Stimulus artefact suppressor for EMG recording during FES by a constant-current stimulator

Low-cost stimulation resistant electromyography

Surface Electromyography (sEMG) is the non-invasive measurement of skeletal muscle contraction bio-potentials. Measuring sEMG of a stimulated muscle can prove particularly difficult due to large scale and long lasting stimulation-induced artefacts: if an sEMG device does not account for such artefacts, its measurements can be swamped and components damaged. sEMG has been used in a wide range of clinical and biomedical fields, providing measures such as muscular fatigue and subject intent. The recording of sEMG can prove difficult due to signal contamination such as movement artefact and mains interference. There are very few commercial sEMG devices that contain protection against large stimulation voltages or measures to reduce artefact transient times. Furthermore, most commercial or research level designs are not open source; these designs are effectively an inflexible black box to researchers and developers. This research presents the design, test and validation of an open source sEMG design, able to record muscle bio-potentials concurrently to electrical stimulation. The open source, low-cost nature of the design provides accessibility to researchers without the time and cost associated with design development. The design has been tested on the forearms of four able-bodied subjects during 25 Hz constant current stimulation, and has been shown to record subject volitional sEMG and M-wave without saturation.

An Anti Stimulation Artifacts and M-waves Surface Electromyography Detector with a Short Blanking Time. ANNUAL INTERNATIONAL CONFERENCE OF THE IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. IEEE ENGINEERING IN MEDICINE AND BIOLOGY SOCIETY. ANNUAL INTERNATIONAL CONFERENCE 2020;2020:4126-4129. [PMID: 33018906 DOI: 10.1109/embc44109.2020.9176373]

A surface electromyography (sEMG) detector, that not only removes stimulation artifacts entirely but also increases the recording time, has been developed in this paper. The sEMG detector consists of an sEMG detection circuit and a stimulation isolator. The sEMG detection circuit employs a stimulus isolate switch (SIS), a blanking (BLK) and non-linear feed-back (NFB) circuit to remove the artifacts and to increase the recording time. In the SIS, the connection between stimulator and stimulation electrodes, along with the stimulation electrodes and the ground are controlled by an opto-isolator, and the connection of instrument amplifier and the recording electrodes are controlled by CMOS-based switches. The mode switches of the BLK and the NFB circuit also employs CMOS-based switches. By an accurate timing adjustment, the voluntary EMG can be recorded during electrical stimulation. Two 6 able-bodied experiments have been performed to test the three anti-artifact sEMG detector: BLK, BLK&SIS, BLK&SIS&NFB. The results indicate that the BLK&SIS&NFB proposed in this work effectively removes stimulus artifacts and M-waves, and has a longer recording time compared with BLK and BLK&SIS circuits.

Real-Time Closed-Loop Functional Electrical Stimulation Control of Muscle Activation with Evoked Electromyography Feedback for Spinal Cord Injured Patients

Functional electrical stimulation (FES) is a neuroprosthetic technique to help restore motor function of spinal cord-injured (SCI) patients. Through delivery of electrical pulses to muscles of motor-impaired subjects, FES is able to artificially induce their muscle contractions. Evoked electromyography (eEMG) is used to record such FES-induced electrical muscle activity and presents a form of [Formula: see text]-wave. In order to monitor electrical muscle activity under stimulation and ensure safe stimulation configurations, closed-loop FES control with eEMG feedback is needed to be developed for SCI patients who lose their voluntary muscle contraction ability. This work proposes a closed-loop FES system for real-time control of muscle activation on the triceps surae and tibialis muscle groups through online modulating pulse width (PW) of electrical stimulus. Subject-specific time-variant muscle responses under FES are explicitly reflected by muscle excitation model, which is described by Hammerstein system with its input and output being, respectively, PW and eEMG. Model predictive control is adopted to compute the PW based on muscle excitation model which can online update its parameters. Four muscle activation patterns are provided as desired control references to validate the proposed closed-loop FES control paradigm. Real-time experimental results on three able-bodied subjects and five SCI patients in clinical environment show promising performances of tracking the aforementioned reference muscle activation patterns based on the proposed closed-loop FES control scheme.

A blink restoration system with contralateral EMG triggered stimulation and real-time artifact blanking

Patients suffering from facial paralysis are on the hazard of disfigurement and loss of vision due to loss of blink function. Functional-electrical stimulation (FES) is one possible way of restoring blink and other functions in these patients. A blink restoration system for uni-lateral facial paralyzed patients is described in this paper. The system achieves restoration of synchronized blink through processing the myoelectric signal of orbicularis oculi at the normal side in real-time as the trigger to stimulate the paralyzed eyelid. Design issues are discussed, including EMG processing, stimulating strategies and real-time artifact blanking. Two artifact removal approaches based on sample and hold and digital filtering technique are proposed and implemented. Finally, the whole system has been verified on rabbit models.

Surface EMG as a fatigue indicator during FES-induced isometric muscle contractions

The electromyogram (EMG) signal has potential as an indicator of stimulated muscle fatigue in applications of functional electrical stimulation (FES). In particular, it could be used to detect near lower limb collapse due to the associated muscle fatigue in FES-aided standing systems and thereby prevent falling. Surface EMG measurement, however, is hampered by stimulation artifact during FES. Modified surface stimulation and EMG detection equipment were designed and built to minimize this artifact and to permit detection of the electrical signal generated by the muscle during contraction. Artifact reduction techniques included shorting stimulator output leads between stimulus pulses and limiting and blanking slew rate in the EMG processing stage. Isometric fatigue experiments were performed by stimulating the quadriceps muscle of 20 able-bodied (a total of 125 trials) and three spinal cord injured (18 trials) subjects. Fatigue-tracking performance indicators were derived from the root-mean-square (RMS) of the EMG amplitude and from the median frequency (MF) of the EMG power spectral content. The results demonstrate that reliable fatigue tracking indicators for practical FES applications will be difficult to obtain, but that amplitude-based measures in spinal cord injured subjects show promise.

An EMG-controlled neuroprosthesis for daily upper limb support: a preliminary study

MUNDUS is an assistive platform for recovering direct interaction capability of severely impaired people based on upper limb motor functions. Its main concept is to exploit any residual control of the end-user, thus being suitable for long term utilization in daily activities. MUNDUS integrates multimodal information (EMG, eye tracking, brain computer interface) to control different actuators, such as a passive exoskeleton for weight relief, a neuroprosthesis for arm motion and small motors for grasping. Within this project, the present work integreted a commercial passive exoskeleton with an EMG-controlled neuroprosthesis for supporting hand-to-mouth movements. Being the stimulated muscle the same from which the EMG was measured, first it was necessary to develop an appropriate digital filter to separate the volitional EMG and the stimulation response. Then, a control method aimed at exploiting as much as possible the residual motor control of the end-user was designed. The controller provided a stimulation intensity proportional to the volitional EMG. An experimental protocol was defined to validate the filter and the controller operation on one healthy volunteer. The subject was asked to perform a sequence of hand-to-mouth movements holding different loads. The movements were supported by both the exoskeleton and the neuroprosthesis. The filter was able to detect an increase of the volitional EMG as the weight held by the subject increased. Thus, a higher stimulation intensity was provided in order to support a more intense exercise. The study demonstrated the feasibility of an EMG-controlled neuroprosthesis for daily upper limb support on healthy subjects, providing a first step forward towards the development of the final MUNDUS platform.

Pulse energy as a reliable reference for twitch forces induced by transcutaneous neuromuscular electrical stimulation

Voltage-controlled neuromuscular electrical stimulation has been considered to be safer in noninvasive applications notwithstanding the fact that voltage-controlled devices purportedly generate forces less predictable than their current-controlled equivalents. This prompted us to evaluate relevant electrical parameters to determine whether forces induced by voltage-controlled stimuli were able to match to those induced by current-controlled ones, which tend to evoke forces that were more predictable. Force magnitudes corresponding to current- and voltage-controlled stimuli were aligned with respect to electric charge (equivalent to average current intensity) and electrical energy (equivalent to average power) of the same stimulation pulse to determine which provided a better coherence. Consistency of forces evaluated with energy was significantly (p < 0.001) better than that evaluated with electric charges, suggesting that electrically stimulated forces can be reliably predicted by monitoring the energy parameter of stimulation pulses. The above results appear to show that electrode-tissue impedance, a factor that makes charge and energy evaluations different, redefined the actual effects of current intensities in generating favorable results. Accordingly, novel schemes that track the energy (or average power) of a stimulation pulse may be used as a reliable benchmark to associate mechanical (force) and electrical (stimulation pulse) characteristics in transcutaneous applications of electrical stimulation.

Design and testing of an advanced implantable neuroprosthesis with myoelectric control

An implantable stimulator-telemeter (IST-12) was developed for applications in neuroprosthetic restoration of limb function in paralyzed individuals. The IST-12 provides 12 stimulation channels and two myoelectric signal (MES) channels. The MES circuitry includes a two-channel multiplexer, preamplifier, variable gain amplifier/bandpass filter, full-wave rectifier, and bin integrator. Power and control signals are transmitted from an external control unit to the IST-12 through an inductive link. Recorded MES signals are telemetered back to the external control unit through the same inductive link. Following bench testing, one device was implanted chronically in a dog for 15 months and evaluated. Conditions were identified in which MES could be recorded with minimal stimulus artifact. The ability to record MES in the presence of stimulation was verified, confirming the potential of the IST-12 to be used as a myoelectric controlled neuroprosthesis.

Enhancement of muscle activity by electrical stimulation in cerebral palsy: a case-control study

The objectives of this study were to compare the effects of low-intensity electrical stimulation of the quadriceps muscle in children with cerebral palsy in the following 2 modes: reconditioning by long-term training of the muscle versus real-time assist to the muscle during motion. To evaluate the force enhancement in the assist mode, we developed a method to dissociate the volitional and the induced components from the total electromyographic signal. The study group, including 5 children with cerebral palsy (mean age, 3.3 years; 0.4 SD), underwent 2 testing sessions: 1 before and 1 after 3-month training by electrical stimulation. Each session included 2 series of trials: 1 with electrical stimulation, as an orthotic assist, and 1 without electrical stimulation. The tests included flexion-extension movements of the knee at a self-selected pace. The results showed that, compared to before training, there was a significant increase in the average motion velocity and a decrease in motion jerk and in knee torque after training in both the electrical stimulation- assisted and -unassisted modes. Of special interest was the significant decrease in quadriceps-hamstrings co-contraction following training by electrical stimulation but not during electrical stimulation-assisted motion. The results obtained for the group with cerebral palsy were statistically different from those of the control group, but this difference decreased after long-term training by electrical stimulation. It was concluded that, in children with cerebral palsy, electrical stimulation is more beneficial in long-term training than when used as a real-time motion assist. Although muscle strength is not affected, more centrally controlled attributes such as co-contraction are improved.

Muscle force augmentation by low-intensity electrical stimulation

In cases of muscle partial deficiency, force augmentation can be achieved by hybrid activation, i.e., by combining electrical stimulation (ES) with volitional activation. In the present study the volitional and electrically-induced torque components are resolved under visual-feedback activation. Isometric contraction of the tibialis anterior (TA) muscle was studied on 5 healthy subjects, using an activation protocol combining ES alone, volitional activation alone and hybrid activation. Ankle torque and TA EMG were measured. A computational algorithm was developed to dissociate the volitional from the overall torque, based on EMG filtering and on pre-measured calibration curves of volitional torque versus EMG. Based on a defined facilitation factor, the results indicate that within the range of stimulation intensities, there exist regions of increased facilitation of the volitional activation of the TA muscle, in which the torque contribution due to the induced activation is higher compared that of the recruitment curve.

Evaluation of methods for extraction of the volitional EMG in dynamic hybrid muscle activation

Background

Hybrid muscle activation is a modality used for muscle force enhancement, in which muscle contraction is generated from two different excitation sources: volitional and external, by means of electrical stimulation (ES). Under hybrid activation, the overall EMG signal is the combination of the volitional and ES-induced components. In this study, we developed a computational scheme to extract the volitional EMG envelope from the overall dynamic EMG signal, to serve as an input signal for control purposes, and for evaluation of muscle forces.

Methods

A "synthetic" database was created from in-vivo experiments on the Tibialis Anterior of the right foot to emulate hybrid EMG signals, including the volitional and induced components. The database was used to evaluate the results obtained from six signal processing schemes, including seven different modules for filtration, rectification and ES component removal. The schemes differed from each other by their module combinations, as follows: blocking window only, comb filter only, blocking window and comb filter, blocking window and peak envelope, comb filter and peak envelope and, finally, blocking window, comb filter and peak envelope.

Results and conclusion

The results showed that the scheme including all the modules led to an excellent approximation of the volitional EMG envelope, as extracted from the hybrid signal, and underlined the importance of the artifact blocking window module in the process.

The results of this work have direct implications on the development of hybrid muscle activation rehabilitation systems for the enhancement of weakened muscles.

Measurement and modeling of stimulus-evoked electromyography in lengthened and shortened muscles for spinal cord injured subjects during an electrically-elicited fatigue process

This study compares the amplitude and temporal features of stimulus-evoked electromyography (EMG) of paralyzed muscle, rectus femoris (RF), in both lengthened and shortened positions of six spinal cord injured (SCI) subjects during an electrically elicited fatigue process. The torque output and evoked EMG were fitted by hyperbolic tangent functions from which their amplitude residual levels and temporal inflection times can be extracted. Furthermore, a structural EMG model of Fuglevand et al (1992 Biol. Cybern. 67 143-53) was modified to include type I (slow twitch) and type II (fast twitch) of motor unit (MU) fibers with viable parameters obtained from paralyzed muscles to observe their amplitude and temporal changes. Our results showed that the amplitude of stimulus-evoked EMG decreased earlier in the lengthened muscle with a shorter inflection time (48.53 +/- 8.7 s versus 55.13 +/- 4.03 s) than that of the shortened position during 120 s of stimulation time (p < 0.05). Similarly, the peak-to-peak duration (PTPd) of the evoked EMG increased faster at an earlier time to a higher asymptotical value in lengthened muscle (2.23 +/- 0.74 versus 1.77 +/- 0.54), compared to that of a shortened one (p < 0.05). These observations coincided with the higher rising rate and larger final value of the temporal coefficients, i.e., longer duration, in both type I and II MUs of lengthened muscles. From the observation of all parameters, the fatigue process in lengthened muscle proceeds faster than that in shortened muscle.

Partition between volitional and induced forces in electrically augmented dynamic isometric muscle contractions

Augmentation of force in partially deficient muscles can be achieved by combining electrical stimulation (ES) with their volitional activation (hybrid activation). However, while the overall torque results from the combination of the volitional and the electrically-induced torque components, the exact share between these components is not known. In a previous work, we described a method to resolve the share between the torque components under isometric static contractions. In this work, we extend our analysis to the case of isometric dynamic contractions. Five healthy subjects were instructed to contract their Tibialis Anterior (TA) muscles according to a typical gait-like dynamic torque pattern, that was visually displayed to them, while monitoring their actual ankle torque and TA electromyography (EMG). These experiments were done with and without augmented activation by means of ES. A computational algorithm was developed to dissociate the volitional from the overall torque, based on EMG signal processing and on precalibration of the dynamic system of the volitional torque versus EMG. The results indicated the quantitative relations between decrease in the volitional torque and the required increase in ES enhancement. The developed method also demonstrated what ES intensity profile is necessary to produce a desired overall torque output. This provides the means for designing an adaptive rehabilitation device for the hybrid activation of deficient muscles.

Muscle enhancement using closed-loop electrical stimulation: volitional versus induced torque

In cases of partial deficiency of muscle activation capacity, force augmentation can be achieved by hybrid activation, i.e., by combining electrical stimulation (ES) with volitional activation. In this activation modality the shares of the volitional and induced torques within the overall hybrid torque are unknown. The purpose of this study was to suggest a computational approach to parcel out the volitional and stimulation induced components of joint torque generated during combined voluntary and electrical activation of the Tibialis Anterior muscle (TA). For this purpose, isometric contraction of the TA was studied on 5 healthy subjects, using an activation protocol involving ES alone, volitional activation alone and hybrid activation. Ankle torque and TA EMG were measured. A computational algorithm was developed to dissociate the volitional from the overall torque, based on EMG filtering and on pre-measured calibration curves of volitional torque versus EMG. The results indicated that for a certain hybrid torque there is a linear decaying relationship between the induced torque and the volitional torque shares. Moreover, based on a defined enhancement ratio, the results indicate that within the range of stimulation intensities, there exist regions of increased facilitation, in which the stimulation efficiency is higher under combined compared to isolated conditions.

Factors affecting the stimulus artifact tail in surface-recorded somatosensory-evoked potentials

Surface-recorded somatosensory-evoked potentials (SEPs) are neural signals elicited by an external stimulus. In the case of electrically induced SEPs, the artifact generated by the stimulation process can severely distort the signal. In some cases, the artifact tail often lasts well into the initiation of the SEP making the determination of absolute latency very difficult. In this work, a new approach was taken to identify factors that affect the tail of the artifact. The methodology adopted was the development of a lumped electrical circuit model of the artifact generation process. While the modeling of the instrumentation hardware is relatively simple, this is not the case with tissue and electrode/skin interface effects. Consequently, this paper describes a novel tissue modeling approach that uses an autoregressive moving average (ARMA) parametric technique and an artificial neural network (ANN) to estimate tissue parameters from experimental data. This coupled with an estimation of the stimulation electrode-skin impedance completes the lumped circuit model. Simulink (The Mathworks Inc.) was used to evaluate the model under several different conditions. These results show that both the stimulation electrode-skin interface impedance and nature of the body tissue directly under the recording electrodes have a profound effect on the appearance of the stimulus artifact tail. This was verified by experimentally recorded data obtained from the median nerve using surface electrodes. Conclusions drawn from this work include that stimulation electrodes with low series capacitance should be used whenever possible to minimize the duration of the artifact tail.

Impact of High-Frequency Stimulation Parameters on the Pattern of Discharge of Subthalamic Neurons

In clinical conditions, high-frequency stimulation (HFS) of subthalamic (STN) neurons in Parkinson's disease is empirically applied at ≥100 Hz (130–185 Hz), with pulses of short duration (60–100 μs) and 1- to 3-mA amplitude. Other parameter values produce no effect or aggravate the symptoms. To gain a better understanding of the mechanisms that underlie the therapeutic action of HFS, we have compared the effects of different combinations of parameter values delivered by clinical stimulators on the activity of STN neurons recorded in whole cell patch-clamp configuration in slices. We showed that none of tested combinations of parameters silenced the neurons. Non-therapeutic combinations i.e., low-frequency pulses (10–50 Hz), even at large amplitude or width, further excited the STN neurons with respect to their spontaneous activity. In contrast, combinations in the therapeutic range (80–185 Hz, 90–200 μs, 500–800 μA) replaced the preexisting activity by spikes, time-locked to the stimuli and thus presenting a striking regularity. When increasing pulse width or amplitude in this high-frequency range, the dual effect was still present but the activity generated became more irregular. We propose that during HFS at clinically relevant parameters, STN neurons behave as stable oscillators entirely driven by the stimulation, giving an average stable STN output that overrides spontaneous activity and introduces high-frequency regular spiking in the basal ganglia network.

Dual effect of high-frequency stimulation on subthalamic neuron activity

Although it is well known that high-frequency stimulation (HFS) of the subthalamic nucleus (STN) alleviates the cardinal symptoms of Parkinson's disease, the underlying mechanisms are not fully understood. We investigated the effect of stimulation from low to high frequencies on rat STN neurons in naive and dopamine-depleted slices using whole-cell, current-clamp techniques and on-line artifact suppression. Stimulation at 10 Hz evoked 10 Hz single spikes but did not significantly modify ongoing STN activity. In contrast, at therapeutically relevant frequencies (80-185 Hz), stimulation had a dual effect: it fully suppressed STN spontaneous activity and generated a robust pattern of recurrent bursts of spikes, with each spike being time-locked to a stimulus pulse. Neither the suppression of spontaneous activity nor the generation of spikes was prevented by the antagonists of the metabotropic and ionotropic receptors of glutamate and gamma-aminobutyric acid. Tetrodotoxin, the Na+ channel blocker, suppressed all HFS-evoked spikes, whereas nifedipin, an L-type Ca2+-channel blocker, abolished the membrane oscillations underlying bursts. Therefore, we conclude that HFS drives the STN neuronal activity by directly activating the neuronal membrane. We suggest that this pattern may remove the deleterious activity of the basal ganglia network in the parkinsonian state and drive target neurons to a high-frequency state of activity, dependent on the characteristics of STN efferent synapses and resonant properties of target membranes.

Stimulus artifact removal using a software-based two-stage peak detection algorithm

The analysis of stimulus evoked neuromuscular potentials or m-waves is a useful technique for improved feedback control in functional electrical stimulation systems. Usually, however, these signals are contaminated by stimulus artifact. A novel software technique, which uses a two-stage peak detection algorithm, has been developed to remove the unwanted artifact from the recorded signal. The advantage of the technique is that it can be used on all stimulation artifact-contaminated electroneurophysiologic data provided that the artifact and the biopotential are non-overlapping. The technique does not require any estimation of the stimulus artifact shape or duration. With the developed technique, it is not necessary to record a pure artifact signal for template estimation, a process that can increase the complexity of experimentation. The technique also does not require the recording of any external hardware synchronisation pulses. The method avoids the use of analogue or digital filtering techniques, which endeavour to remove certain high frequency components of the artifact signal, but invariably have difficulty, resulting in the removal of frequencies in the same spectrum as the m-wave. With the new technique the signal is sampled at a high frequency to ensure optimum fidelity. Instrumentation saturation effects due to the artifact can be avoided with careful electrode placement. The technique was fully tested with a wide variety of electrical stimulation parameters (frequency and pulse width) applied to the common peroneal nerve to elicit contraction in the tibialis anterior. The program was also developed to allow batch processing of multiple files, using closed loop feedback correction. The two-stage peak detection artifact removal algorithm is demonstrated as an efficient post-processing technique for acquiring artifact free m-waves.

An artefact suppressing fast-recovery myoelectric amplifier

An amplifier for recording myoelectric signals using surface electrodes has been developed. The special features are suppression of stimulation artefacts and motion artefacts from electrodes. It is designed for recording of myoelectric signals from a muscle that is being stimulated with short impulses. The artifact suppression is achieved by using fast-recovery instrumentation amplifiers and having a nonlinear feedback loop for automatic compensation of changes in DC-offset.

Artefact cancellation in motor-sensory evoked potentials: two approaches using adaptive filtration and exponential approximation

Adaptive filtering for artefact cancellation in motor-sensory evoked potentials using signals obtained by subtraction methods (double-stimulus, off-nerve and subthreshold) is proposed. This is advantageous as inherent non-linear distortions can be overcome in an easier way by adaptive filtering. Efficiency is assessed with reference signals synthesised by varying the shape and reducing the amplitude of a 'pure' evoked potential in the range from 10% to 50%. The experiments show virtually identical shapes of the 'pure' and the filtered signal. The time shift between them is insignificant if a causal filter and small number of Widrow coefficients, e.g. N = 8, are used. Further, two-exponential artefact approximation is applied with subsequent direct subtraction from the contaminated signal by a specially designed PC-controlled system for data acquisition and processing. For a fast procedure convergence, one-parametric optimisation of the time-constant tau is used, starting with tau = 0.5 ms. The results obtained with artefact-corrupted evoked potentials from several subjects prove the efficiency of the approach. It has the substantial advantage of avoiding the need for reference signals. Both methods have advantages compared with other known software techniques.

A comparison between control methods for implanted FES hand-grasp systems

Implanted neuroprostheses employing functional electrical stimulation (FES) provide grasp and release to individuals with tetraplegia. This paper describes and compares three methods of controlling the stimulated hand movement: shoulder position, wrist position and myoelectric activity from the wrist extensors. Three experienced neuroprosthesis users were evaluated with each of the control methods by performing a grasp release test (GRT). A significant improvement was found between each functional electrical stimulation (FES) method and tenodesis without FES. No significant difference in overall performance was found between the three FES methods of control. Each method of control demonstrated advantages and disadvantages which depend upon characteristics of the individual patient. Factors which must be considered are injury level, voluntary wrist strength, proximal upper limb strength, the level of cognition of the patient, hand-grasp characteristics, cosmeses, importance of using both arms, and personal preference. Due to the unique characteristics of each controller type, it is advantageous to have each type available for the FES patients to adapt the system to the needs and desires of the individual patient.

Convergence characteristics of two algorithms in non-linear stimulus artefact cancellation for electrically evoked potential enhancement

Somatosensory evoked potentials (SEPs) are a sub-class of evoked potentials (EPs) that are very useful in diagnosing various neuromuscular disorders and in spinal cord and peripheral-nerve monitoring. Most often, the measurements of these signals are contaminated by stimulus-evoked artefact. Conventional stimulus-artifact (SA) reduction schemes are primarily hardware-based and rely on some form of input blanking during the SA phase. This procedure can result in partial SEP loss if the tail of the SA interferes with the SEP. Adaptive filters offer an attractive solution to this problem by iteratively reducing the SA waveform while leaving the SEP intact. Owing to the inherent non-linearities in the SA generation system, non-linear adaptive filters (NAFs) are most suitable. SA reduction using NAFs based on truncated second-order Volterra expansion series is investigated. The focus is on the performance of two main adaptation algorithms, the least mean square (LMS) and recursive least squares (RLS) algorithms, in the context of non-linear adaptive filtering. A comparison between the convergence and performance characteristics of these two algorithms is made by processing both simulated and experimental SA data. It is found that, in high artefact-to-noise ratio (ANR) SA cancellation, owing to the large eigenvalue spreads, the RLS-based NAF is more efficient than the LMS-based NAF. However, in low-ANR scenarios, the RLS- and LMS-based NAFs exhibit similar convergence properties, and the computational simplicity of the LMS-based NAFs makes them the preferred option.

Using evoked EMG as a synthetic force sensor of isometric electrically stimulated muscle

A method for the estimation of the force generated by electrically stimulated muscle during isometric contraction is developed here. It is based upon measurements of the evoked electromyogram (EMG) [EEMG] signal. Muscle stimulation is provided to the quadriceps muscle of a paralyzed human subject using percutaneous intramuscular electrodes, and EEMG signals are collected using surface electrodes. Through the use of novel signal acquisition and processing techniques, as well as a mathematical model that reflects both the excitation and activation phenomena involved in isometric muscle force generation, accurate prediction of stimulated muscle forces is obtained for large time horizons. This approach yields synthetic muscle force estimates for both unfatigued and fatigued states of the stimulated muscle. In addition, a method is developed that accomplishes automatic recalibration of the model to account for day-to-day changes in pickup electrode mounting as well as other factors contributing to EEMG gain variations. It is demonstrated that the use of the measured EEMG as the input to a predictive model of muscle torque generation is superior to the use of the electrical stimulation signal as the model input. This is because the measured EEMG signal captures all of the neural excitation, whereas stimulation-to-torque models only reflect that portion of the neural excitation that results directly from stimulation. The time-varying properties of the excitation process cannot be captured by existing stimulation-to-torque models, but they are tracked by the EEMG-to-torque models that are developed here. This work represents a promising approach to the real-time estimation of stimulated muscle force in functional neuromuscular stimulation applications.

Adaptive stimulus artifact reduction in noncortical somatosensory evoked potential studies

Somatosensory evoked potentials (SEP's) are an important class of bioelectric signals which contain clinically valuable information. The surface measurements of these potentials are often contaminated by a stimulus evoked artifact. The stimulus artifact (SA), depending upon the stimulator and measurement system characteristics, may obscure some of the information carried by the SEP's. Conventional methods for SA reduction employ hardware-based circuits which attempt to eliminate the SA by blanking the input during SA period. However, there is a danger of losing some of the important SEP information, especially if the stimulating and recording electrodes are close together. In this paper, we apply both linear and nonlinear adaptive filtering techniques to the problem of SA reduction. Nonlinear adaptive filters (NAF's) based on truncated second-order Volterra series expansion are discussed and their applicability to SA cancellation is explored through processing both simulated and in vivo SEP data. The performances of the NAF and the finite impulse response (FIR) linear adaptive filter (LAF) are compared by processing experimental SEP data collected from different recording sites. Due to the inherent nonlinearities in the generation of the SA, the NAF is shown to achieve significantly better SA cancellation compared to the LAF.

Functional neuromuscular stimulation controlled by surface electromyographic signals produced by volitional activation of the same muscle: adaptive removal of the muscle response from the recorded EMG-signal

In order to use the volitional electromyography (EMG) as a control signal for the stimulation of the same muscle, it is necessary to eliminate the stimulation artifacts and the muscle responses caused by the stimulation. The stimulation artifacts, caused by the electric field in skin and tissue generated by the stimulation current, are relatively easy to eliminate by shutting down the EMG-amplifier at the onset of the stimulation pulses. The muscle response is a nonstationary signal, therefore, an adaptive linear prediction filter is proposed. The filter is implemented and for three filter lengths tested on both simulated and real data. The filter performance is compared with a conventional fixed comb filter. The simulations indicate that the adaptive filter is relatively insensitive to variations in amplitude of the muscle responses, and for all filter lengths produces a good filtering. For variations in shape of the muscle responses and for real data, an increased filter performance can be achieved by increasing the filter length. Using a filter length of up to seven stimulation periods, it is possible to reduce real muscle responses to a level comparable with the background noise. Using the shut-down circuit and the adaptive filter both the stimulation artifacts and the muscle responses can be effectively eliminated from the EMG signal from a stimulated muscle. It is therefore possible to extract the volitional EMG from a partly paralyzed muscle and use it for controlling the stimulation of the same muscle.

The validity of stimulus-evoked EMG for studying muscle fatigue characteristics of paraplegic subjects during dynamic cycling movement

The fatigue characteristics of paralyzed muscles were investigated during dynamic cycling movement induced by functional electrical stimulation (FES). The peak-to-peak (PTP) amplitude of stimulus-evoked electromyogram (EMG), after suppression of stimulus artifact, was adopted as fatigue indicator. Compared to static contraction, the effects of dynamic movement factors on the stimulus-evoked EMG, such as the intermittent stimulation, joint angle, and contraction speed, were first evaluated in separate experiments. The results of isolated tests laid the foundation for interpreting the data obtained in two FES-cycling experiments, performed under maximum stimulation or in controlled cycling speeds. The effects of intermittent stimulation and joint angle caused periodic changes in PTP amplitude which can be alleviated by averaging the PTP amplitude of one cycle. Under the same stimulation intensity, our results indicated that slower muscle contraction speed would have larger PTP amplitude and vice versa. For the limited number of subjects with paraplegia studied, our results showed that the use of EMG PTP as reliable muscle fatigue indicator during dynamic movement is only valid at the same cycling speed or corresponding contraction speed. The decline of the PTP amplitude decreased with the decay of muscle force can be observed during cycling movement; however, reduction of cycling speed had the opposite effect on PTP amplitude. Observations from the hyperbolic modeling of fatigue process demonstrated that the EMG PTP of a fatigued muscle under dynamic movement decreased at a slower rate.

Stimulus artefact in somatosensory evoked potential measurement

When an electrical stimulus is used to evoke action potentials in peripheral nerves or the spinal cord, the stimulus causes an artefact which may interfere with measurement of the evoked potentials. This artefact, unlike all other sources of noise in the measurement, cannot be reduced by ensemble averaging. Confusion about the origin and transmission of stimulus artefact has led to considerable frustration in spinal somatosensory evoked potential (SSEP) measurements. The three mechanisms by which stimulus artefact is coupled into the measuring system are identified, and means for their reduction are discussed.

Stimulus artifact reduction in evoked potential measurements

OBJECTIVE

To investigate the main coupling mechanisms involved in stimulus artifact contamination of evoked potential recordings and to suggest techniques that minimize this interference.

DESIGN

A before-after trial of a single subject.

SETTING

Measurements were obtained at a university biomedical engineering laboratory.

PARTICIPANTS

Data were obtained from one volunteer subject.

INTERVENTION

An electrical stimulus was used to depolarize the posterior tibial nerve at the ankle. Various recording electrode configurations were used to demonstrate stimulus artifact recordings.

RESULTS

Three mechanisms are defined as contributing significantly to stimulus artifact contamination of evoked potential data. These are: the volume conducted component, the displacement current component, and the electromagnetic coupling component. When each component is maximally controlled, the problem of stimulus artifact is greatly reduced.

CONCLUSION

Three major factors that contribute to stimulus artifact contamination of the evoked potential waveform can be identified and minimized by relatively simple clinical techniques.

Stimulus artifact removal in EMG from muscles adjacent to stimulated muscles

When stimulating muscles, EMG signals recorded in neighboring muscles can be contaminated by stimulus artifacts, and artifact deletion is necessary. We have devised a digital technique for removing stimulus artifacts from rectified EMG recordings in muscles which lie close to a stimulated muscle. This artifact deletion method replaces the rectified EMG during the artifact interval with an estimate of the rectified EMG. Our research requires detection of very small changes in EMG levels. Therefore, the artifact deletion technique described in this paper was designed to leave less than 10 microV of artifact in the rectified EMG post-processing. This technique relies on being able to estimate the artifact duration. Since stimulated muscles have M-waves that can overlap with artifacts, our technique is only appropriate for removing artifacts in muscles which are not being stimulated. Unlike other artifact elimination techniques, our technique does not change the mean value of the rectified EMG, regardless of artifact width. In addition, it provides a more accurate estimate of the rectified EMG during the artifact interval as opposed to sample-and-hold techniques.