Determining appropriate models for joint control using surface electrical stimulation of soleus in spinal cord injury

The mechanical impedance of the ankle joint during electrical stimulation of the soleus is studied by applying constant-velocity 10° angular perturbations to the ankle and measuring the resultant torque. Both neurologically intact subjects and spinal cord injured subjects are tested. Lumped, piecewise linear models are developed to predict the torque from the measured displacement and acceleration signals. The commonly used second-order mass-spring-dashpot model fails to predict the changes in torque that occur following imposed movements. A fiveelement, directionally-dependent piecewise linear model is much better at predicting the measured responses for velocities up to 50° s⁻¹. Numerical least squared error indentification techniques are used to estimate the model parameters for three neurologically intact and three spinal cord injured subjects. The average error between the model’s response and the measured response across all subjects is 10·9%. There is some evidence that a velocity-dependent non-linear model could produce better results than the directionally-dependent piecewise linear model.     

Reliability of neural-network functional electrical stimulation gait-control system

Functional electrical stimulation (FES) has been used for restoring walking in spinal-cord injured (SCI) persons. Using artificial intelligence (Al), FES controllers have been developed that allow the automatic phasing of stimulation, to replace the function of hand or heel switches. However, there has been no study to evaluate the reliability of these Al systems. Neural networks were used to construct FES controllers to control the timing of stimulation. Different numbers of sensors in the sensor set and different numbers of data points from each sensor were used. Two incomplete-SCI subjects were recruited, and each was tested on three separate occasions. The results show the neural-network controllers can maintain a high accuracy (around 90% for the two- and three-sensor groups and 80% for the onesensor group) over a period of six months. Two or three sensors were sufficient to provide enough information to construct a reliable FES control system, and the number of data points did not have any effect on the reliability of the system.     

Reliability of neural-network functional electrical stimulation gait-control system

Functional electrical stimulation (FES) has been used for restoring walking in spinal-cord injured (SCI) persons. Using artificial intelligence (AI), FES controllers have been developed that allow the automatic phasing of stimulation, to replace the function of hand or heel switches. However, there has been no study to evaluate the reliability of these AI systems. Neural networks were used to construct FES controllers to control the timing of stimulation. Different numbers of sensors in the sensor set and different numbers of data points from each sensor were used. Two incomplete-SCI subjects were recruited, and each was tested on three separate occasions. The results show the neural-network controllers can maintain a high accuracy (around 90% for the two- and three-sensor groups and 80% for the one-sensor group) over a period of six months. Two or three sensors were sufficient to provide enough information to construct a reliable FES control system, and the number of data points did not have any effect on the reliability of the system.     

Regulating knee joint position by combining electrical stimulation with a controllable friction brake

Hybrid FES gait restoration systems which combine stimulation with controllable mechanical damping elements at the joints show promise for providing good control of limb motion despite variations in muscle properties. In this paper we compared three controllers for position tracking of the free swinging shank in able-bodied subjects. The controllers were open-loop (OL), proportional-derivative closed-loop (PD), and bang-bang plus controlled-brake control (CB). Both OL and PD controllers contained a forward path element, which inverted a model of the electrically stimulated muscle and limb system. The CB control was achieved by maximally activating the appropriate muscle group and controlling the brake to be a “moving-wall” against which the limb pushed. The CB control resulted in superior tracking performance for a wide range of position tracking tasks and muscle fatigue states but required no calibration or knowledge of muscle properties. The disadvantages of CB control include excess mechanical power dissipation in the brake and impact forces applied to the skeletal system.     

Gait control system for functional electrical stimulation using neural networks

In functional electrical stimulation (FES) systems for restoring walking in spinal cord injured (SCI) individuals, hand switches are the preferred method for controlling stimulation timing. Through practice the user becomes an ‘expert’ in determining when stimulation should be applied. Neural networks have been used to ‘clone’ this expertise but these applications have used small numbers of sensors, and their structure has used a binary output, giving rise to possible controller oscillations. It was proposed that a threelayer structure neural network with continuous function, using a larger number of sensors, including ‘virtual’ sensors, can be used to ‘clone’ this expertise to produce good controllers. Using a sensor set of ten force sensors and another of 13 ‘virtual’ kinematic sensors, a good FES control system was constructed using a three-layer neural network with five hidden nodes. The sensor set comprising three sensors showed the best performance. The accuracy of the optimum three-sensor set for the force sensors and the virtual kinematic sensors was 90% and 93%, respectively, compared with 81% and 77% for a heel switch. With 32 synchronised sensors, binary neural networks and continuous neural networks were constructed and compared. The networks using continuous function had significantly fewer oscillations. Continuous neural networks offer the ability to generate good FES controllers.     

功能性电刺激在呼吸功能重建中的应用进展

目的:总结功能性电刺激技术在呼吸功能重建方面的应用进展。方法:以"膈神经刺激器""膈肌起搏""呼吸起搏器""脊髓电刺激""脊髓损伤""功能性电刺激""呼吸功能不全"以及"phrenic nerve stimulator""diaphragm pacing""respiratory pacemaker""spinal c...     

Biomechanical and reflex responses to joint perturbations during electrical stimulation of muscle: instrumentation and measurement techniques

A test device is developed to measure ankle joint compliance and muscle activity when the ankle is subjected to perturbations in angular position (or torque) from bias positions achieved volitionally or via electrical stimulation. The ankle measurement system uses a pivoting footplate and is operable with the subject sitting or supine. A companion platform for the knee is developed that uses a rotary arm and attached leg brace and is operable with the subject’s leg in the horizontal or vertical plane. The knee fixture’s pivoting arm can slide to account for the cam-like movement of the knee during rotation. The devices use similar hardware and share common instrumentation and control. Precise torque or position perturbations are delivered by a computer-controlled torque motor to the ankle or knee. Angular displacement, torque, acceleration, knee fixture moment arm and electromyographic data are collected on analogue tape and simultaneously digitised and stored. A special stimulator/recording amplifier permits the recording of electromyographic signals from the stimulated muscle. Experimental data indicate that the ankle and knee devices, operated horizontally, are purely inertial systems. Sample ankle and knee joint responses to perturbations are presented.     

Improvement in step clearance via calf muscle stimulation

The aim is to study the influence of electrically stimulated calf muscles on the effectiveness of the swinging leg movement. The study is carried out with a group of patients with incomplete spinal cord injuries both under stationary conditions and during cruth-assisted walking. Before stimulation is applied to the ankle plantar flexors, the knee extensors are inactivated. In each cycle, after ankle plantar flexor stimulation, peroneal stimulation is started, triggering the flexion reflex. From a biomechanical point of view. functional electrical stimulation (FES) of the ankle plantar flexors results in increased ground clearance of the lower extremity. Additionally, the FES-assisted lifting of the heel results in the elimination of extensor tone and thus shortens the swing time.     

Field distribution of epidural electrical stimulation

Epidural electrical stimulation has been applied in clinics for many years. However, there is still a concern about possible injury to spinal nerves. This study investigated electrical field and current density distribution during direct epidural electrical stimulation. Field distribution models were theoretically deduced, while the distribution of potentials and current were analyzed. The current density presented an increase of 70–80%, with one peak value ranging from −85° to 85° between the two stimulated poles. The effect of direct epidural electrical stimulation is mainly on local tissue surrounding the electrodes, concentrated around the two stimulated positions.     

Characterization and control of muscle response to electrical stimulation

The maintenance of upright posture in neurologically intact human subjects is mediated by two major nervous pathways. The first, leading from the cerebral cortex through the spinal cord to motor neurons, activates muscles which produce postural movements. The second, leading from various sensory organs to higher centers, provides sensory feedback regarding the postural state. The path through the spinal cord is no longer intact in victims of spinal cord injury and loss of normal control of muscle activity results. Functional neuromuscular stimulation (FNS) has been shown as a feasible method for obtaining muscle contraction in paraplegic and has been proposed as a means for control of antero-posterior sway to make upright posture possible for these individuals. Before muscle can be controlled through the use of FNS, the response of muscle to electrical stimulation must be understood. In past studies, linear control theory has been applied to the analysis of this response and to the testing of various controllers. The aim of this study was to demonstrate some control issues in FNS using linear control theory, as it applies to electrical stimulation of muscle for stabilization of posture. The linearity of the muscle response was improved through closed-loop control using pole compensation techniques. The excess phase shift of the system due to the time delay in the muscle response, however, limits the ability to increase the open-loop gain in order to obtain improved performance. A suggestion for further study is the application of this methodology for uses in posture control.     

New algorithm to control a cycle ergometer using electrical stimulation

Data were collected from four male subjects to determine the relationships between load, speed and muscle use during cycle ergometry. These data were then used to construct equations to govern the stimulation of muscle in paralysed individuals, during cycle ergometry induced by functional electrical stimulation (FES) of the quadriceps, gluteus maximus and hamstring muscles. The algorithm was tested on four subjects who were paralysed owing to a complete spinal cord injury between T4 and T11. Using the multivariate equation, the control of movement was improved, and work was accomplished that was double (2940 Nm min⁻¹ compared with 5880 Nm min⁻¹) that of traditional FES cycle ergometry, when muscle stimulation was also controlled by electrical stimulation. Stress on the body, assessed by cardiac output, was increased almost two-fold during maximum work with the new algorithm (8l min⁻¹ compared with 15l min⁻¹ with the new algorithm). These data support the concept that the limitation to workload that a person can achieve on FES cycle ergometry is in the control equations and not in the paralysed muscle.     

功能性电刺激技术在截瘫行走中的应用研究进展

截瘫是由脊髓损伤所造成的双下肢严重残疾,近年来的发病率呈显著上升趋势。截瘫最主要的病症就是行走能力的丧失。经过40多年的研究表明,功能性电刺激技术能成功地恢复截瘫患者的部分运动功能,是现代康复工程领域很有应用前景的一项新技术,正在受到越来越多的重视。本文专门针对用于截瘫行走的功能性电刺激技术,介绍了与之相关的背景知识和研究进展。     

Frequency response characteristics of ankle plantar flexors in humans following spinal cord injury: relation to degree of spasticity

Frequency response characteristics of the ankle plantar flexors were studied in adults both with and without spinal cord injury (SCI) to determine how the muscle contractile properties change after SCI, and to see if there is a relation between the severity of spasticity and how the properties change. Ten controls and ten complete, chronic spinal cord injured subjects were tested, where the tibial nerve was stimulated electrically in a stochastic manner with the ankle fixed isometrically at various joint angles. A nonparametric linear frequency response function was derived, from which a second-order transfer function was calculated. The contractile dynamics were then characterized by the three classic second-order parameters: gain, damping ratio, and natural frequency. We found that in subjects with low degrees of spasticity (as determined by clinical evaluation), the contractile dynamics presented the largest changes, in which the speed of contraction increased significantly while there were no statistical differences in the gains between the two groups. This similarity emerged even though there was noticeable atrophy in the SCI patient group. Differences between the controls and subjects with high levels of spasticity were markedly different, in that these SCI subjects had slower contractile speeds than the controls, but significantly lower gains. Moderately spastic subjects fell somewhere in between, where the speed of muscle contraction increased modestly yet the gain was significantly smaller than that of the control subjects. These findings indicate that in subjects with chronic spinal cord injury, the severity of spasticity can significantly influence the degree of change in muscle contractile properties. It appears that high degrees of spasticity tend to preserve contractile dynamics, while in less spastic subjects, muscle contractile properties may display faster response characteristics. © 2002 Biomedical Engineering Society. PAC2002: 8719Ff, 8717Nn, 8754Dt, 8719Rr     

The relationship between exercise work intervals and duration of exercise on lower extremity training induced by electrical stimulation in humans with spinal cord injuries

A group of 90 male paraplegics were studied to determine the optimal training protocol for isokinetic exercise induced by functional electrical stimulation of the quadriceps muscles. The parameters that were varied were the number of training sessions a week, the length of the training sessions each day, and the work-rest intervals in each training session. Training for 3 days a week for 30 min a day with 6 s of exercise and 6 s of rest proved the optimal protocol. Training for 5 days or for 1 day a week was not as effective in training strength or endurance. A combination of 50% work and 50% rest produced a much greater gain in strength and endurance than work:rest ratios of 66%:33% or 25%:75%. When training was conducted for 5 min, 15 min or 30 min each day, the greatest increase was found when the muscles were exercised for 30 min each day. While more variables need to be examined, this study has provided some initial guidelines for isokinetic training of humans using electrical stimulation. Accepted: 9 April 2000     

Short-term effects of functional electrical stimulation on motor-evoked potentials in ankle flexor and extensor muscles

Stimulating sensory afferents can increase corticospinal excitability. Intensive use of a particular part of the body can also induce reorganization of neural circuits (use-dependent plasticity) in the central nervous system (CNS). What happens in the CNS when the nerve stimulation is applied in concert with the use of particular muscle groups? The purpose of this study was to investigate short-term effects of electrical stimulation of the common peroneal (CP) nerve during walking on motor-evoked potentials (MEPs) in the ankle flexors and extensors in healthy subjects. Since the stimulation was applied during the swing phase of the step cycle when the ankle flexors are active, this is referred to as functional electrical stimulation (FES). The following questions were addressed: (1) can FES during walking increase corticospinal excitability more effectively than passively received repetitive nerve stimulation and (2) does walking itself improve the descending connection. FES was delivered using a foot drop stimulator that activates ankle dorsiflexors during the swing phase of the step cycle. MEPs in the tibialis anterior (TA) and soleus muscles were measured before, between, and after periods of walking with or without FES, using transcranial magnetic stimulation. After 30 min of walking with FES, the half-maximum peak-to-peak MEP (MEP_h) in the TA increased in amplitude and this facilitatory effect lasted for at least 30 min. In contrast, walking had no effects on the TA MEP_h without FES. The increase in the TA MEP_h with FES (~40%) was similar to that with repetitive CP nerve stimulation at rest. The soleus MEP_h was also increased after walking with FES, but not without FES, which differs from the previous observation with CP nerve stimulation at rest. With FES, the TA silent period at MEP_h was unchanged or slightly decreased, while it increased after walking without FES. Increased cortical excitability accompanied by unchanged cortical inhibition (no changes in the silent period with FES) suggests that FES did not simply increase general excitability of the cortex, but had specific effects on particular cortical neurons.     

Initiating extension of the lower limbs in subjects with complete spinal cord injury by epidural lumbar cord stimulation

We provide evidence that the human spinal cord is able to respond to external afferent input and to generate a sustained extension of the lower extremities when isolated from brain control. The present study demonstrates that sustained, nonpatterned electrical stimulation of the lumbosacral cord—applied at a frequency in the range of 5–15 Hz and a strength above the thresholds for twitches in the thigh and leg muscles—can initiate and retain lower-limb extension in paraplegic subjects with a long history of complete spinal cord injury. We hypothesize that the induced extension is due to tonic input applied by the epidural stimulation to primary sensory afferents. The induced volleys elicit muscle twitches (posterior root muscle-reflex responses) at short and constant latency times and coactivate the configuration of the lumbosacral interneuronal network, presumably via collaterals of the primary sensory neurons and their connectivity with this network. We speculate that the volleys induced externally to the lumbosacral network at a frequency of 5–15 Hz initiate and retain an extension pattern generator organization. Once established, this organization would recruit a larger population of motor units in the hip and ankle extensor muscles as compared to the flexors, resulting in an extension movement of the lower limbs. In the electromyograms of the lower-limb muscle groups, such activity is reflected as a characteristic spatiotemporal pattern of compound motor-unit potentials.Abbreviations C Cervical - CMUP Compound motor-unit potential - EMG Potential - CNS Central nervous system - EMG Electromyography, electromyographic - H Hamstring - L Lumbar - MLR Mesencephalic locomotor region - PARA Paraspinal muscles - Q Quadriceps - S Sacral - SCI Spinal cord injury, spinal cord-injured - SCS Spinal cord stimulation - T Thoracic - TA Tibialis anterior - TS Triceps surae     

Functional electrical stimulation for the upper limb in tetraplegic spinal cord injury: a systematic review

Abstract

Technological advances have helped to improve functional ability in spinal cord injury survivors. The aim of this study is to systematically review the evidence for functional electrical stimulation (FES) on functional tasks involving the upper limb in people with spinal cord injuries. The authors systematically searched from September 2009 to September 2014 in relevant databases using a combination of keywords covering spinal cord injury and FES. Studies were selected using pre-determined criteria. The search yielded 144 studies. Only five studies met the inclusion criteria. All five reported improvements immediately and at follow-up in functional ability as a result of FES or FES combined with conventional therapy. There is some preliminary evidence that FES may reduce disability due to upper limb-related activity limitations in tetraplegic spinal cord injury. Further work needs to examine the role of FES in more detail and in combination with other treatments.     

Reduction in inflammation-induced sensitization of dorsal horn neurons by transcutaneous electrical nerve stimulation in anesthetized rats

Transcutaneous electrical nerve stimulation (TENS) is utilized to treat a variety of painful conditions. Inflamed animals present with an increased response to noxious stimuli, i.e., hyperalgesia, at the site of injury (primary hyperalgesia) and outside the site of injury (secondary hyperalgesia). Further, following acute inflammation, dorsal horn neurons show an increased responsiveness to peripherally applied stimuli, which has been termed sensitization. Previous studies demonstrate a reduction in dorsal horn neuron activity following TENS treatment in normal animals and a reduction in primary and secondary hyperalgesia in acutely inflamed animals. The purpose of this study was to examine the effects of TENS on dorsal horn neurons sensitized by acute inflammation. Extracellular recordings from wide dynamic range (WDR), high threshold (HT) and low threshold (LT) dorsal horn neurons in anesthetized rats were assessed for spontaneous activity, responses to innocuous and noxious mechanical stimulation and receptive field size. Responses were measured before and 3 h after induction of inflammation, and immediately and 1 h after application of either high (100 Hz) or low (4 Hz) frequency TENS (motor intensity, pulse duration = 100 microseconds). TENS was applied to the inflamed paw to encompass the receptive field of the neuron for 20 min. WDR and HT dorsal horn neurons sensitized to mechanical stimulation after induction of inflammation. Application of either high or low frequency TENS to the inflamed paw reduced both innocuous and noxious evoked responses of WDR and HT dorsal horn neurons immediately and 1 h after treatment with TENS. Comparison of responses after TENS with baseline responses showed that the evoked responses in the majority of WDR and HT cells returned to or fell below baseline responses. TENS had no effect on responses of LT neurons. In summary, central neuron sensitization is reduced by TENS and may underlie the reduction in hyperalgesia observed after treatment with TENS.     

Identification of ankle plantar-flexors dynamics in response to electrical stimulation

Modeling the muscle response to functional electrical stimulation (FES) is an essential step in the design of closed-loop controlled neuroprostheses. This study was aimed at identifying the dynamic response of ankle plantar-flexors to FES during quiet standing. Thirteen healthy subjects stood in a standing frame that locked the knee and hip joints. The ankle plantar-flexors were stimulated bilaterally through surface electrodes and the generated ankle torque was measured. The pulse amplitude was sinusoidally modulated at five different frequencies. The pulse amplitude and the measured ankle torque fitted by a sine function were considered as input and output, respectively. First-order and critically-damped second-order linear models were fitted to the experimental data. Both models fitted similarly well to the experimental data. The coefficient of variation of the time constant among subjects was smaller in the case of the second-order model compared to the first-order model (18.1% vs. 79.9%, p < 0.001). We concluded that the critically-damped second-order model more consistently described the relationship between isometric ankle torque and surface FES pulse amplitude, which was applied to the ankle plantar-flexors during quiet standing.     

Non-intrusive real-time breathing pattern detection and classification for automatic abdominal functional electrical stimulation

Abdominal Functional Electrical Stimulation (AFES) has been shown to improve the respiratory function of people with tetraplegia. The effectiveness of AFES can be enhanced by using different stimulation parameters for quiet breathing and coughing. The signal from a spirometer, coupled with a facemask, has previously been used to differentiate between these breath types. In this study, the suitability of less intrusive sensors was investigated with able-bodied volunteers. Signals from two respiratory effort belts, positioned around the chest and the abdomen, were used with a Support Vector Machine (SVM) algorithm, trained on a participant by participant basis, to classify, in real-time, respiratory activity as either quiet breathing or coughing. This was compared with the classification accuracy achieved using a spirometer signal and an SVM. The signal from the belt positioned around the chest provided an acceptable classification performance compared to the signal from a spirometer (mean cough (c) and quiet breath (q) sensitivity (Se) of Se^c = 92.9% and Se^q = 96.1% vs. Se^c = 90.7% and Se^q = 98.9%). The abdominal belt and a combination of both belt signals resulted in lower classification accuracy. We suggest that this novel SVM classification algorithm, combined with a respiratory effort belt, could be incorporated into an automatic AFES device, designed to improve the respiratory function of the tetraplegic population.