Distributed low-frequency functional electrical stimulation delays muscle fatigue compared to conventional stimulation

A systematic review on functional electrical stimulation based rehabilitation systems for upper limb post-stroke recovery

Background

Stroke is one of the most common neurological conditions that often leads to upper limb motor impairments, significantly affecting individuals' quality of life. Rehabilitation strategies are crucial in facilitating post-stroke recovery and improving functional independence. Functional Electrical Stimulation (FES) systems have emerged as promising upper limb rehabilitation tools, offering innovative neuromuscular reeducation approaches.

Objective

The main objective of this paper is to provide a comprehensive systematic review of the start-of-the-art functional electrical stimulation (FES) systems for upper limb neurorehabilitation in post-stroke therapy. More specifically, this paper aims to review different types of FES systems, their feasibility testing, or randomized control trials (RCT) studies.

Methods

The FES systems classification is based on the involvement of patient feedback within the FES control, which mainly includes "Open-Loop FES Systems" (manually controlled) and "Closed-Loop FES Systems" (brain-computer interface-BCI and electromyography-EMG controlled). Thus, valuable insights are presented into the technological advantages and effectiveness of Manual FES, EEG-FES, and EMG-FES systems.

Results and discussion

The review analyzed 25 studies and found that the use of FES-based rehabilitation systems resulted in favorable outcomes for the stroke recovery of upper limb functional movements, as measured by the FMA (Fugl-Meyer Assessment) (Manually controlled FES: mean difference = 5.6, 95% CI (3.77, 7.5), P < 0.001; BCI-controlled FES: mean difference = 5.37, 95% CI (4.2, 6.6), P < 0.001; EMG-controlled FES: mean difference = 14.14, 95% CI (11.72, 16.6), P < 0.001) and ARAT (Action Research Arm Test) (EMG-controlled FES: mean difference = 11.9, 95% CI (8.8, 14.9), P < 0.001) scores. Furthermore, the shortcomings, clinical considerations, comparison to non-FES systems, design improvements, and possible future implications are also discussed for improving stroke rehabilitation systems and advancing post-stroke recovery. Thus, summarizing the existing literature, this review paper can help researchers identify areas for further investigation. This can lead to formulating research questions and developing new studies aimed at improving FES systems and their outcomes in upper limb rehabilitation.

Co-activation of the diaphragm and external intercostal muscles through an adaptive closed-loop respiratory pacing controller

Introduction

Respiratory pacing is a promising alternative to traditional mechanical ventilation that has been shown to significantly increase the survival and quality of life after the neural control of the respiratory system has been compromised. However, current pacing approaches to achieve adequate ventilation tend to target only the diaphragm without pacing external intercostal muscles that are also activated during normal inspiration. Furthermore, the pacing paradigms do not allow for intermittent sighing, which carries an important physiological role. We hypothesized that simultaneous activation of the diaphragm and external intercostal muscles would improve the efficiency of respiratory pacing compared to diaphragm stimulation alone.

Materials and Methods

We expanded an adaptive, closed-loop diaphragm pacing paradigm we had previously developed to include external intercostal muscle activation and sigh generation. We then investigated, using a rodent model for respiratory pacing, if simultaneous activation would delay the fatigability of the diaphragm during pacing and allow induction of appropriate sigh-like behavior in spontaneously breathing un-injured anesthetized rats (n = 8) with pacing electrodes implanted bilaterally in the diaphragm and external intercostal muscles, between 2nd and 3rd intercostal spaces.

Results

With this novel pacing system, we show that fatigability of the diaphragm was lower when using combined muscle stimulation than diaphragm stimulation alone (p = 0.014) and that combined muscle stimulation was able to induce sighs with significantly higher tidal volumes compared to diaphragm stimulation alone (p = 0.014).

Conclusion

Our findings demonstrate that simultaneous activation of the inspiratory muscles could be used as a suitable strategy to delay stimulation-induced diaphragmatic fatigue and to induce a sigh-like behavior that could improve respiratory health.

Electrical stimulation for investigating and improving neuromuscular function in vivo: Historical perspective and major advances

This historical review summarizes the major advances - particularly from the last 50 years - in transcutaneous motor-level electrical stimulation, which can be used either as a tool to investigate neuromuscular function and its determinants (electrical stimulation for testing; EST) or as a therapeutic/training modality to improve neuromuscular and physical function (neuromuscular electrical stimulation; NMES). We focus on some of the most important applications of electrical stimulation in research and clinical settings, such as the investigation of acute changes, chronic adaptations and pathological alterations of neuromuscular function with EST, as well as the enhancement, preservation and restoration of muscle strength and mass with NMES treatment programs in various populations. For both EST and NMES, several major advances converge around understanding and optimizing motor unit recruitment during electrically-evoked contractions, also taking into account the influence of stimulation site (e.g., muscle belly vs nerve trunk) and type (e.g., pulse duration, frequency, and intensity). This information is equally important both in the context of mechanistic research of neuromuscular function as well as for clinicians who believe that improvements in neuromuscular function are required to provide health-related benefits to their patients.

Asynchronous axonal firing patterns evoked via continuous subthreshold kilohertz stimulation

Objective.Transcutaneous electrical stimulation of peripheral nerves is a common technique to assist or rehabilitate impaired muscle activation. However, conventional stimulation paradigms activate nerve fibers synchronously with action potentials time-locked with stimulation pulses. Such synchronous activation limits fine control of muscle force due to synchronized force twitches. Accordingly, we developed a subthreshold high-frequency stimulation waveform with the goal of activating axons asynchronously.Approach.We evaluated our waveform experimentally and through model simulations. During the experiment, we delivered continuous subthreshold pulses at frequencies of 16.67, 12.5, or 10 kHz transcutaneously to the median and ulnar nerves. We obtained high-density electromyographic (EMG) signals and fingertip forces to quantify the axonal activation patterns. We used a conventional 30 Hz stimulation waveform and the associated voluntary muscle activation for comparison. We modeled stimulation of biophysically realistic myelinated mammalian axons using a simplified volume conductor model to solve for extracellular electric potentials. We compared the firing properties under kHz and conventional 30 Hz stimulation.Main results.EMG activity evoked by kHz stimulation showed high entropy values similar to voluntary EMG activity, indicating asynchronous axon firing activity. In contrast, we observed low entropy values in EMG evoked by conventional 30 Hz stimulation. The muscle forces evoked by kHz stimulation also showed more stable force profiles across repeated trials compared with 30 Hz stimulation. Our simulation results provide direct evidence of asynchronous firing patterns across a population of axons in response to kHz frequency stimulation, while 30 Hz stimulation elicited synchronized time-locked responses across the population.Significance.We demonstrate that the continuous subthreshold high-frequency stimulation waveform can elicit asynchronous axon firing patterns, which can lead to finer control of muscle forces.

Torque and Discomfort During Electrically Evoked Muscle Contractions in Healthy Young Adults: Influence of Stimulation Current and Pulse Frequency

OBJECTIVE

To investigate (1) how current and pulse frequency of electrical stimulation (ES) as well as contraction mode (isometric, concentric, and eccentric) influence torque output and discomfort and (2) how familiarization by repeated ES sessions influences ratings of perceived discomfort.

DESIGN

An experimental study, 3 sessions.

SETTING

A university laboratory.

PARTICIPANTS

Eight healthy participants (5 men, 3 women; mean age 25.2 years; N=8).

INTERVENTIONS

Participants completed 3 trial days, each including 17 electrically evoked thigh muscle contractions. On each trial day, the first 6 contractions consisted of 2 isometric, 2 concentric, and 2 eccentric muscle contractions randomly ordered with a fixed stimulation current and pulse frequency (200 mA, 20 Hz), while the remaining 11 muscle contractions were all isometric with randomly ordered combinations of current (100-250 mA) and pulse frequency (20-100 Hz).

MAIN OUTCOME MEASURES

Torque and perceived discomfort were measured for each ES-evoked contraction.

RESULTS

Overall, the findings revealed that a higher stimulation frequency was associated with an increased torque without increased discomfort, while higher currents were associated with increases of both torque and discomfort. Contraction type did not influence level of discomfort, despite eccentric contractions eliciting higher torque compared with concentric and isometric contractions (P<.001). Finally, a significant familiarization to ES (P<.001) was observed after just 1 of 3 identical stimulation sessions.

CONCLUSIONS

The outlined data suggest that to elicit high torque levels while minimizing levels of discomfort in young subjects, eccentric muscle contractions evoked with a low stimulation current, and a high pulse frequency are preferable. Furthermore, a single familiarization session significantly lowers rating of perceived discomfort during ES.

Emergence of flexible technology in developing advanced systems for post-stroke rehabilitation: a comprehensive review

OBJECTIVE

Stroke is one of the most common neural disorders, which causes physical disabilities and motor impairments among its survivors. Several technologies have been developed for providing stroke rehabilitation and to assist the survivors in performing their daily life activities. Currently, the use of flexible technology (FT) for stroke rehabilitation systems is on a rise that allows the development of more compact and lightweight wearable systems, which stroke survivors can easily use for long-term activities.

APPROACH

For stroke applications, FT mainly includes the "flexible/stretchable electronics", "e-textile (electronic textile)" and "soft robotics". Thus, a thorough literature review has been performed to report the practical implementation of FT for post-stroke application.

MAIN RESULTS

In this review, the highlights of the advancement of FT in stroke rehabilitation systems are dealt with. Such systems mainly involve the "biosignal acquisition unit", "rehabilitation devices" and "assistive systems". In terms of biosignals acquisition, electroencephalography (EEG) and electromyography (EMG) are comprehensively described. For rehabilitation/assistive systems, the application of functional electrical stimulation (FES) and robotics units (exoskeleton, orthosis, etc.) have been explained.

SIGNIFICANCE

This is the first review article that compiles the different studies regarding flexible technology based post-stroke systems. Furthermore, the technological advantages, limitations, and possible future implications are also discussed to help improve and advance the flexible systems for the betterment of the stroke community.

A Generic Sequential Stimulation Adapter for Reducing Muscle Fatigue during Functional Electrical Stimulation

Background: Clinical applications of conventional functional electrical stimulation (FES) administered via a single electrode are limited by rapid onset neuromuscular fatigue. “Sequential” (SEQ) stimulation, involving the rotation of pulses between multiple active electrodes, has been shown to reduce fatigue compared to conventional FES. However, there has been limited adoption of SEQ in research and clinical settings. Methods: The SEQ adapter is a small, battery-powered device that transforms the output of any commercially available electrical stimulator into SEQ stimulation. We examined the output of the adaptor across a range of clinically relevant stimulation pulse parameters to verify the signal integrity preservation ability of the SEQ adapter. Pulse frequency, amplitude, and duration were varied across discrete states between 4 and 200 Hz, 10 and100 mA, and 50 and 2000 μs, respectively. Results: A total of 420 trials were conducted, with 80 stimulation pulses per trial. The SEQ adapter demonstrated excellent preservation of signal integrity, matching the pulse characteristics of the originating stimulator within 1% error. The SEQ adapter operates as expected at pulse frequencies up to 160 Hz, failing at a frequency of 200 Hz. Conclusion: The SEQ adapter represents an effective and low-cost solution to increase the utilization of SEQ in existing rehabilitation paradigms.

A Comparison Between Body Weight-Supported Treadmill Training and Conventional Over-Ground Training in Dogs With Incomplete Spinal Cord Injury

In human medicine there was no evidence registered of a significant difference in recovery between body weight-supported treadmill training (BWSTT) and conventional over-ground (COGI). There isn't any similar study in veterinary medicine. Thus, this study aimed to compare the locomotor recovery obtained in incomplete SCI (T11–L3 Hansen type I) post-surgical dogs following BWSTT or COGI protocols, describing their evolution during 7 weeks in regard to OFS classifications. At admission, dogs were blindly randomized in two groups but all were subjected to the same protocol (underwater treadmill training) for the first 2 weeks. After, they were divided in the BWSTT group (n = 10) and the COGI group (n = 10) for the next 2 weeks, where they performed different training. In both groups locomotor training was accompanied by functional electrical stimulation (FES) protocols. Results reported statistically significant differences between all OFS evaluations time-points (p < 0.001) and between the two groups (p < 0.001). In particular with focus on T1 to T3 a two-way repeated measures ANOVA was performed and similar results were obtained (p = 0.007). Functional recovery was achieved in 90% (17/19) of all dogs and 100% recovered bladder function. The BWSTT group showed 100% (10/10) recovery within a mean time of 4.6 weeks, while the COGI group had 78% (7/9) within 6.1 weeks. Therefore, BWSTT leads to a faster recovery with a better outcome in general.

Does increasing the number of channels during neuromuscular electrical stimulation reduce fatigability and produce larger contractions with less discomfort?

PURPOSE

Neuromuscular electrical stimulation (NMES) is often delivered at frequencies that recruit motor units (MUs) at unphysiologically high rates, leading to contraction fatigability. Rotating NMES pulses between multiple electrodes recruits subpopulations of MUs from each site, reducing MU firing rates and fatigability. This study was designed to determine whether rotating pulses between an increasing number of stimulation channels (cathodes) reduces contraction fatigability and increases the ability to generate torque during NMES. A secondary outcome was perceived discomfort.

METHODS

Fifteen neurologically intact volunteers completed four sessions. NMES was delivered over the quadriceps through 1 (NMES₁), 2 (NMES₂), 4 (NMES₄) or 8 (NMES₈) channels. Fatigability was assessed over 100 contractions (1-s on/1-s off) at an initial contraction amplitude that was 20% of a maximal voluntary contraction. Torque-frequency relationships were characterized over six frequencies from 20 to 120 Hz.

RESULTS

NMES₄ and NMES₈ resulted in less decline in peak torque (42 and 41%) over the 100 contractions than NMES₁ and NMES₂ (53 and 50% decline). Increasing frequency from 20 to 120 Hz increased torque by 7, 13, 21 and 24% MVC, for NMES₁, NMES₂, NMES₄ and NMES₈, respectively. Perceived discomfort was highest during NMES₈.

CONCLUSION

NMES₄ and NMES₈ reduced contraction fatigability and generated larger contractions across a range of frequencies than NMES₁ and NMES₂. NMES₈ produced the most discomfort, likely due to small electrodes and high current density. During NMES, more is not better and rotating pulses between four channels may be optimal to reduce contraction fatigability and produce larger contractions with minimal discomfort compared to conventional NMES configurations.

Joint angle based motor point tracking stimulation for surface FES: A Study on biceps brachii

Functional electrical stimulation (FES) has been an effective treatment option in clinical rehabilitation such as motor function recovery after stroke. The main limitation of FES is the lack of stimulation efficiency in motor unit recruitment compared with voluntary contractions, which may cause the early onset of muscle fatigue. The stimulation efficiency of FES can be improved by optimizing electrode positions to target the motor point (MP). However, the location of MP relative to the skin may shift with the change of muscle geometry during dynamic exercise. Hence, the purpose of this study is to maintain the stimulation efficiency of FES in dynamic exercise by switching the stimulation position to follow the shift of MP. We first measured the shift of the MP of the biceps brachii with respect to the elbow joint angle, and then conducted an experiment to compare four stimulation methods: 2-channel simultaneous stimulation (SS), 2-channel time based shifting stimulation (TSS), 2-channel joint angle based shifting stimulation (JASS), and 3-channel JASS. TSS and JASS were designed as two different MP tracking strategies. The experimental results show that the 3-channel JASS caused the smallest decrease in the maximal elbow angle and the angular velocity. The results also suggest that MP tracking stimulation based on joint angle is effective for the sustainable induction of muscle contraction. Both tracking selectivity and tracking density were shown to be important to improve the stimulation efficiency of FES.

Standardizing fatigue-resistance testing during electrical stimulation of paralysed human quadriceps muscles, a practical approach

Background

Rapid onset of muscular fatigue is still one of the main issues of functional electrical stimulation (FES). A promising technique, known as distributed stimulation, aims to activate sub-units of a muscle at a lower stimulation frequency to increase fatigue-resistance. Besides a general agreement on the beneficial effects, the great heterogeneity of evaluation techniques, raises the demand for a standardized method to better reflect the requirements of a practical application.

Methods

This study investigated the fatigue-development of 6 paralysed quadriceps muscles over the course of 180 dynamic contractions, evaluating different electrode-configurations (conventional and distributed stimulation). For a standardized comparison, fatigue-testing was performed at 40% of the peak-torque during a maximal evoked contraction (MEC). Further, we assessed the isometric torque for each electrode-configuration at different knee-extension-angles (70°–170°, 10° steps).

Results

Our results showed no significant difference in the fatigue-index for any of the tested electrode-configurations, compared to conventional-stimulation. We conjecture that the positive effects of distributed stimulation become less pronounced at higher stimulation amplitudes. The isometric torque produced at different knee-extension angles was similar for most electrode-configurations. Maximal torque-production was found at 130°–140° knee-extension-angle, which correlates with the maximal knee-flexion-angles during running.

Conclusion

In most practical applications, FES is intended to initiate dynamic movements. Therefore, it is crucial to assess fatigue-resistance by using dynamic contractions. Reporting the relationship between produced torque and knee-extension-angle can help to observe the stability of a chosen electrode-configuration for a targeted range-of-motion. Additionally, we suggest to perform fatigue testing at higher forces (e.g. 40% of the maximal evoked torque) in pre-trained subjects with SCI to better reflect the practical demands of FES-applications.

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.

Contraction fatigability during interleaved neuromuscular electrical stimulation of the ankle dorsiflexors does not depend on contraction amplitude

Interleaved neuromuscular electrical stimulation (iNMES) involves alternating stimulus pulses between the tibialis anterior muscle and common peroneal nerve. The current investigation aimed to characterize the relationship between contraction amplitude, motor unit (MU) “overlap”, and contraction fatigability during iNMES. It was hypothesized that as iNMES generates progressively larger contractions, more MUs would be recruited from both sites (i.e., more MU overlap), resulting in more fatigability for larger than smaller contractions. Fourteen participants completed 3 sessions. Fatigability was assessed as the decline in torque over 180 contractions (0.3 s “on”, 0.7 s “off”) when iNMES was delivered to produce initial contractions of ∼5%, 15%, or 30% of a maximal voluntary contraction. Although MU overlap increased significantly with contraction amplitude, the relative (percent) decline in torque was not different between the contraction amplitudes and torque declined on average by 23%. Contraction fatigability was not significantly correlated with either MU overlap or initial contraction amplitude. In conclusion, iNMES can produce fatigue-resistant contractions across a functionally-meaningful range of contraction amplitudes for rehabilitation.

Novelty Interleaved neuromuscular electrical stimulation progressively recruits MUs as contraction amplitude increases. However, the relative amount of fatigability of recruited MUs was not different as contraction amplitude increased. This suggests iNMES can be used effectively to produce fatigue-resistant and functionally meaningful contractions.

Interleaved intramuscular stimulation with minimally overlapping electrodes evokes smooth and fatigue resistant forces

OBJECTIVE

It is known that multi-site interleaved stimulation generates less muscle fatigue compared to single-site synchronous stimulation. However, in the limited number of studies in which intramuscular electrodes were used, the fatigue reduction associated with interleaved stimulation could not consistently be achieved. We hypothesize that this could be due to the inability to place the intramuscular electrodes used in interleaved stimulation in locations that minimize overlap amongst the motor units activated by the electrodes. Our objective in the present study was to use independent intramuscular electrodes to compare fatigue induced by interleaved stimulation with that generated by synchronous stimulation at the same initial force and ripple.

APPROACH

In the medial gastrocnemius muscle of an anesthetized rabbit (n = 3), ten intramuscular hook wire electrodes were inserted at different distances from the nerve entry. Overlap was measured using the refractory technique and only three electrodes were found to be highly independent. After ensuring that forces obtained by both stimulation modalities had the same ripple and magnitude, fatigue induced during interleaved stimulation across three independent distal electrodes was compared to that obtained by synchronously delivering pulses to a single proximal electrode.

MAIN RESULTS

Contractions evoked by interleaved stimulation exhibited less fatigue than those evoked by synchronous stimulation. Twitch force recruitment curves collected from each of the ten intramuscular electrodes showed frequent intermediate plateaus and the force value at these plateaus decreased as the distance between the electrode and nerve entry increased.

SIGNIFICANCE

The results indicate that interleaved intramuscular stimulation is preferred over synchronous intramuscular stimulation when fatigue-resistant and smooth forces are desired. In addition, the results suggest that the large muscle compartments innervated by the primary intramuscular nerve branches give rise to progressively smaller independent compartments in subsequent nerve divisions.

Comparing Spatially Distributed and Single Electrode Stimulation on Individuals with Spinal Cord Injury

It is still a challenge to delay the onset of fatigue on muscle contraction induced by Functional Electrical Stimulation (FES). We explored the use of two stimulation methods with the same total area, single electrode stimulation (SES), and spatially distributed electrical stimulation (SDSS) during isometric knee extension with spinal cord injured (SCI) volunteers. We applied stimulation on the left and right quadriceps of two SCI participants with both methods and recorded isometric force and evoked electromyography (eEMG). We calculated the force-time integral (FTI) and eEMG-time integral (eTI) for each stimulation series and used a linear regression as a measure of decay ratio. Moreover, we also estimated the contribution from each channel from eEMG.

Advances in neuroprosthetic management of foot drop: a review

This paper reviews the technological advances and clinical results obtained in the neuroprosthetic management of foot drop. Functional electrical stimulation has been widely applied owing to its corrective abilities in patients suffering from a stroke, multiple sclerosis, or spinal cord injury among other pathologies. This review aims at identifying the progress made in this area over the last two decades, addressing two main questions: What is the status of neuroprosthetic technology in terms of architecture, sensorization, and control algorithms?. What is the current evidence on its functional and clinical efficacy? The results reveal the importance of systems capable of self-adjustment and the need for closed-loop control systems to adequately modulate assistance in individual conditions. Other advanced strategies, such as combining variable and constant frequency pulses, could also play an important role in reducing fatigue and obtaining better therapeutic results. The field not only would benefit from a deeper understanding of the kinematic, kinetic and neuromuscular implications and effects of more promising assistance strategies, but also there is a clear lack of long-term clinical studies addressing the therapeutic potential of these systems. This review paper provides an overview of current system design and control architectures choices with regard to their clinical effectiveness. Shortcomings and recommendations for future directions are identified.

Distributed stimulation increases force elicited with functional electrical stimulation

OBJECTIVE

The maximum muscle forces that can be evoked using functional electrical stimulation (FES) are relatively modest. The reason for this weakness is not fully understood but could be partly related to the widespread distribution of motor nerve branches within muscle. As such, a single stimulating electrode (as is conventionally used) may be incapable of activating the entire array of motor axons supplying a muscle. Therefore, the objective of this study was to determine whether stimulating a muscle with more than one source of current could boost force above that achievable with a single source.

APPROACH

We compared the maximum isometric forces that could be evoked in the anterior deltoid of anesthetized monkeys using one or two intramuscular electrodes. We also evaluated whether temporally interleaved stimulation between two electrodes might reduce fatigue during prolonged activity compared to synchronized stimulation through two electrodes.

MAIN RESULTS

We found that dual electrode stimulation consistently produced greater force (~50% greater on average) than maximal stimulation with single electrodes. No differences, however, were found in the fatigue responses using interleaved versus synchronized stimulation.

SIGNIFICANCE

It seems reasonable to consider using multi-electrode stimulation to augment the force-generating capacity of muscles and thereby increase the utility of FES systems.

A foot drop compensation device based on surface multi-field functional electrical stimulation-Usability study in a clinical environment

Introduction

Functional electrical stimulation applies electrical pulses to the peripheral nerves to artificially achieve a sensory/motor function. When applied for the compensation of foot drop it provides both assistive and therapeutic effects. Multi-field electrodes have shown great potential but may increase the complexity of these systems. Usability aspects should be checked to ensure their success in clinical environments.

Methods

We developed the Fesia Walk device, based on a surface multi-field electrode and an automatic calibration algorithm, and carried out a usability study to check the feasibility of integrating this device in therapeutic programs in clinical environments. The study included 4 therapists and 10 acquired brain injury subjects (8 stroke and 2 traumatic brain injury).

Results

Therapists and users were “very satisfied” with the device according to the Quebec User Evaluation of Satisfaction with Assistive Technology scale, with average scores of 4.1 and 4.2 out of 5, respectively. Therapists considered the Fesia Walk device as “excellent” according to the System Usability Scale with an average score of 85.6 out of 100.

Conclusions

This study showed us that it is feasible to include surface multi-field technology while keeping a device simple and intuitive for successful integration in common neurorehabilitation programs.

Fatigue and Discomfort During Spatially Distributed Sequential Stimulation of Tibialis Anterior

Neuromuscular electrical stimulation is conventionally applied through a single pair of electrodes over the muscle belly, denominated single electrode stimulation (SES). SES is limited by discomfort and incomplete motor-unit recruitment, restricting electrically-evoked torque and promoting premature fatigue-induced torque-decline. Sequential stimulation involving rotation of pulses between multiple pairs of electrodes has been proposed as an alternative, denominated spatially distributed sequential stimulation (SDSS). The present aim was to compare discomfort, maximal-tolerated torque, and fatigue-related outcomes between SES and SDSS of tibialis anterior. Ten healthy participants completed two experimental sessions. The self-reported discomfort at sub-maximal torque, the maximal-tolerated torque, fatigue-induced torque-decline during, and doublet-twitch torque at 10- and 100-Hz before and after, 300 intermittent (0.6-s-ON-0.6-s-OFF) isokinetic contractions were compared between SES and SDSS. SDSS stimulation improved fatigue-related outcomes, whereas increased discomfort and reduced maximal-tolerated torque. SDSS holds promise for reducing fatigue. However, limited torque production and associated discomfort may limit its utility for rehabilitation/training.

Investigation of different stimulation patterns with doublet pulses to reduce muscle fatigue

Introduction: Functional electrical stimulation is a common technique used in the rehabilitation of individuals with a spinal cord injury to produce functional movement of paralysed muscles. However, it is often associated with rapid muscle fatigue which limits its applications. Methods: The objective of this study is to investigate the effects on the onset of fatigue of different multi-electrode patterns of stimulation via multiple pairs of electrodes using doublet pulses: Synchronous stimulation is compared to asynchronous stimulation patterns which are activated sequentially (AsynS) or randomly (AsynR), mimicking voluntary muscle activation by targeting different motor units. We investigated these three different approaches by applying stimulation to the gastrocnemius muscle repeatedly for 10 min (300 ms stimulation followed by 700 ms of no-stimulation) with 40 Hz effective frequency for all protocols and doublet pulses with an inter-pulse-interval of 6 ms. Eleven able-bodied volunteers (28 ± 3 years old) participated in this study. Ultrasound videos were recorded during stimulation to allow evaluation of changes in muscle morphology. The main fatigue indicators we focused on were the normalised fatigue index, fatigue time interval and pre-post twitch–tetanus ratio. Results: The results demonstrate that asynchronous stimulation with doublet pulses gives a higher normalised fatigue index (0.80 ± 0.08 and 0.87 ± 0.08) for AsynS and AsynR, respectively, than synchronous stimulation (0.62 ± 0.06). Furthermore, a longer fatigue time interval for AsynS (302.2 ± 230.9 s) and AsynR (384.4 ± 279.0 s) compared to synchronous stimulation (68.0 ± 30.5 s) indicates that fatigue occurs later during asynchronous stimulation; however, this was only found to be statistically significant for one of two methods used to calculate the group mean. Although no significant difference was found in pre-post twitch–tetanus ratio, there was a trend towards these effects. Conclusion: In this study, we proposed an asynchronous stimulation pattern for the application of functional electrical stimulation and investigated its suitability for reducing muscle fatigue compared to previous methods. The results show that asynchronous multi-electrode stimulation patterns with doublet pulses may improve fatigue resistance in functional electrical stimulation applications in some conditions.

Non-invasive, Brain-controlled Functional Electrical Stimulation for Locomotion Rehabilitation in Individuals with Paraplegia

Spinal cord injury (SCI) impairs the flow of sensory and motor signals between the brain and the areas of the body located below the lesion level. Here, we describe a neurorehabilitation setup combining several approaches that were shown to have a positive effect in patients with SCI: gait training by means of non-invasive, surface functional electrical stimulation (sFES) of the lower-limbs, proprioceptive and tactile feedback, balance control through overground walking and cue-based decoding of cortical motor commands using a brain-machine interface (BMI). The central component of this new approach was the development of a novel muscle stimulation paradigm for step generation using 16 sFES channels taking all sub-phases of physiological gait into account. We also developed a new BMI protocol to identify left and right leg motor imagery that was used to trigger an sFES-generated step movement. Our system was tested and validated with two patients with chronic paraplegia. These patients were able to walk safely with 65–70% body weight support, accumulating a total of 4,580 steps with this setup. We observed cardiovascular improvements and less dependency on walking assistance, but also partial neurological recovery in both patients, with substantial rates of motor improvement for one of them.

Stimulation of paralysed quadriceps muscles with sequentially and spatially distributed electrodes during dynamic knee extension

BACKGROUND

During functional electrical stimulation (FES) tasks with able-bodied (AB) participants, spatially distributed sequential stimulation (SDSS) has demonstrated substantial improvements in power output and fatigue properties compared to conventional single electrode stimulation (SES). The aim of this study was to compare the properties of SDSS and SES in participants with spinal cord injury (SCI) in a dynamic isokinetic knee extension task simulating knee movement during recumbent cycling.

METHOD

Using a case-series design, m. vastus lateralis and medialis of four participants with motor and sensory complete SCI (AIS A) were stimulated for 6 min on both legs with both electrode setups. With SES, target muscles were stimulated by a pair of electrodes. In SDSS, the distal electrodes were replaced by four small electrodes giving the same overall stimulation frequency and having the same total surface area. Torque was measured during knee extension by a dynamometer at an angular velocity of 110 deg/s. Mean power of the left and right sides (P_meanL,R) was calculated from all stimulated extensions for initial, final and all extensions. Fatigue is presented as an index value with respect to initial power from 1 to 0, whereby 1 means no fatigue.

RESULTS

SDSS showed higher P_meanL,R values for all four participants for all extensions (increases of 132% in participant P1, 100% in P2, 36% in P3 and 18% in P4 compared to SES) and for the initial phase (increases of 84%, 59%, 66%, and 16%, respectively). Fatigue resistance was better with SDSS for P1, P2 and P4 but worse for P3 (0.47 vs 0.35, 0.63 vs 0.49, 0.90 vs 0.82 and 0.59 vs 0.77, respectively).

CONCLUSION

Consistently higher P_meanL,R was observed for all four participants for initial and overall contractions using SDSS. This supports findings from previous studies with AB participants. Fatigue properties were better in three of the four participants. The lower fatigue resistance with SDSS in one participant may be explained by a very low muscle activation level in this case. Further investigation in a larger cohort is warranted.

Muscle Fatigue Post-stroke Elicited From Kilohertz-Frequency Subthreshold Nerve Stimulation

Purpose: Rapid muscle fatigue limits clinical applications of functional electrical stimulation (FES) for individuals with motor impairments. This study aimed to characterize the sustainability of muscle force elicited with a novel transcutaneous nerve stimulation technique.

Method: A hemiplegic chronic stroke survivor was recruited in this case study. Clustered subthreshold pulses of 60-μs with kilohertz (12.5 kHz) carrier frequency (high-frequency mode, HF) were delivered transcutaneously to the proximal segment of the median/ulnar nerve bundles to evaluate the finger flexor muscle fatigue on both sides of the stroke survivor. Conventional nerve stimulation technique with 600-μs pulses at 30 Hz (low-frequency mode, LF) served as the control condition. Fatigue was evoked by intermittently delivering 3-s stimulation trains with 1-s resting. For fair comparison, initial contraction forces (approximately 30% of the maximal voluntary contraction) were matched between the HF and LF modes. Muscle fatigue was evaluated through elicited finger flexion forces (amplitude and fluctuation) and muscle activation patterns quantified by high-density electromyography (EMG).

Result: Compared with those from the LF stimuli, the elicited forces declined more slowly, and the force plateau was higher under the HF stimulation for both the affected and contralateral sides, resulting in a more sustainable force output at higher levels. Meanwhile, the force fluctuation under the HF stimulation increased more slowly, and, thus, was smaller after successive stimulation trains compared with the LF stimuli, indicating a less synchronized activation of muscle fibers. The efficiency of the muscle activation, measured as the force-EMG ratio, was also higher in the HF stimulation mode.

Conclusion: Our results indicated that the HF nerve stimulation technique can reduce muscle fatigue in stroke survivors by maintaining a higher efficiency of muscle activations compared with the LF stimulation. The technique can help improve the performance of neurorehabilitation methods based on electrical stimulation, and facilitate the utility of FES in clinical populations.

Mitigation of excessive fatigue associated with functional electrical stimulation

OBJECTIVE

Restoration of motor function in paralyzed limbs using functional electrical stimulation (FES) is undermined by rapid fatigue associated with artificial stimulation. Typically, single electrodes are used to activate muscles with FES. However, due to the highly distributed branching of muscle nerves, a single electrode may not be able to activate the entire array of motor axons supplying a muscle. Therefore, stimulating muscle with multiple electrodes might enable access to a larger volume of muscle and thereby reduce fatigue.

APPROACH

Accordingly, we compared the endurance times that ankle dorsiflexion could be sustained at 20% maximum voluntary force using feedback controlled stimulation (25 Hz) of human tibialis anterior (TA) using one or four percutaneous intramuscular electrodes. In addition, we measured endurance times in response to direct stimulation of the nerve supplying TA and during voluntary contraction. In all sessions involving electrical stimulation, an anesthetic nerve block proximal to the site of stimulation was used to isolate the effects of stimulation and alleviate discomfort.

MAIN RESULTS

Endurance time associated with stimuli delivered by a single intramuscular electrode (84  ±  19 s) was significantly smaller than that elicited by four intramuscular electrodes (232  ±  123 s). Moreover, endurance time in response to nerve stimulation (787  ±  201 s) was not significantly different that that produced during voluntary contraction (896  ±  272 s).

SIGNIFICANCE

Therefore, excessive fatigue associated with FES is probably due to the inability of conventional FES systems to enlist the full complement of motor axons innervating muscle and can be mitigated using multiple electrodes or nerve-based electrodes.

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.

Influence of Elbow Flexion and Stimulation Site on Neuromuscular Electrical Stimulation of the Biceps Brachii

Functional electrical stimulation (FES) can help individuals with physical disabilities by assisting limb movement; however, the change in muscle geometry associated with limb movement may affect the response to stimulation. The aim of this paper was to quantify the effects of elbow flexion and stimulation site on muscle torque production. Contraction torque about the elbow was measured in 12 healthy individuals using a custom elbow flexion testbed and a transcutaneous electrode array. Stimulation was delivered to six distinct sites along the biceps brachii over 11 elbow flexion angles. Flexion angle was found to significantly influence the optimal (i.e., torque-maximizing) stimulation site ( ), with post hoc analysis indicating a proximal shift in optimal stimulation site with increased flexion. Similarly, the biceps stimulation site was found to significantly influence the flexion angle at which peak torque occurred ( ), with post hoc analysis indicating an increase in peak-torque flexion angle as stimulation site is moved proximally up the biceps. Since maximizing muscle force per unit stimulation is a common goal in rehabilitative FES, future efforts could examine methods which compensate for the shift in optimal stimulation site during FES-induced limb movement.

Improved muscle activation using proximal nerve stimulation with subthreshold current pulses at kilohertz-frequency

OBJECTIVE

Transcutaneous electrical nerve stimulation can help individuals with neurological disorders to regain their motor function by activating muscles externally. However, conventional stimulation technique often induces near-simultaneous recruitment of muscle fibers, leading to twitch forces time-locked to the stimulation.

APPROACH

To induce less synchronized activation of finger flexor muscles, we delivered clustered narrower pulses to the proximal segment of the median and ulnar nerves at a carrier frequency of either 10 kHz (with an 80 µs pulse width) or 7.14 kHz (with a 120 µs pulse width) (high-frequency mode, HF), and different clustered pulses were delivered at a frequency of 30 or 40 Hz. Conventional stimulation with pulse frequency of 30 or 40 Hz (low-frequency mode, LF) was used for comparison. With matched elicited muscle forces between the HF and LF modes, the force variation, the high-density electromyogram (EMG) signals recorded at the finger flexor muscles and stimulation-induced-pain levels were compared.

MAIN RESULTS

The compound action potentials in the 10 kHz HF mode revealed a significant difference (i.e. a lower amplitude and area, and a wider duration) compared with the LF mode, indicating cancellations of asynchronized action potentials. A smaller fluctuation in the elicited forces in the 10 kHz mode further demonstrated the less synchronized activation of different motor units. These effects tended to be weaker in the 7.14 kHz HF condition. However, the levels of pain sensation was not reduced in the HF mode potentially due to the high charge density used in the HF mode. Our findings indicated that different nerve fibers were recruited asynchronously through summations of different numbers of subthreshold depolarizations in the HF mode.

SIGNIFICANCE

Compared with the LF mode, the HF mode stimulation was capable of activating the nerve fibers in a less synchronized way, which is more similar to the physiological activation pattern.

Functional electrical stimulation cycling strategies tested during preparation for the First Cybathlon Competition - a practical report from team ENS de Lyon

Whether it is from the patient’s or the physical therapist’s point of view, FES cycling can be considered either as a recreational activity, or an engaging rehabilitation tool. In both cases, it keeps patients with lower-limb paralysis motivated to sustain a regular physical activity. Thus, it is not surprising that it was selected as one of the six disciplines of the first Cybathlon competition held on October 8, 2016. However, many unresolved issues prevent FES cycling from being an activity practiced outdoors on a daily basis; such as, low power production, rapid muscle fatigue, precise electrode positioning, lack of systematic procedures to determine stimulation patterns, and the difficulty of transferring disabled riders from their wheelchair to the tricycle. This article documents the challenges we faced during preparation for the Cybathlon 2016 FES cycling race, and provides results obtained during different phases of the process. A particular specificity of our team was that, unlike most other teams where pilots were mainly paraplegic, both the primary and backup pilots for team ENS de Lyon are C6/C7 tetraplegics, with neither voluntary control of their abdominal muscles nor hand grip, and only partial use of their arms.

Utilizing Physiological Principles of Motor Unit Recruitment to Reduce Fatigability of Electrically-Evoked Contractions: A Narrative Review

Neuromuscular electrical stimulation (NMES) is used to produce contractions to restore movement and reduce secondary complications for individuals experiencing motor impairment. NMES is conventionally delivered through a single pair of electrodes over a muscle belly or nerve trunk using short pulse durations and frequencies between 20 and 40Hz (conventional NMES). Unfortunately, the benefits and widespread use of conventional NMES are limited by contraction fatigability, which is in large part because of the nonphysiological way that contractions are generated. This review provides a summary of approaches designed to reduce fatigability during NMES, by using physiological principles that help minimize fatigability of voluntary contractions. First, relevant principles of the recruitment and discharge of motor units (MUs) inherent to voluntary contractions and conventional NMES are introduced, and the main mechanisms of fatigability for each contraction type are briefly discussed. A variety of NMES approaches are then described that were designed to reduce fatigability by generating contractions that more closely mimic voluntary contractions. These approaches include altering stimulation parameters, to recruit MUs in their physiological order, and stimulating through multiple electrodes, to reduce MU discharge rates. Although each approach has unique advantages and disadvantages, approaches that minimize MU discharge rates hold the most promise for imminent translation into rehabilitation practice. The way that NMES is currently delivered limits its utility as a rehabilitative tool. Reducing fatigability by delivering NMES in ways that better mimic voluntary contractions holds promise for optimizing the benefits and widespread use of NMES-based programs.

Power output and fatigue properties using spatially distributed sequential stimulation in a dynamic knee extension task

Purpose

The low power output and fatigue resistance during functional electrical stimulation (FES) limits its use for functional applications. The aim of this study was to compare the power output and fatigue properties of spatially distributed sequential stimulation (SDSS) against conventional single electrode stimulation (SES) in an isokinetic knee extension task simulating knee movement during recumbent cycling.

Methods

M. vastus lateralis and m. vastus medialis of eight able-bodied subjects were stimulated for 6 min on both legs with both setups. In the SES setup, target muscles were each stimulated by a pair of electrodes. In SDSS, four small electrodes replaced the SES active electrodes, but reference electrodes were the same. Torque was measured during knee extension movement by a dynamometer at an angular velocity of 110°/s. Mean power (P_mean) was calculated from stimulated extensions for the first 10 extensions, the final 20 extensions and overall. Fatigue is presented as an index, calculated as the decrease with respect to initial power.

Results

P_mean was significantly higher for SDSS than for SES in the final phase (9.9 ± 4.0 vs. 7.4 ± 4.3 W, p = 0.035) and overall (11.5 ± 4.0 vs. 9.2 ± 4.5 W, p =  0.037). With SDSS, the reduction in P_mean was significantly smaller compared to SES (from 14.9 to 9.9 vs. 14.6 to 7.4 W, p = 0.024). The absolute mean pulse width was substantially lower with SDSS (62.5 vs. 90.0 µs).

Conclusion

Although less stimulation was applied, SDSS showed a significantly higher mean power output than SES. SDSS also had improved fatigue resistance when compared to conventional stimulation. The SDSS approach may provide substantial performance benefits for cyclical FES applications.

A decision support system for electrode shaping in multi-pad FES foot drop correction

BACKGROUND

Functional electrical stimulation (FES) can be applied as an assistive and therapeutic aid in the rehabilitation of foot drop. Transcutaneous multi-pad electrodes can increase the selectivity of stimulation; however, shaping the stimulation electrode becomes increasingly complex with an increasing number of possible stimulation sites. We described and tested a novel decision support system (DSS) to facilitate the process of multi-pad stimulation electrode shaping. The DSS is part of a system for drop foot treatment that comprises a custom-designed multi-pad electrode, an electrical stimulator, and an inertial measurement unit.

METHODS

The system was tested in ten stroke survivors (3-96 months post stroke) with foot drop over 20 daily sessions. The DSS output suggested stimulation pads and parameters based on muscle twitch responses to short stimulus trains. The DSS ranked combinations of pads and current amplitudes based on a novel measurement of the quality of the induced movement and classified them based on the movement direction (dorsiflexion, plantar flexion, eversion and inversion) of the paretic foot. The efficacy of the DSS in providing satisfactory pad-current amplitude choices for shaping the stimulation electrode was evaluated by trained clinicians. The range of paretic foot motion was used as a quality indicator for the chosen patterns.

RESULTS

The results suggest that the DSS output was highly effective in creating optimized FES patterns. The position and number of pads included showed pronounced inter-patient and inter-session variability; however, zones for inducing dorsiflexion and plantar flexion within the multi-pad electrode were clearly separated. The range of motion achieved with FES was significantly greater than the corresponding active range of motion (p < 0.05) during the first three weeks of therapy.

CONCLUSIONS

The proposed DSS in combination with a custom multi-pad electrode design covering the branches of peroneal and tibial nerves proved to be an effective tool for producing both the dorsiflexion and plantar flexion of a paretic foot. The results support the use of multi-pad electrode technology in combination with automatic electrode shaping algorithms for the rehabilitation of foot drop.

TRIAL REGISTRATION

This study was registered at the Current Controlled Trials website with ClinicalTrials.gov ID NCT02729636 on March 29, 2016.

Switched Tracking Control of the Lower Limb During Asynchronous Neuromuscular Electrical Stimulation: Theory and Experiments

Neuromuscular electrical stimulation (NMES) induces muscle contractions via electrical stimuli. NMES can be used for rehabilitation and to enable functional movements; however, a fundamental limitation is the early onset of fatigue. Asynchronous stimulation is a method that can reduce fatigue by utilizing multiple stimulation channels to segregate and switch between different sets of recruited motor units. However, switching between stimulation channels is challenging due to each channel's differing response to stimulation. To address this challenge, a switched systems analysis is used in the present work to design a controller that allows for instantaneous switching between stimulation channels. The developed controller yields semi-global exponential tracking of a desired angular trajectory for a person's knee-joint. Experiments were conducted in six able-bodied individuals. Compared to conventional stimulation, the results indicate that asynchronous stimulation with the developed controller yields longer durations of successful tracking despite different responses between the stimulation channels.

Interleaved neuromuscular electrical stimulation after spinal cord injury

INTRODUCTION

Neuromuscular electrical stimulation (NMES) over a muscle belly (mNMES) recruits superficial motor units (MUs) preferentially, whereas NMES over a nerve trunk (nNMES) recruits MUs evenly throughout the muscle. We performed tests to determine whether "interleaving" pulses between the mNMES and nNMES sites (iNMES) reduces the fatigability of contractions for people experiencing paralysis because of chronic spinal cord injury.

METHODS

Plantar flexion torque and soleus electromyography (M-waves) were recorded from 8 participants. A fatigue protocol (75 contractions; 2 s on/2 s off for 5 min) was delivered by iNMES. The results were compared with previously published data collected with mNMES and nNMES in the same 8 participants.

RESULTS

Torque declined ∼40% more during mNMES than during nNMES or iNMES. M-waves declined during mNMES but not during nNMES or iNMES.

DISCUSSION

To reduce fatigability of electrically evoked contractions of paralyzed plantar flexors, iNMES is equivalent to nNMES, and both are superior to mNMES. Muscle Nerve 56: 989-993, 2017.

Fatigue reduction during aggregated and distributed sequential stimulation

INTRODUCTION

Transcutaneous neuromuscular electrical stimulation (NMES) can generate muscle contractions for rehabilitation and exercise. However, NMES-evoked contractions are limited by fatigue when they are delivered "conventionally" (CONV) using a single active electrode. Researchers have developed "sequential" (SEQ) stimulation, involving rotation of pulses between multiple "aggregated" (AGGR-SEQ) or "distributed" (DISTR-SEQ) active electrodes, to reduce fatigue (torque-decline) by reducing motor unit discharge rates. The primary objective was to compare fatigue-related outcomes, "potentiation," "variability," and "efficiency" between CONV, AGGR-SEQ, and DISTR-SEQ stimulation of knee extensors in healthy participants.

METHODS

Torque and current were recorded during testing with fatiguing trains using each NMES type under isometric and isokinetic (180°/s) conditions.

RESULTS

Compared with CONV stimulation, SEQ techniques reduced fatigue-related outcomes, increased potentiation, did not affect variability, and reduced efficiency.

CONCLUSIONS

SEQ techniques hold promise for reducing fatigue during NMES-based rehabilitation and exercise; however, optimization is required to improve efficiency. Muscle Nerve 56: 271-281, 2017.

A flexible standalone system with integrated sensor feedback for multi-pad electrode FES of the hand

Functional electrical stimulation aims to help patients suffering from stroke or spinal cord injury to supplement lost motor function. Effective functional electrical stimulation requires precise placement of the stimulation electrode. Finding the correct placement, however, can be difficult and time consuming. Another common problem with functional electrical stimulation is early occurrence of muscle fatigue upon repetitive stimulation, limiting treatment efficiency. Both, precise electrode placement as well as the reduction of muscle fatigue can be achieved using multi-pad electrodes. Here we present a new standalone device for multi-pad functional electrical stimulation. The device is easy to use and designed to help patients recovering from stroke to train and perform opening of the hand.

Advances in selective activation of muscles for non-invasive motor neuroprostheses

Non-invasive neuroprosthetic (NP) technologies for movement compensation and rehabilitation remain with challenges for their clinical application. Two of those major challenges are selective activation of muscles and fatigue management. This review discusses how electrode arrays improve the efficiency and selectivity of functional electrical stimulation (FES) applied via transcutaneous electrodes. In this paper we review the principles and achievements during the last decade on techniques for artificial motor unit recruitment to improve the selective activation of muscles. We review the key factors affecting the outcome of muscle force production via multi-pad transcutaneous electrical stimulation and discuss how stimulation parameters can be set to optimize external activation of body segments. A detailed review of existing electrode array systems proposed by different research teams is also provided. Furthermore, a review of the targeted applications of existing electrode arrays for control of upper and lower limb NPs is provided. Eventually, last section demonstrates the potential of electrode arrays to overcome the major challenges of NPs for compensation and rehabilitation of patient-specific impairments.

Strategies for Rapid Muscle Fatigue Reduction during FES Exercise in Individuals with Spinal Cord Injury: A Systematic Review

Background

Rapid muscle fatigue during functional electrical stimulation (FES)-evoked muscle contractions in individuals with spinal cord injury (SCI) is a significant limitation to attaining health benefits of FES-exercise. Delaying the onset of muscle fatigue is often cited as an important goal linked to FES clinical efficacy. Although the basic concept of fatigue-resistance has a long history, recent advances in biomedical engineering, physiotherapy and clinical exercise science have achieved improved clinical benefits, especially for reducing muscle fatigue during FES-exercise. This review evaluated the methodological quality of strategies underlying muscle fatigue-resistance that have been used to optimize FES therapeutic approaches. The review also sought to synthesize the effectiveness of these strategies for persons with SCI in order to establish their functional impacts and clinical relevance.

Methods

Published scientific literature pertaining to the reduction of FES-induced muscle fatigue was identified through searches of the following databases: Science Direct, Medline, IEEE Xplore, SpringerLink, PubMed and Nature, from the earliest returned record until June 2015. Titles and abstracts were screened to obtain 35 studies that met the inclusion criteria for this systematic review.

Results

Following the evaluation of methodological quality (mean (SD), 50 (6) %) of the reviewed studies using the Downs and Black scale, the largest treatment effects reported to reduce muscle fatigue mainly investigated isometric contractions of limited functional and clinical relevance (n = 28). Some investigations (n = 13) lacked randomisation, while others were characterised by small sample sizes with low statistical power. Nevertheless, the clinical significance of emerging trends to improve fatigue-resistance during FES included (i) optimizing electrode positioning, (ii) fine-tuning of stimulation patterns and other FES parameters, (iii) adjustments to the mode and frequency of exercise training, and (iv) biofeedback-assisted FES-exercise to promote selective recruitment of fatigue-resistant motor units.

Conclusion

Although the need for further in-depth clinical trials (especially RCTs) was clearly warranted to establish external validity of outcomes, current evidence was sufficient to support the validity of certain techniques for rapid fatigue-reduction in order to promote FES therapy as an integral part of SCI rehabilitation. It is anticipated that this information will be valuable to clinicians and other allied health professionals administering FES as a treatment option in rehabilitation and aid the development of effective rehabilitation interventions.

Non-invasive Central and Peripheral Stimulation: New Hope for Essential Tremor?

Essential tremor (ET) is among the most frequent movement disorders. It usually manifests as a postural and kinematic tremor of the arms, but may also involve the head, voice, lower limbs, and trunk. An oscillatory network has been proposed as a neural correlate of ET, and is mainly composed of the olivocerebellar system, thalamus, and motor cortex. Since pharmacological agents have limited benefits, surgical interventions like deep brain stimulation are the last-line treatment options for the most severe cases. Non-invasive brain stimulation techniques, particularly transcranial magnetic or direct current stimulation, are used to ameliorate ET. Their non-invasiveness, along with their side effects profile, makes them an appealing treatment option. In addition, peripheral stimulation has been applied in the same perspective. Hence, the aim of the present review is to shed light on the emergent use of non-invasive central and peripheral stimulation techniques in this interesting context.

Method to Reduce Muscle Fatigue During Transcutaneous Neuromuscular Electrical Stimulation in Major Knee and Ankle Muscle Groups

BACKGROUND

A critical limitation with transcutaneous neuromuscular electrical stimulation as a rehabilitative approach is the rapid onset of muscle fatigue during repeated contractions. We have developed a method called spatially distributed sequential stimulation (SDSS) to reduce muscle fatigue by distributing the center of electrical field over a wide area within a single stimulation site, using an array of surface electrodes.

OBJECTIVE

To extend the previous findings and to prove feasibility of the method by exploring the fatigue-reducing ability of SDSS for lower limb muscle groups in the able-bodied population, as well as in individuals with spinal cord injury (SCI).

METHODS

SDSS was delivered through 4 active electrodes applied to the knee extensors and flexors, plantarflexors, and dorsiflexors, sending a stimulation pulse to each electrode one after another with 90° phase shift between successive electrodes. Isometric ankle torque was measured during fatiguing stimulations using SDSS and conventional single active electrode stimulation lasting 2 minutes.

RESULTS

We demonstrated greater fatigue-reducing ability of SDSS compared with the conventional protocol, as revealed by larger values of fatigue index and/or torque peak mean in all muscles except knee flexors of able-bodied individuals, and in all muscles tested in individuals with SCI.

CONCLUSIONS

Our study has revealed improvements in fatigue tolerance during transcutaneous neuromuscular electrical stimulation using SDSS, a stimulation strategy that alternates activation of subcompartments of muscles. The SDSS protocol can provide greater stimulation times with less decrement in mechanical output compared with the conventional protocol.

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

Neuromuscular electrical stimulation (NMES) is commonly used in rehabilitation, but electrically evoked muscle activation is in several ways different from voluntary muscle contractions. These differences lead to challenges in the use of NMES for restoring muscle function. We investigated the use of low-current, high-frequency nerve stimulation to activate the muscle via the spinal motoneuron (MN) pool to achieve more natural activation patterns. Using a novel stimulation protocol, the H-reflex responses to individual stimuli in a train of stimulation pulses at 100 Hz were reliably estimated with surface EMG during low-level contractions. Furthermore, single motor unit recruitment by afferent stimulation was analyzed with intramuscular EMG. The results showed that substantially elevated H-reflex responses were obtained during 100-Hz stimulation with respect to a lower stimulation frequency. Furthermore, motor unit recruitment using 100-Hz stimulation was not fully synchronized, as it occurs in classic NMES, and the discharge rates differed among motor units because each unit was activated only after a specific number of stimuli. The most likely mechanism behind these observations is the temporal summation of subthreshold excitatory postsynaptic potentials from Ia fibers to the MNs. These findings and their interpretation were also verified by a realistic simulation model of afferent stimulation of a MN population. These results suggest that the proposed stimulation strategy may allow generation of considerable levels of muscle activation by motor unit recruitment that resembles the physiological conditions.