. The magnitude of F f(t) modulation, ΔF f, depended on the percentage of ACh release nodes and ΔACh. The spatial pattern of ACh release did not have an impact on F¯f and only a mild impact on ΔF f. The f-wave frequency, being indicative of vagal activity, has the potential to drive autonomic-based therapeutic actions and could replace HRV markers not quantifiable from AF patients.

Exercise following immobility increases lower motor neuron excitability: F-wave and H-reflex studies.

OBJECTIVES: The excitability of lower motor neurons can be explored non-invasively by several neurophysiological techniques, e.g., F-wave and H-reflex studies after a period of immobility and then after subsequent exercise. The aim of this study is to investigate the impact of exercise and high frequency repetitive nerve stimulation (RNS) following changes induced by 75 min of immobility. METHODS: We studied 10 healthy subjects following 75 min lower limb immobility, then randomized to RNS or cycling on different days. The neurophysiological studies of M-response, F-wave latency, F/M amplitude ratio and persistence; H-reflex threshold and latency, H/M amplitude ratio, and homosynaptic depression were performed at baseline, after immobility and immediately following the intervention, using stimulation of posterior tibial and peroneal nerves. RESULTS: After immobility F-wave latencies were delayed and homosynaptic depression at 2 Hz was increased (p < 0.025). RNS had no effect, but cycling exercise reduced H-reflex latencies (p = 0.025) and decreased homosynaptic depression at 2 Hz. DISCUSSION: Our findings suggest that both proprioceptive stimulation and supraspinal pathways modulate intraspinal physiological changes after immobility. These observations suggest that specific exercise protocols may be useful in managing patients recovering from periods of immobility.

Are nerve conduction studies altered in functional neurological disorders?

Background: Functional neurological disorders represent conditions without a readily identifiable origin or laboratory-supported diagnostic. We report a case of functional neurological disorder, presenting with muscle weakness with alterations in F-waves on the affected side. Case report: A retrospective case review of a patient seen in clinic. Electrophysiological evaluation included nerve conduction studies, including recording of F-waves in lower limbs, and needle EMG. A patchy sensory loss and unilateral muscle weakness of the left lower limb persisted nine days after a 40-year-old female patient developed bilateral lower limb weakness following a laparoscopic surgery. MRI was negative for radicular compression, myelopathy, or lumbosacral plexopathy. F-waves of the peroneal and tibial nerves on the left were absent or of reduced persistence and amplitude compared to the asymptomatic right side. Significance: The observation of unilateral alterations of F-wave parameters could be interpreted as an asymmetrical decrease of alpha motor neuron excitability on L4 - S2 segments. In the absence of peripheral nervous system dysfunction or a structural lesion, the results here suggest a central control dysfunction or point to a more complex peripheral role. Further research is necessary to determine the frequency of these findings in a larger group of patients while incorporating other late responses, such as H (Hoffman) reflex, and measures of cortical excitability.

A new waveform analysis method reflecting the diversity of F-wave Waveforms-Waveform types in healthy subjects based on the combined use of the additive averaging method and histograms.

BACKGROUND: F-waves, which are an indicator of the excitability of spinal cord anterior horn cells, are characterized by diverse waveforms. However, no analytical method has yet been development that fully reflects the diversity of such waveforms. The present study examined whether or not the change in the amplitude by the additive averaging process reflects the dispersion of the peak. NEW METHOD: The average amplitude of each waveform and the decrease in the amplitude after the additive averaging process were determined. The correlation between the decrease in the amplitude and the density of the peak was then examined. The histogram was also used to classify the type of waveform dispersion based on the characteristics of the peak latency. RESULTS: No correlation was found between the change in the amplitude and the peak density. However, the F-waves obtained from the ulnar nerve of healthy subjects were able to be classified into five types. COMPARISON WITH EXISTING METHOD: The parameters of an F-wave analysis are the rise latency, the amplitude and the persistence, and many reports have examined F-waves based on the changes in these values. The present study explored new parameters focusing on the waveform of F-waves reflecting the motor unit. CONCLUSION: The results of this study may help to establish a standard of comparison when using the F wave to evaluate spasticity due to upper motor neuron disorders.

Precision pinch force control via brain and spinal motor neuron excitability during motor imagery.

This study presents a novel approach for identifying neural substrates underlying the beneficial effects of motor imagery. For motor imagery, participants were instructed to imagine contraction of the left thenar muscle at 50 % maximal voluntary contraction (MVC). The participants then performed isometric contractions of the thumb and index finger at 50 % MVC as accurately as possible after motor imagery and without motor imagery. F-waves and oxygen-hemoglobin levels were examined with and without motor imagery relative to the resting condition. These data were analyzed using structural equation modeling. The degree of changes in the excitability of spinal motor neurons using F-waves during motor imagery may be modulated by inputs from the supplementary motor area. F-waves were analyzed with respect to persistence and the F-wave/maximum M-wave amplitude ratio. We found an association between precision pinch force control after motor imagery and spinal motor neuron excitability during motor imagery. The excitability of the supplementary motor area was not directly associated with precision pinch force control. However, spinal motor neuron excitability was adjusted by the supplementary motor area. Thus, the ability to perform precision pinch force control may be influenced by the supplementary motor area through the excitability of spinal motor neurons.