Physiological recruitment of motor units by high-frequency electrical stimulation of afferent pathways

Motoneuron persistent inward current contribution to increased torque responses to wide-pulse high-frequency neuromuscular electrical stimulation

PURPOSE

To assess the effect of a remote handgrip contraction during wide-pulse high-frequency (WPHF) neuromuscular electrical stimulation (NMES) on the magnitude of extra torque, progressive increase in torque during stimulation, and estimates of the persistent inward current (PIC) contribution to motoneuron firing in the plantar flexors.

METHODS

Ten participants performed triangular shaped contractions to 20% of maximal plantar flexion torque before and after WPHF NMES with and without a handgrip contraction, and control conditions. Extra torque, the relative difference between the initial and final torque during stimulation, and sustained electromyographic (EMG) activity were assessed. High-density EMG was recorded during triangular shaped contractions to calculate ∆F, an estimate of PIC contribution to motoneuron firing, and its variation before vs after the intervention referred to as ∆F change score.

RESULTS

While extra torque was not significantly increased with remote contraction (WPHF + remote) vs WPHF (+ 37 ± 63%, p = 0.112), sustained EMG activity was higher in this condition than WPHF (+ 3.9 ± 4.3% MVC EMG, p = 0.017). Moreover, ∆F was greater (+ 0.35 ± 0.30 Hz) with WPHF + remote than control (+ 0.03 ± 0.1 Hz, p = 0.028). A positive correlation was found between ∆F change score and extra torque in the WPHF + remote (r = 0.862, p = 0.006).

DISCUSSION

The findings suggest that the addition of remote muscle contraction to WPHF NMES enhances the central contribution to torque production, which may be related to an increased PIC contribution to motoneuron firing. Gaining a better understanding of these mechanisms should enable NMES intervention optimization in clinical and rehabilitation settings, improving neuromuscular function in clinical populations.

Relief of post-stroke spasticity with acute vibrotactile stimulation: controlled crossover study of muscle and skin stimulus methods

Background

Prior work suggests that vibratory stimulation can reduce spasticity and hypertonia. It is unknown which of three predominant approaches (stimulation of the spastic muscle, antagonist muscle, or cutaneous regions) most reduces these symptoms.

Objective

Determine which vibrotactile stimulation approach is most effective at reducing spastic hypertonia among post-stroke patients.

Methods

Sham-controlled crossover study with random assignment of condition order in fourteen patients with post-stroke hand spasticity. All patients were studied in four conditions over four visits: three stimulation conditions and a sham control. The primary outcome measure was the Modified Ashworth Scale, and the secondary outcome measure was the Modified Tardieu Scale measured manually and using 3D motion capture. For each condition, measures of spastic hypertonia were taken at four time points: baseline, during stimulation, after stimulation was removed, and after a gripping exercise.

Results

A clinically meaningful difference in spastic hypertonia was found during and after cutaneous stimulation of the hand. Modified Ashworth and Modified Tardieu scores were reduced by a median of 1.1 (SD = 0.84, p = 0.001) and 0.75 (SD = 0.65, p = 0.003), respectively, during cutaneous stimulation, and by 1.25 (SD = 0.94, p = 0.001) and 0.71 (SD = 0.67, p = 0.003), respectively, at 15 min after cutaneous stimulation. Symptom reductions with spastic muscle stimulation and antagonist muscle stimulation were non-zero but not significant. There was no change with sham stimulation.

Conclusions

Cutaneous vibrotactile stimulation of the hand provides significant reductions in spastic hypertonia, compared to muscle stimulation.

Clinical trial registration

www.ClinicalTrials.gov, identifier: NCT03814889.

Bilateral high-frequency noninvasive peroneal nerve stimulation evokes tonic leg muscle activation for sleep-compatible reduction of restless legs syndrome symptoms

STUDY OBJECTIVES

Restless legs syndrome (RLS) is a prevalent sleep disorder with limited treatment options. Bilateral high-frequency noninvasive peroneal nerve stimulation (NPNS) reduces RLS symptoms. Here, we sought to characterize the mechanism of action for NPNS and identify predictors of treatment response. We hypothesized that, similar to voluntary leg movements, NPNS reduces RLS symptoms by activating leg muscles.

METHODS

For 20 adults with moderate-severe RLS, we tested this hypothesis by recording surface electromyography (EMG) from the tibialis anterior leg muscle while administering NPNS at varying amplitudes to determine the minimum NPNS amplitude that evoked EMG activity (motor threshold) and maximal NPNS amplitude that was not distracting (therapeutic intensity level). Afterwards, participants self-administered NPNS (at the therapeutic intensity level) and sham control for 14 days, each in randomized order. Efficacy was defined as International RLS Study Group Rating Scale (IRLS) score difference for NPNS compared with sham.

RESULTS

NPNS consistently activated leg muscles; NPNS evoked EMG activity at the therapeutic intensity level for 19 of 20 participants (mean TIL: 26.6 mA, mean MT: 18.3 mA). Evoked EMG activity was tonic (not phasic) and sustained over time. Evoked EMG activity predicted efficacy; participants with lower motor thresholds had greater IRLS improvement (r = .45, P = .046). NPNS treatment did not interfere with self-reported sleep onset (NPNS: 16% of nights; sham: 11%; P = .629) and frequently improved self-reported sleep onset (NPNS: 52% of nights; sham: 15%; P = .002).

CONCLUSIONS

These results demonstrate that NPNS reduces RLS symptoms by activating afferent pathways, thereby generating tonic and sustained leg muscle activity without interfering with sleep.

CLINICAL TRIAL REGISTRATION

Registry: ClinicalTrials.gov; Name: Noninvasive Peripheral Nerve Stimulation for Restless Legs Syndrome; URL: https://clinicaltrials.gov/ct2/show/NCT04700683; Identifier: NCT04700683.

CITATION

Charlesworth JD, Adlou B, Singh H, Buchfuhrer MJ. Bilateral high-frequency noninvasive peroneal nerve stimulation evokes tonic leg muscle activation for sleep-compatible reduction of restless legs syndrome symptoms. J Clin Sleep Med. 2023;19(7):1199-1209.

Modulation of spinal circuits following phase-dependent electrical stimulation of afferent pathways

Objective.Peripheral electrical stimulation (PES) of afferent pathways is a tool commonly used to induce neural adaptations in some neural disorders such as pathological tremor or stroke. However, the neuromodulatory effects of stimulation interventions synchronized with physiological activity (closed-loop strategies) have been scarcely researched in the upper-limb. Here, the short-term spinal effects of a 20-minute stimulation protocol where afferent pathways were stimulated with a closed-loop strategy named selective and adaptive timely stimulation (SATS) were explored in 11 healthy subjects.Approach. SATS was applied to the radial nerve in-phase (INP) or out-of-phase (OOP) with respect to the muscle activity of the extensor carpi radialis (ECR). The neural adaptations at the spinal cord level were assessed for the flexor carpi radialis (FCR) by measuring disynaptic Group I inhibition, Ia presynaptic inhibition, Ib facilitation from the H-reflex and estimation of the neural drive before, immediately after, and 30 minutes after the intervention.Main results.SATS strategy delivered electrical stimulation synchronized with the real-time muscle activity measured, with an average delay of 17 ± 8 ms. SATS-INP induced increased disynaptic Group I inhibition (77 ± 23% of baseline conditioned FCR H-reflex), while SATS-OOP elicited the opposite effect (125 ± 46% of baseline conditioned FCR H-reflex). Some of the subjects maintained the changes after 30 minutes. No other significant changes were found for the rest of measurements.Significance.These results suggest that the short-term modulatory effects of phase-dependent PES occur at specific targeted spinal pathways for the wrist muscles in healthy individuals. Importantly, timely recruitment of afferent pathways synchronized with specific muscle activity is a fundamental principle that shall be considered when tailoring PES protocols to modulate specific neural circuits. (NCT number 04501133).

Acute Effects of Neuromuscular Electrical Stimulation on Contralateral Plantar Flexor Neuromuscular Function

Contralateral facilitation, i.e., the increase in contralateral maximal voluntary strength that is observed when neuromuscular electrical stimulation (NMES) is applied to the ipsilateral homonymous muscle, has previously been reported for the knee extensors but the neurophysiological mechanisms remain to be investigated. The aim of this study was to compare plantar flexor contralateral facilitation between a submaximal voluntary contraction (~10% MVC torque) and two evoked contractions (conventional and wide-pulse high-frequency NMES) of the ipsilateral plantar flexors, with respect to a resting condition. Contralateral MVC torque and voluntary activation level were measured in 22 healthy participants while the ipsilateral plantar flexors were at rest, voluntarily contracted or stimulated for 15 s. Additional neurophysiological parameters (soleus H-reflex and V-wave amplitude and tibialis anterior coactivation level) were quantified in a subgroup of 12 participants. Conventional and wide-pulse high-frequency NMES of the ipsilateral plantar flexors did not induce any contralateral facilitation of maximal voluntary strength and activation with respect to the resting condition. Similarly, no alteration of neurophysiological parameters was observed in the different conditions. This absence of contralateral facilitation contrasts with some results previously obtained on the knee extensors but is consistent with the absence of neurophysiological changes on the contralateral soleus.

Non-invasive electrical stimulation of peripheral nerves for the management of tremor

Pathological tremor in patients with essential tremor and Parkinsons disease is typically treated using medication or neurosurgical interventions. There is a widely recognized need for new treatments that avoid the side effects of current medications and do not carry the risks of surgical interventions. Building on decades of research and engineering development, non-invasive electrical stimulation of peripheral nerves has emerged as a safe and effective strategy for reducing pathologic tremor in essential tremor. This review surveys the peripheral electrical stimulation (PES) literature and summarizes effectiveness, safety, clinical translatability, and hypothesized tremor-reduction mechanisms of various PES approaches. The review also proposes guidelines for assessing tremor in the context of evaluating new therapies that combine the strengths of clinician assessments, patient evaluations, and novel motion sensing technology. The review concludes with a summary of future directions for PES, including expanding clinical access for patients with Parkinson's disease and leveraging large, at-home datasets to learn more about tremor physiology and treatment effect that will better characterize the state of tremor management and accelerate discovery of new therapies. Growing evidence suggests that non-invasive electrical stimulation of afferent neural pathways provides a viable new option for management of pathological tremor, with one specific PES therapy cleared for prescription and home use, suggesting that PES be considered along with medication and neurosurgical interventions for treatment of tremor. This article is part of the Special Issue "Tremor" edited by Daniel D. Truong, Mark Hallett, and Aasef Shaikh.

Optimization framework for the model-based estimation of in vivo α-motoneuron properties in the intact human

The in vivo estimation of α-motoneuron (MN) properties in humans is crucial to characterize the effect that neurorehabilitation technologies may elicit over the composite neuro-musculoskeletal system. Here, we combine biophysical neuronal modelling, high-density electromyography and convolutive blind-source separation along with numerical optimization to estimate geometrical and electrophysiological properties of in vivo decoded human MNs. The proposed methodology implements multi-objective optimization to automatically tune ionic channels conductance and soma size of MN models for minimizing the error between several features of simulated and in vivo decoded MN spike trains. This approach will open new avenues for the closed-loop control of motor restorative technologies such as wearable robots and neuromodulation devices.Clinical Relevance- This work proposes a non-invasive framework for the in vivo estimation of person-specific α-motoneuron properties. This will enable predicting neuronal adaptations in response to neurorehabilitation therapies in the intact human.

CONTRACTION FATIGUE, STRENGTH ADAPTATIONS, AND DISCOMFORT DURING CONVENTIONAL VERSUS WIDE-PULSE, HIGH-FREQUENCY, NEUROMUSCULAR ELECTRICAL STIMULATION: A SYSTEMATIC REVIEW

Neuromuscular electrical stimulation (NMES) can be delivered in a conventional form (CONVNMES) and using relatively wide-pulses and high-frequencies (WPHFNMES). WPHFNMES is proposed to reduce contraction fatigability and generate larger contractions with less discomfort than CONVNMES, however, there are no systematic reviews to guide the selection of NMES types. This systematic review compared the effects of CONVNMES versus WPHFNMES on contraction fatigability, strength adaptations, and perceived discomfort in clinical and non-clinical populations. Eight studies were included. When averaged across all non-clinical participants in individual short- and long-term studies, there was either no difference between CONVNMES and WPHFNMES for all outcomes or WPHFNMES produced more fatigability. In a subset of non-clinical participants ("responders"), however, WPHFNMES reduced contraction fatigability during a single session. Long-term studies found no differences between protocols for strength adaptations in non-clinical participants and those with multiple sclerosis. We concluded that WPHFNMES reduces contraction fatigability only in the short-term and in non-clinical responder participants and may exacerbate fatigability in non-responders. This review was registered in the prospective international registry of systematic reviews/PROSPERO (Registration Number: CRD42020153907, accessed at https://www.crd.york.ac.uk/PROSPERO/). Novelty bullets: • WPHF NMES may reduce fatigue in some participants and exacerbate fatigue in others. • There were no differences in long-term between WPHF and CONV NMES on strength adaptations. • Future high-quality research is needed to optimize outcomes of NMES-based programs.

Influence of wide-pulse neuromuscular electrical stimulation frequency and superimposed tendon vibration on occurrence and magnitude of extra torque

Low-frequency and high-frequency wide-pulse neuromuscular electrical stimulation (NMES) can generate extra torque (ET) via afferent pathways. Superimposing tendon vibration (TV) to NMES can increase the activation of these afferent pathways and favor ET generation. Knowledge of the characteristics of ET is essential to implement these stimulation paradigms in clinical practice. Thus, we aimed to investigate the effects of frequency and TV superimposition on the occurrence and magnitude of ET in response to wide-pulse NMES. NMES-induced isometric plantar flexion torque was recorded in 30 healthy individuals who performed five NMES protocols: wide-pulse low-frequency (1 ms; 20 Hz; WPLF) and wide-pulse high-frequency (1 ms; 100 Hz; WPHF) without and with superimposed TV (1 mm; 100 Hz) and conventional NMES (50 µs; 20 Hz; reference protocol). Each NMES protocol consisted of three 20-s trains interspersed by 90 s of rest, with NMES intensity being adjusted to reach 10% of maximal voluntary contraction. The ET occurrence was similar for WPLF and WPHF (P = 0.822). In the responders, the ET magnitude was greater for WPHF than WPLF (P < 0.001). There was no effect of superimposed TV on ET characteristics. This study reported an effect of NMES frequency on ET magnitude, whereas TV superimposition did not affect this parameter. In the context of our experimental design decisions, the present findings question the clinical use of wide-pulse NMES and its combination with superimposed TV. Yet, further research is needed to maximize force production through the occurrence and magnitude of ET.NEW & NOTEWORTHY This study is the first to assess the effect of stimulation frequency and superimposed tendon vibration on extra torque characteristics generated by wide-pulse neuromuscular electrical stimulation. The percentage of subjects showing extra torque (i.e., considered as responders) was similar for low-frequency and high-frequency wide-pulse neuromuscular electrical stimulation. In the responders, the extra torque was greater for high-frequency than for low-frequency wide-pulse neuromuscular electrical stimulation. The superimposition of tendon vibration had no effect on extra torque occurrence or magnitude.

Intramuscular Stimulation of Muscle Afferents Attains Prolonged Tremor Reduction in Essential Tremor Patients

This study proposes and clinically tests intramuscular electrical stimulation below motor threshold to achieve prolonged reduction of wrist flexion/extension tremor in Essential Tremor (ET) patients. The developed system consisted of an intramuscular thin-film electrode structure that included both stimulation and electromyography (EMG) recording electrodes, and a control algorithm for the timing of intramuscular stimulation based on EMG (closed-loop stimulation). Data were recorded from nine ET patients with wrist flexion/extension tremor recruited from the Gregorio Marañón Hospital (Madrid, Spain). Patients participated in two experimental sessions comprising: 1) sensory stimulation of wrist flexors/extensors via thin-film multichannel intramuscular electrodes; and 2) surface stimulation of the nerves innervating the same target muscles. For each session, four of these patients underwent random 60-s trials of two stimulation strategies for each target muscle: 1) selective and adaptive timely stimulation (SATS) - based on EMG of the antagonist muscle; and 2) continuous stimulation (CON) of target muscles. Two patients underwent SATS stimulation trials alone while the other three underwent CON stimulation trials alone in each session. Kinematics of wrist, elbow, and shoulder, together with clinical scales, were used to assess tremor before, right after, and 24 h after each session. Intramuscular SATS achieved, on average, 32% acute (during stimulation) tremor reduction on each trial, while continuous stimulation augmented tremorgenic activity. Furthermore, tremor reduction was significantly higher using intramuscular than surface stimulation. Prolonged reduction of tremor amplitude (24 h after the experiment) was observed in four patients. These results showed acute and prolonged (24 h) tremor reduction using a minimally invasive neurostimulation technology based on SATS of primary sensory afferents of wrist muscles. This strategy might open the possibility of an alternative therapeutic approach for ET patients.

Modulation of torque evoked by wide-pulse, high-frequency neuromuscular electrical stimulation and the potential implications for rehabilitation and training

The effectiveness of neuromuscular electrical stimulation (NMES) for rehabilitation is proportional to the evoked torque. The progressive increase in torque (extra torque) that may develop in response to low intensity wide-pulse high-frequency (WPHF) NMES holds great promise for rehabilitation as it overcomes the main limitation of NMES, namely discomfort. WPHF NMES extra torque is thought to result from reflexively recruited motor units at the spinal level. However, whether WPHF NMES evoked force can be modulated is unknown. Therefore, we examined the effect of two interventions known to change the state of spinal circuitry in opposite ways on evoked torque and motor unit recruitment by WPHF NMES. The interventions were high-frequency transcutaneous electrical nerve stimulation (TENS) and anodal transcutaneous spinal direct current stimulation (tsDCS). We show that TENS performed before a bout of WPHF NMES results in lower evoked torque (median change in torque time-integral: - 56%) indicating that WPHF NMES-evoked torque might be modulated. In contrast, the anodal tsDCS protocol used had no effect on any measured parameter. Our results demonstrate that WPHF NMES extra torque can be modulated and although the TENS intervention blunted extra torque production, the finding that central contribution to WPHF NMES-evoked torques can be modulated opens new avenues for designing interventions to enhance WPHF NMES.

Peripheral electrical stimulation to reduce pathological tremor: a review

Interventions to reduce tremor in essential tremor (ET) and Parkinson’s disease (PD) clinical populations often utilize pharmacological or surgical therapies. However, there can be significant side effects, decline in effectiveness over time, or clinical contraindications for these interventions. Therefore, alternative approaches must be considered and developed. Some non-pharmacological strategies include assistive devices, orthoses and mechanical loading of the tremorgenic limb, while others propose peripheral electrical stimulation. Specifically, peripheral electrical stimulation encompasses strategies that activate motor and sensory pathways to evoke muscle contractions and impact sensorimotor function. Numerous studies report the efficacy of peripheral electrical stimulation to alter tremor generation, thereby opening new perspectives for both short- and long-term tremor reduction. Therefore, it is timely to explore this promising modality in a comprehensive review. In this review, we analyzed 27 studies that reported the use of peripheral electrical stimulation to reduce tremor and discuss various considerations regarding peripheral electrical stimulation: the stimulation strategies and parameters, electrodes, experimental designs, results, and mechanisms hypothesized to reduce tremor. From our review, we identified a high degree of disparity across studies with regard to stimulation patterns, experimental designs and methods of assessing tremor. Having standardized experimental methodology is a critical step in the field and is needed in order to accurately compare results across studies. With this review, we explore peripheral electrical stimulation as an intervention for tremor reduction, identify the limitations and benefits of the current state-of-the-art studies, and provide ideas to guide the development of novel approaches based on the neural circuitries and mechanical properties implied in tremor generation.

The effect of gait training with low-intensity neuromuscular electrical stimulation of hip abductor muscles in two patients following surgery for hip fracture: Two case reports

Background: The rate of force development (RFD) is an indicator of muscle strength. A previous study reported that the RFD of hip abductor muscles was increased by neuromuscular electrical stimulation (NMES) to gluteus medius (GM) during gait in healthy adults. However, the effects for patients following femoral head replacement for hip fracture are unclear.Purpose: The aim of this case report was to investigate the effects of gait training with sub-motor threshold NMES on RFD of hip abductor muscles in two patients following femoral head replacement for hip fracture compared to gait training without NMES.Case description: Two elderly patients following femoral head replacement for hip fracture received both interventions of gait training with sub-motor threshold NMES to GM and without NMES. Intervention phases involved 14 sessions each, for 28 sessions total.Outcomes: The RFD of hip abductor muscles, maximum walking speed, six-minute walk distance (6MWD), Berg Balance Scale, one-leg standing time (OLST), functional independence measure, and Numeric Pain Rating Scale (NPRS) were used as outcome measures. In both patients, RFD, 6MWD, OLST, and NPRS were improved by gait training with NMES compared to without NMES.Conclusion: Our results suggest the potential of NMES as a treatment methodology for these two patients undergoing femoral head replacement for hip fracture.

Architectural Changes in Superficial and Deep Compartments of the Tibialis Anterior During Electrical Stimulation Over Different Sites

Electrical stimulation is widely used in rehabilitation to prevent muscle weakness and to assist the functional recovery of neural deficits. Its application is however limited by the rapid development of muscle fatigue due to the non-physiological motor unit (MU) recruitment. This issue can be mitigated by interleaving muscle belly (mStim) and nerve stimulation (nStim) to distribute the temporal recruitment among different MU groups. To be effective, this approach requires the two stimulation modalities to activate minimally-overlapped groups of MUs. In this manuscript, we investigated spatial differences between mStim and nStim MU recruitment through the study of architectural changes of superficial and deep compartments of tibialis anterior (TA). We used ultrasound imaging to measure variations in muscle thickness, pennation angle, and fiber length during mStim, nStim, and voluntary (Vol) contractions at 15% and 25% of the maximal force. For both contraction levels, architectural changes induced by nStim in the deep and superficial compartments were similar to those observed during Vol. Instead, during mStim superficial fascicles underwent a greater change compared to those observed during nStim and Vol, both in absolute magnitude and in their relative differences between compartments. These observations suggest that nStim results in a distributed MU recruitment over the entire muscle volume, similarly to Vol, whereas mStim preferentially activates the superficial muscle layer. The diversity between spatial recruitment of nStim and mStim suggests the involvement of different MU populations, which justifies strategies based on interleaved nerve/muscle stimulation to reduce muscle fatigue during electrically-induced contractions of TA.

Burst-modulated wide-pulse neuromuscular electrical stimulation enhances H-reflex recruitment in rats

INTRODUCTION

In this study we investigated whether burst-modulated wide-pulse neuromuscular electrical stimulation (NMES) can improve the H-reflex and activation efficiency of sensory fibers.

METHODS

NMES-induced electromyography (EMG) was recorded from hindpaw plantar muscles in 11 anesthetized rats. A burst-modulated wide pulse (mWP) with three carrier frequencies (2 kHz, 5 kHz, and 10 kHz) and a continuous wide-pulse (WP) were delivered to the tibial nerve of each rat. The evoked Hoffman (H)-reflexes were measured to evaluate nerve activation efficiency using the H-reflex recruitment curve (HRC).

RESULTS

Relative to WP simulation, mWP stimulation required less electrical charge to excite sensory fibers and improved the H-reflex recruitment. Greater electrical charge and smaller recruitment gains were obtained with increased carrier frequency of mWP.

DISCUSSION

mWP NMES can improve stimulation efficiency and improve recruitment of sensory fibers on tibial nerve stimulation, which may help to optimize NMES stimulus parameters.

The relative strength of common synaptic input to motor neurons is not a determinant of the maximal rate of force development in humans

Correlation between motor unit discharge times, often referred to as motor unit synchronization, is determined by common synaptic input to motor neurons. Although it has been largely speculated that synchronization should influence the rate of force development, the association between the degree of motor unit synchronization and rapid force generation has not been determined. In this study, we examined this association with both simulations and experimental motor unit recordings. The analysis of experimental motor unit discharges from the tibialis anterior muscle of 20 healthy individuals during rapid isometric contractions revealed that the average motor unit discharge rate was associated with the rate of force development. Moreover, the extent of motor unit synchronization was entirely determined by the average motor unit discharge rate ( R > 0.7, P < 0.0001). The simulation model demonstrated that the relative proportion of common synaptic input received by motor neurons, which determines motor unit synchronization, does not influence the rate of force development ( R = 0.03, P > 0.05). Nonetheless, the estimates of correlation between motor unit spike trains were significantly correlated with the rate of force generation ( R > 0.8, P < 0.0001). These results indicate that the average motor unit discharge rate, but not the degree of motor unit synchronization, contributes to most of the variance of human contractile speed among individuals. In addition, estimates of correlation between motor unit discharge times depend strongly on the number of identified motor units and therefore are not indicative of the strength of common input.

NEW & NOTEWORTHY It is commonly assumed that motor unit synchronization has an impact on the rate of force development of a muscle. Here we present computer simulations and experimental data of human tibialis anterior motor units during rapid contractions that show that motor unit synchronization is not a determinant of the rate of force production. This conclusion clarifies the neural determinants of rapid force generation.

Delayed fatigue in finger flexion forces through transcutaneous nerve stimulation

OBJECTIVE

Weakness of the hand is a major impairment which limits independent living. Neuromuscular electrical stimulation (NMES) is a common approach to help restore muscle strength. Traditional NMES directly over the muscle often leads to a rapid onset of muscle fatigue. In this study, we investigated the force sustainability of finger flexor muscles using a transcutaneous nerve stimulation approach.

APPROACH

Finger flexion forces and high-density electromyogram (HD EMG) signals were obtained while electrical stimulation was applied to the ulnar and median nerve bundles through a stimulation grid on the proximal arm segment. Stimulation was also applied to the finger flexor muscle belly targeting the motor point, serving as a control condition. The force produced from the two stimulation approaches were initially matched, and muscle fatigue was subsequently induced with 5 min of continuous stimulation. The rate of decay of the force and EMG amplitude were quantified, and the spatial distribution of the muscle activation during the sustained contraction was also evaluated.

MAIN RESULTS

The proximal nerve stimulation approach induced a slower decay in both force and EMG, compared with the stimulation at the motor point. The spatial distribution of the elicited muscle activation showed that the proximal nerve stimulation led to a distributed activation across the intrinsic and extrinsic finger flexor muscles and also activated a wider area within the extrinsic muscle.

SIGNIFICANCE

Our findings demonstrated that the stimulation of the proximal nerve bundles can elicit sustained force output and delayed decrease in the rate of force decline. This is potentially due to a spatially distributed activation of the muscle fibers, compared with the traditional motor point stimulation. Future development of our nerve stimulation approach may enable prolonged usage during rehabilitation or assistance for better functional outcomes.

Effects of Very High Stimulation Frequency and Wide-Pulse Duration on Stimulated Force and Fatigue of Quadriceps in Healthy Participants

Objective

To determine the effect of very high stimulation frequency (150 and 200 Hz) with wide pulse duration versus 50 Hz with wide pulse duration on stimulated force and fatigue of quadriceps femoris in healthy participants.

Methods

Thirty-four healthy participants underwent fatigue test using three stimulation frequency conditions (50, 150, and 200 Hz) with pulse duration of 0.9 ms. Normalized force values at the end of each fatigue protocol and curve fitting patterns were compared among stimulated frequencies.

Results

Very high stimulation frequency (150 and 200 Hz) conditions showed a trend of having more decline in normalized stimulated force during fatigue test compared to a low stimulation frequency at 50 Hz. However, the difference was not statistically significant. Responder group showed the same slope of a linear fitting pattern, implying the same pattern of muscle fatigue among three stimulation frequency conditions (−3.32 in 50 Hz, −2.88 in 150 Hz, and −3.14 in 200 Hz, respectively).

Conclusion

There were high inter-subject variations in the response to different frequency stimulation conditions. However, very high stimulation frequency generated the same fatigue pattern as the low stimulation frequency in the responder group. Further research is needed to explore the mechanism involved.

Investigation of the Relationship Between Electrical Stimulation Frequency and Muscle Frequency Response Under Submaximal Contractions

Neuromuscular electrical stimulation (NMES) is a common tool that is used in clinical and laboratory experiments and can be combined with mechanomyography (MMG) for biofeedback in neuroprostheses. However, it is not clear if the electrical current applied to neuromuscular tissues influences the MMG signal in submaximal contractions. The objective of this study is to investigate whether the electrical stimulation frequency influences the mechanomyographic frequency response of the rectus femoris muscle during submaximal contractions. Thirteen male participants performed three maximal voluntary isometric contractions (MVIC) recorded in isometric conditions to determine the maximal force of knee extensors. This was followed by the application of nine modulated NMES frequencies (20, 25, 30, 35, 40, 45, 50, 75, and 100 Hz) to evoke 5% MVIC. Muscle behavior was monitored by the analysis of MMG signals, which were decomposed into frequency bands by using a Cauchy wavelet transform. For each applied electrical stimulus frequency, the mean MMG spectral/frequency response was estimated for each axis (X, Y, and Z axes) of the MMG sensor with the values of the frequency bands used as weights (weighted mean). Only with respect to the Z (perpendicular) axis of the MMG signal, the stimulus frequency of 20 Hz did not exhibit any difference with the weighted mean (P = 0.666). For the frequencies of 20 and 25 Hz, the MMG signal displayed the bands between 12 and 16 Hz in the three axes (P < 0.050). In the frequencies from 30 to 100 Hz, the muscle presented a higher concentration of the MMG signal between the 22 and 29 Hz bands for the X and Z axes, and between 16 and 34 Hz bands for the Y axis (P < 0.050 for all cases). We observed that MMG signals are not dependent on the applied NMES frequency, because their frequency contents tend to mainly remain between the 20- and 25-Hz bands. Hence, NMES does not interfere with the use of MMG in neuroprosthesis.

Exploration of Hand Grasp Patterns Elicitable Through Non-Invasive Proximal Nerve Stimulation

Various neurological conditions, such as stroke or spinal cord injury, result in an impaired control of the hand. One method of restoring this impairment is through functional electrical stimulation (FES). However, traditional FES techniques often lead to quick fatigue and unnatural ballistic movements. In this study, we sought to explore the capabilities of a non-invasive proximal nerve stimulation technique in eliciting various hand grasp patterns. The ulnar and median nerves proximal to the elbow joint were activated transcutanously using a programmable stimulator, and the resultant finger flexion joint angles were recorded using a motion capture system. The individual finger motions averaged across the three joints were analyzed using a cluster analysis, in order to classify the different hand grasp patterns. With low current intensity (<5 mA and 100 µs pulse width) stimulation, our results show that all of our subjects demonstrated a variety of consistent hand grasp patterns including single finger movement and coordinated multi-finger movements. This study provides initial evidence on the feasibility of a proximal nerve stimulation technique in controlling a variety of finger movements and grasp patterns. Our approach could also be developed into a rehabilitative/assistive tool that can result in flexible movements of the fingers.

Electrical Stimulation of Afferent Pathways for the Suppression of Pathological Tremor

Pathological tremors are involuntary oscillatory movements which cannot be fully attenuated using conventional treatments. For this reason, several studies have investigated the use of neuromuscular electrical stimulation for tremor suppression. In a recent study, however, we found that electrical stimulation below the motor threshold also suppressed tremor, indicating involvement of afferent pathways. In this study, we further explored this possibility by systematically investigating how tremor suppression by afferent stimulation depends on the stimulation settings. In this way, we aimed at identifying the optimal stimulation strategy, as well as to elucidate the underlying physiological mechanisms of tremor suppression. Stimulation strategies varying the stimulation intensity and pulse timing were tested in nine tremor patients using either intramuscular or surface stimulation. Significant tremor suppression was observed in six patients (tremor suppression > 75% was observed in three patients) and the average optimal suppression level observed across all subjects was 52%. The efficiency for each stimulation setting, however, varied substantially across patients and it was not possible to identify a single set of stimulation parameters that yielded positive results in all patients. For example, tremor suppression was achieved both with stimulation delivered in an out-of-phase pattern with respect to the tremor, and with random timing of the stimulation. Overall, these results indicate that low-current stimulation of afferent fibers is a promising approach for tremor suppression, but that further research is required to identify how the effect can be maximized in the individual patient.

Relation Between the Frequency of Short-Pulse Electrical Stimulation of Afferent Nerve Fibers and Evoked Muscle Force

Objective: Functional electrical stimulation (FES) is conventionally performed by the stimulation of motor axons causing the muscle fibers innervated by these axons to contract. An alternative strategy that may evoke contractions with more natural motor unit behavior is to stimulate afferent fibers (primarily type Ia) to excite the motor neurons at the spinal level. The aim of the study was to investigate the range of forces that can be evoked in this way and the degree to which the torque can be controlled. Methods: We stimulated the tibial nerve of ten healthy participants at amplitudes at which the highest H-reflex with minimal M-wave was present. The evoked plantar flexion torque was recorded following short stimulation pulses (0.4 ms) with frequencies ranging from 20 to 200 Hz. Results: Across all subjects, the median highest evocable torque was 38.3% (quartiles: 16.9-51.0) of the maximum voluntary contraction torque (MVC). The average torque variability (standard deviation) was 1.7 +/- 0.7% MVC. For most subjects, the relation between stimulation frequency and evoked torque was well characterized by sigmoidal curves (median root mean square error: 6.4% MVC). The plateau of this sigmoid curve (indicating the range of frequencies over which torque amplitude could be modulated) was reached at 56.0 (quartiles: 29.4-81.9) Hz. Conclusion: Using the proposed method for FES, substantial evoked torques that could be controlled by stimulation frequency were achieved. Significance: Stimulation of afferent fibers could be a useful and fatigue-resistant strategy for several applications of FES.Objective: Functional electrical stimulation (FES) is conventionally performed by the stimulation of motor axons causing the muscle fibers innervated by these axons to contract. An alternative strategy that may evoke contractions with more natural motor unit behavior is to stimulate afferent fibers (primarily type Ia) to excite the motor neurons at the spinal level. The aim of the study was to investigate the range of forces that can be evoked in this way and the degree to which the torque can be controlled. Methods: We stimulated the tibial nerve of ten healthy participants at amplitudes at which the highest H-reflex with minimal M-wave was present. The evoked plantar flexion torque was recorded following short stimulation pulses (0.4 ms) with frequencies ranging from 20 to 200 Hz. Results: Across all subjects, the median highest evocable torque was 38.3% (quartiles: 16.9-51.0) of the maximum voluntary contraction torque (MVC). The average torque variability (standard deviation) was 1.7 +/- 0.7% MVC. For most subjects, the relation between stimulation frequency and evoked torque was well characterized by sigmoidal curves (median root mean square error: 6.4% MVC). The plateau of this sigmoid curve (indicating the range of frequencies over which torque amplitude could be modulated) was reached at 56.0 (quartiles: 29.4-81.9) Hz. Conclusion: Using the proposed method for FES, substantial evoked torques that could be controlled by stimulation frequency were achieved. Significance: Stimulation of afferent fibers could be a useful and fatigue-resistant strategy for several applications of FES.

The human motor neuron pools receive a dominant slow-varying common synaptic input

KEY POINTS

Motor neurons in a pool receive both common and independent synaptic inputs, although the proportion and role of their common synaptic input is debated. Classic correlation techniques between motor unit spike trains do not measure the absolute proportion of common input and have limitations as a result of the non-linearity of motor neurons. We propose a method that for the first time allows an accurate quantification of the absolute proportion of low frequency common synaptic input (<5 Hz) to motor neurons in humans. We applied the proposed method to three human muscles and determined experimentally that they receive a similar large amount (>60%) of common input, irrespective of their different functional and control properties. These results increase our knowledge about the role of common and independent input to motor neurons in force control.

ABSTRACT

Motor neurons receive both common and independent synaptic inputs. This observation is classically based on the presence of a significant correlation between pairs of motor unit spike trains. The functional significance of different relative proportions of common input across muscles, individuals and conditions is still debated. One of the limitations in our understanding of correlated input to motor neurons is that it has not been possible so far to quantify the absolute proportion of common input with respect to the total synaptic input received by the motor neurons. Indeed, correlation measures of pairs of output spike trains only allow for relative comparisons. In the present study, we report for the first time an approach for measuring the proportion of common input in the low frequency bandwidth (<5 Hz) to a motor neuron pool in humans. This estimate is based on a phenomenological model and the theoretical fitting of the experimental values of coherence between the permutations of groups of motor unit spike trains. We demonstrate the validity of this theoretical estimate with several simulations. Moreover, we applied this method to three human muscles: the abductor digiti minimi, tibialis anterior and vastus medialis. Despite these muscles having different functional roles and control properties, as confirmed by the results of the present study, we estimate that their motor pools receive a similar and large (>60%) proportion of common low frequency oscillations with respect to their total synaptic input. These results suggest that the central nervous system provides a large amount of common input to motor neuron pools, in a similar way to that for muscles with different functional and control properties.

Responders to Wide-Pulse, High-Frequency Neuromuscular Electrical Stimulation Show Reduced Metabolic Demand: A 31P-MRS Study in Humans

Conventional (CONV) neuromuscular electrical stimulation (NMES) (i.e., short pulse duration, low frequencies) induces a higher energetic response as compared to voluntary contractions (VOL). In contrast, wide-pulse, high-frequency (WPHF) NMES might elicit--at least in some subjects (i.e., responders)--a different motor unit recruitment compared to CONV that resembles the physiological muscle activation pattern of VOL. We therefore hypothesized that for these responder subjects, the metabolic demand of WPHF would be lower than CONV and comparable to VOL. 18 healthy subjects performed isometric plantar flexions at 10% of their maximal voluntary contraction force for CONV (25 Hz, 0.05 ms), WPHF (100 Hz, 1 ms) and VOL protocols. For each protocol, force time integral (FTI) was quantified and subjects were classified as responders and non-responders to WPHF based on k-means clustering analysis. Furthermore, a fatigue index based on FTI loss at the end of each protocol compared with the beginning of the protocol was calculated. Phosphocreatine depletion (ΔPCr) was assessed using 31P magnetic resonance spectroscopy. Responders developed four times higher FTI's during WPHF (99 ± 37 × 10(3) N.s) than non-responders (26 ± 12 × 10(3) N.s). For both responders and non-responders, CONV was metabolically more demanding than VOL when ΔPCr was expressed relative to the FTI. Only for the responder group, the ∆PCr/FTI ratio of WPHF (0.74 ± 0.19 M/N.s) was significantly lower compared to CONV (1.48 ± 0.46 M/N.s) but similar to VOL (0.65 ± 0.21 M/N.s). Moreover, the fatigue index was not different between WPHF (-16%) and CONV (-25%) for the responders. WPHF could therefore be considered as the less demanding NMES modality--at least in this subgroup of subjects--by possibly exhibiting a muscle activation pattern similar to VOL contractions.

Wide-pulse-high-frequency neuromuscular electrical stimulation in cerebral palsy

OBJECTIVE

The present study assesses whether wide-pulse-high-frequency (WPHF) neuromuscular electrical stimulation (NMES) could result in extra-force production in cerebral palsy (CP) patients as previously observed in healthy individuals.

METHODS

Ten CP and 10 age- and sex-matched control participants underwent plantar flexors NMES. Two to three 10-s WPHF (frequency: 100 Hz, pulse duration: 1 ms) and conventional (CONV, frequency 25 Hz, pulse duration: 50 μs) trains as well as two to three burst-like stimulation trains (2s at 25 Hz, 2s at 100 Hz, 2s at 25 Hz; pulse duration: 1 ms) were evoked. Resting soleus and gastrocnemii maximal H-reflex amplitude (Hmax) was normalized by maximal M-wave amplitude (Mmax) to quantify α-motoneuron modulation.

RESULTS

Similar Hmax/Mmax ratio was found in CP and control participants. Extra-force generation was observed both in CP (+18 ± 74%) and control individuals (+94 ± 124%) during WPHF (p<0.05). Similar extra-forces were found during burst-like stimulations in both groups (+108 ± 110% in CP and +65 ± 85% in controls, p>0.05).

CONCLUSION

Although the mechanisms underlying extra-force production may differ between WPHF and burst-like NMES, similar increases were observed in patients with CP and healthy controls.

SIGNIFICANCE

Development of extra-forces in response to WPHF NMES evoked at low stimulation intensity might open new possibilities in neuromuscular rehabilitation.