Functional electrical stimulation using microstimulators to correct foot drop: a case study.

This paper presents a case study that tested the feasibility and efficacy of using injectable microstimulators (BIONs) in a functional electrical stimulation (FES) device to correct foot drop. Compared with surface stimulation of the common peroneal nerve, stimulation with BIONs provides more selective activation of specific muscles. For example, stimulation of the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles with BIONs produces ankle flexion without excessive inversion or eversion of the foot (i.e., balanced flexion). Efficacy was assessed using a 3-dimensional motion analysis of the ankle and foot trajectories during walking with and without stimulation. Without stimulation, the toe on the affected leg drags across the ground. BION stimulation of the TA muscle and deep peroneal nerve (which innervates TA and EDL) elevates the foot such that the toe clears the ground by 3 cm, which is equivalent to the toe clearance in the less affected leg. The physiological cost index (PCI) measured effort during walking. The PCI equals the change in heart rate (from rest to activity) divided by the walking speed; units are beats per metre. The PCI is high without stimulation (2.29 +/- 0.37, mean +/- SD) and greatly reduced with surface (1.29 +/- 0.10) and BIONic stimulation (1.46 +/- 0.24). Also, walking speed increased from 9.4 +/- 0.4 m/min without stimulation to 19.6 +/- 2.0 m/min with surface and 17.8 +/- 0.7 m/min with BIONic stimulation. These results suggest that FES delivered by a BION is an alternative to surface stimulation and provides selective control of muscle activation.

The role of neuromuscular properties in determining the end-point of a movement.

How does the activation of several muscles combine to produce reliable multijoint movements? To study this question, we stimulated up to six sites in muscles, nerves, and the spinal cord. Flexion and extension of the hip, knee, and ankle were elicited in anesthetized and decerebrate cats. The movements occurred largely in the sagittal plane against a constant spring load and covered most of the passive range of motion of the cat's limb. The movements of the end-point (foot) were compared with predictions based on vectorial summation of end-point movements elicited by stimulating single electrodes. The lengths of the movements produced by stimulating more than one site exceeded what was expected from linear summation for small movements (<3 cm) and showed a less than linear summation for large movements (>11 cm). The data were compared with muscle and limb models. Since the deviations from linearity were predictable as a function of distance, adjustments might easily be learned by trial and error. The summation was less complete for spinal stimulation, compared to nerve and muscle stimulation, so spinal circuits do not appear to compensate for the nonlinearities. Movements were elicited from positions of the limb not only in a neutral position, but also in front and behind the neutral position. A degree of convergence was seen, even with stimulation of some individual muscles, but the convergence increased as more muscles were stimulated and more joints were actively involved. This suggests that convergence to an equilibrium-point arises at least partly from muscle properties. In conclusion, there are deviations from linear vectorial summation, and these deviations increase when more muscles are stimulated. The convergence to an equilibrium-point may simplify the computations needed to produce movements involving many muscles.

A wheelchair modified for leg propulsion using voluntary activity or electrical stimulation.

A commercially available wheelchair has been modified for propulsion by movements of the lower legs. The feet are attached securely to a foot rest that can rotate around the knee joint. Movement is generated either with residual voluntary activation of the quadriceps (knee extensor) and hamstring (knee flexor) muscles, or with electrical stimulation of these muscles, if voluntary control is absent. Either a chain or a lever can couple the movements through a gearbox to the wheel to propel the wheelchair forward. Control of a wheelchair with the legs is more efficient than using the arms and has the potential to increase the mobility and whole-body fitness of many wheelchair users, but there is considerable variability between subjects. To address this variability, we measured for individual subjects the passive properties of the legs and foot at rest (effective stiffness and viscosity), the length-tension (torque-angle) properties of the active muscle groups, as well as their force-velocity curve and their activation and fatigue rates. The measured values were then inserted into a model of the leg-propelled wheelchair. The purpose of this paper is to test whether the model could predict the performance of individual subjects accurately and could be used, for example, to optimize the speed of the wheelchair for a given subject.

Limb movements generated by stimulating muscle, nerve and spinal cord.

We have compared the movements generated by stimulation of muscle, nerve, spinal roots and spinal cord in anesthetized, decerebrate and spinalized cats. Each method produced a full range of movements of the cat's hind limb in the sagittal plane against a spring load, except for stimulation of the roots. Stimulation of the dorsal roots produced movements that were mainly up and forward, whereas stimulation of the ventral roots produced complementary movements (down and backward). Results from stimulation in the intermediate areas of the spinal cord were compared to predictions of the "movement primitives" hypothesis. We could not confirm that the directions were independent of stimulus amplitude or the state of descending inputs. Pros and cons of stimulating at some sites were provisionally considered for the reliable control of limb movements with functional electrical stimulation (FES) in clinical conditions.

Improved efficiency with a wheelchair propelled by the legs using voluntary activity or electric stimulation.

OBJECTIVE: To determine whether a new leg-propelled wheelchair provides enhanced efficiency and mobility to wheelchair users. DESIGN: Observational; subjects were tested while wheeling with the arms and legs and while walking (where possible) for 4-minute periods in random order with approximately 10-minute rest periods between exercise sets. SETTING: Tests were done on an indoor 200-meter track. PATIENTS: Group 1, 13 controls; group 2, 9 persons with complete spinal cord injury (SCI); group 3, 13 persons with other motor disorders (retaining some voluntary control of the legs). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Physiological Cost Index (PCI), (computed as change in heart rate divided by velocity of movement) and oxygen consumption (VO(2)) RESULTS: Arm wheeling took significantly more effort (mean PCI =.52 beats/m) than walking (.33 beats/m) in control subjects. Leg wheeling was most efficient (.23), requiring less than half the effort of arm wheeling and 30% less effort than walking. For SCI subjects, leg wheeling with functional electric stimulation (FES) required less than half the effort (.18) of arm wheeling (.40). The FES group could not walk. Subjects in group 3 could walk, but with substantial effort (1.81) compared with arm (.76) or leg wheeling (.64). Results for VO(2) were similar. CONCLUSIONS: Better wheelchair efficiency can be obtained for many disabled individuals, by moving the leg muscles voluntarily or with FES.

Electrical stimulation: can it increase muscle strength and reverse osteopenia in spinal cord injured individuals?

OBJECTIVE: To study the extent to which atrophy of muscle and progressive weakening of the long bones after spinal cord injury (SCI) can be reversed by functional electrical stimulation (FES) and resistance training. DESIGN: A within-subject, contralateral limb, and matching design. SETTING: Research laboratories in university settings. PARTICIPANTS: Fourteen patients with SCI (C5 to T5) and 14 control subjects volunteered for this study. INTERVENTIONS: The left quadriceps were stimulated to contract against an isokinetic load (resisted) while the right quadriceps contracted against gravity (unresisted) for 1 hour a day, 5 days a week, for 24 weeks. MAIN OUTCOME MEASURES: Bone mineral density (BMD) of the distal femur, proximal tibia, and mid-tibia obtained by dual energy x-ray absorptiometry, and torque (strength). RESULTS: Initially, the BMD of SCI subjects was lower than that of controls. After training, the distal femur and proximal tibia had recovered nearly 30% of the bone lost, compared with the controls. There was no difference in the mid-tibia or between the sides at any level. There was a large strength gain, with the rate of increase being substantially greater on the resisted side. CONCLUSION: Osteopenia of the distal femur and proximal tibia and the loss of strength of the quadriceps can be partly reversed by regular FES-assisted training.

Minimizing discomfort with surface neuromuscular stimulation.

The purpose of this study was to evaluate the effects of stimulus parameters, electrode types, and electrode positions on the perception of discomfort during lower extremity surface neuromuscular stimulation. Ten normal and eight neurologically impaired (four incomplete spinal cord and four stroke) subjects were enrolled. Neurologically impaired subjects had some sensation, although it was often reduced. Parameters of the stimulation were varied in a way that produced the same level of ankle dorsiflexion, as measured with a goniometer. Discomfort was assessed after each stimulation with a 0-10 verbal scale (0, no discomfort; 10, worst pain). Increasing the pulse frequency was associated with increased discomfort for subjects in both groups (p > 0.05). Increasing the pulse duration was associated with increased discomfort in the neurologically impaired subjects (p > 0.05), but not in the normal subjects (p > 0.05). The electrode size and type had no effects on discomfort (p > 0.05). Stimulation of the peroneal nerve over the fibular head was better tolerated than the direct motor point stimulation of the tibialis anterior motor point (p < 0.05). The data suggest that to minimize discomfort, surface stimulation should be applied over nerves rather than motor points, and frequency and pulse duration should be set as low as possible for a given degree of contraction.

Multicenter evaluation of electrical stimulation systems for walking.

OBJECTIVE: To test the long-term benefits of several noninvasive systems for functional electrical stimulation (FES) during walking. DESIGN: Forty subjects (average years since injury, 5.4) were studied in four centers for an average time of 1 year. Gait parameters were tested for all subjects with and without FES. Thus, subjects served as their own controls, since the specific effect of using FES could be separated from improvements resulting from other factors (e.g., training). SETTING: Subjects used the devices in the community, but were tested in a university or hospital setting. PATIENTS: Subjects with spinal cord injury (n = 31) were compared to subjects with cerebral damage (n = 9). MAIN OUTCOME MEASURES: Gait parameters (speed, cycle time, stride length). Acceptance was studied by means of a questionnaire. RESULTS: Some initial improvement in walking speed (average increase of >20%) occurred, and continuing gains were seen (average total improvement, 45%). The largest relative gains were seen in the slowest walkers (speeds of <0.3 m/sec). Acceptance of the FES systems was good and improved systems have been developed using feedback from the subjects. CONCLUSIONS: Based on the improvements in speed and the acceptance of these FES systems, a greatly increased role for FES in treating gait disorders is suggested.

Optimal control of walking with functional electrical stimulation: a computer simulation study.

Bipedal locomotion was simulated to generate a pattern of activating muscles for walking using electrical stimulation in persons with spinal cord injury (SCI) or stroke. The simulation presented in this study starts from a model of the body determined with user-specific parameters, individualized with respect to the lengths, masses, inertia, muscle and joint properties. The trajectory used for simulation was recorded from an able-bodied subject while walking with ankle-foot orthoses. A discrete mathematical model and dynamic programming were used to determine the optimal control. A cost function was selected as the sum of the squares of the tracking errors from the desired trajectories, and the weighted sum of the squares of agonist and antagonist activations of the muscle groups acting around the hip and knee joints. The aim of the simulation was to study plausible trajectories keeping in mind the limitations imposed by the spinal cord injury or stroke (e.g., spasticity, decreased range of movements in some joints, limited strength of paralyzed, externally activated muscles). If the muscles were capable of generating the movements required and the trajectory was achieved, then the simulation provided two kinds of information: 1) timing of the onset and offset of muscle activations with respect to the various gait events and 2) patterns of activation with respect to the maximum activation. These results are important for synthesizing a rule-based controller.

Estimating mechanical parameters of leg segments in individuals with and without physical disabilities.

Methods are described for estimating the inertia, viscosity, and stiffness of the lower leg around the knee and of the whole leg around the hip that are applicable even to persons with considerable spasticity. These involve: 1) a "pull" test in which the limb is slowly moved throughout its range of motion while measuring angles (with an electrogoniometer) and torques (with a hand-held dynamometer) to determine passive stiffness and 2) a "pendulum" test in which the limb is moved against gravity and then dropped, while again measuring angles and torques. By limiting the extent of the movement and choosing a direction (flexion or extension) that minimizes reflex responses, the mechanical parameters can be determined accurately and efficiently using computer programs. In the sample of subjects studied (nine with disability related to spinal cord injury, head injury, or stroke, and nine with no neurological disability), the inertia of the lower leg was significantly reduced in the subjects with disability (p < 0.05) as a result of atrophy, but the stiffness and viscosity were within normal limits. The values of inertia were also compared with anthropometric data in the literature. The identification of these passive parameters is particularly important in designing systems for functional electrical stimulation of paralyzed muscles, but the methods may be widely applicable in rehabilitation medicine.

Neural signals for command control and feedback in functional neuromuscular stimulation: a review.

In current functional neuromuscular stimulation systems (FNS), control and feedback signals are usually provided by external sensors and switches, which pose problems such as donning and calibration time, cosmesis, and mechanical vulnerability. Artificial sensors are difficult to build and are insufficiently biocompatible and reliable for implantation. With the advent of methods for electrical interfacing with nerves and muscles, natural sensors are being considered as an alternative source of feedback and command signals for FNS. Decision making methods for higher level control can perform equally well with natural or artificial sensors. Recording nerve cuff electrodes have been developed and tested in animals and demonstrated to be feasible in humans for control of dorsiflexion in foot-drop and grasp in quadriplegia. Electromyographic signals, being one thousand times larger than electroneurograms, are easier to measure but have not been able to provide reliable indicators (e.g., of muscle fatigue) that would be useful in FNS systems. Animal studies have shown that information about the shape and movement of arm trajectories can be extracted from brain cortical activity, suggesting that FNS may ultimately be directly controllable from the central nervous system.

Machine learning in control of functional electrical stimulation systems for locomotion.

Two machine learning techniques were evaluated for automatic design of a rule-based control of functional electrical stimulation (FES) for locomotion of spinal cord injured humans. The task was to learn the invariant characteristics of the relationship between sensory information and the FES-control signal by using off-line supervised training. Sensory signals were recorded using pressure sensors installed in the insoles of a subject's shoes and goniometers attached across the joints of the affected leg. The FES-control consisted of pulses corresponding to time intervals when the subject pressed on the manual push-button to deliver the stimulation during FES-assisted ambulation. The machine learning techniques used were the adaptive logic network (ALN) [1] and the inductive learning algorithm (IL) [2]. Results to date suggest that, given the same training data, the IL learned faster than the ALN, while both performed the test rapidly. The generalization was estimated by measuring the test errors and it was better with an ALN, especially if past points were used to reflect the time dimension. Both techniques were able to predict future stimulation events. An advantage of the ALN over the IL was that ALN's can be retrained with new data without losing previously collected knowledge. The advantages of the IL over the ALN were that the IL produces small, explicit, comprehensible trees and that the relative importance of each sensory contribution can be quantified.

Sensory nerve recording for closed-loop control to restore motor functions.

A method is developed for using neural recordings to control functional electrical stimulation (FES) to nerves and muscles. Experiments were done in chronic cats with a goal of designing a rule-based controller to generate rhythmic movements of the ankle joint during treadmill locomotion. Neural signals from the tibial and superficial peroneal nerves were recorded with cuff electrodes and processed simultaneously with muscular signals from ankle flexors and extensors in the cat's hind limb. Cuff electrodes are an effective method for long-term chronic recording in peripheral nerves without causing discomfort or damage to the nerve. For real-time operation we designed a low-noise amplifier with a blanking circuit to minimize stimulation artifacts. We used threshold detection to design a simple rule-based control and compared its output to the pattern determined using adaptive neural networks. Both the threshold detection and adaptive networks are robust enough to accommodate the variability in neural recordings. The adaptive logic network used for this study is effective in mapping transfer functions and therefore applicable for determination of gait invariants to be used for closed-loop control in an FES system. Simple rule-bases will probably be chosen for initial applications to human patients. However, more complex FES applications require more complex rule-bases and better mapping of continuous neural recordings and muscular activity. Adaptive neural networks have promise for these more complex applications.

Electrical systems for improving locomotion after incomplete spinal cord injury: an assessment.

Simple systems for electrical stimulation (1-4 channels) with either surface, percutaneous, or implanted electrodes during locomotion were assessed in 10 subjects who had chronic, incomplete spinal cord injury (SCI). On average, the speed of locomotion was increased by 4 m/min independently of the subject's speed of locomotion without stimulation (0-50 m/min) while oxygen consumption was reduced somewhat. These simple systems can provide practical help, particularly for incomplete SCI subjects who can stand but are lacking or have very limited ability to walk. Further improvement in locomotion requires stabilization and reduction in the duration of the stance phase of locomotion.

Influence of electrical stimulation on the morphological and metabolic properties of paralyzed muscle.

Selected morphological and metabolic properties of single fibers were studied in biopsy samples from the tibialis anterior of normal control and spinal cord-injured (SCI) subjects. In the SCI subjects, one muscle was electrically stimulated progressively over 24 wk, in 6-wk blocks for less than or equal to 8 h/day, while the contralateral muscle remained untreated. The percentage of fibers classified as type I [qualitative alkaline preincubation myofibrillar adenosinetriphosphatase (ATPase)] was significantly less in the unstimulated paralyzed muscles than in the muscles of normal control subjects. Electrical stimulation increased the proportion of type I fibers in the SCI subjects. For both type I and type II fibers, the cross-sectional area, activities of myofibrillar ATPase and succinate dehydrogenase, and the capillary-to-fiber ratio were also significantly less in the paralyzed muscles than in the normal control muscles. Electrical stimulation increased only the activity of succinate dehydrogenase in both fiber types of the SCI subjects. These data are discussed in relation to the electromechanical properties of the respective muscles described in an accompanying paper (J. Appl. Physiol. 72: 1393-1400, 1992). In general, the electrical stimulation protocol used in this study enhanced the oxidative capacity and endurance properties of the paralyzed muscles but had no effect on fiber size and strength.

Optimal stimulation of paralyzed muscle after human spinal cord injury.

Muscle properties change profoundly as a result of disuse after spinal cord injury. To study the extent to which these changes can be reversed by electrical stimulation, tibialis anterior muscles in complete spinal cord-injured subjects were stimulated for progressively longer times (15 min, 45 min, 2 h, and 8 h/day) in 6-wk intervals. An index of muscle endurance to repetitive stimulation doubled (from 0.4 to 0.8), contraction and half-relaxation times increased markedly (from 70 to approximately 100 ms), but little or no change was measured in twitch or tetanic tension with increasing amounts of stimulation. The changes observed with 2 h/day of stimulation brought the physiological values close to those for normal (control) subjects. A decrease in the stimulation period produced a reversal of the changes. No effects were observed in the contralateral (unstimulated) muscle at any time, nor was there evidence of decreased numbers of motor units in these subjects secondary to spinal cord injury. Motor unit properties changed in parallel with those of the whole muscle. The occasional spasms occurring in these subjects are not sufficient to maintain normal muscle properties, but these properties can largely be restored by 1-2 h/day of electrical stimulation.

Motor units in incomplete spinal cord injury: electrical activity, contractile properties and the effects of biofeedback.

The electrical and contractile properties of hand muscles in a selected population of quadriplegic subjects were studied intensively before and after EMG biofeedback. Spontaneously active motor units and units that could only be slowly and weakly activated were observed in these subjects, in addition to units that were voluntarily activated normally. This suggests a considerable overlap of surviving motor neurons to a single muscle that are below, near or above the level of a lesion. Despite the common occurrence of polyphasic potentials and other signs of neuromuscular reinnervation, the average twitch tension of single motor units in hand muscles of quadriplegic subjects was not significantly different from that in control subjects. Nor did it increase after biofeedback training that typically increased the peak surface EMG by a factor of 2-5 times. The percentage of spontaneously active units was also constant. The surface EMG may be increased during biofeedback by using higher firing rates in motor units that can already be activated, rather than by recruiting previously unavailable motor units.

[Improvement of prostheses and orthotic aids for the handicapped using electric stimulation and the registration of bioelectric signals]. / Amélioration des prothèses et aides orthotiques pour les handicapés au moyen de stimulation électrique et d'enregistrement des signaux bioélectriques.

Electro-mechanical devices can help a variety of patients with motor disabilities. Surface EMG from remaining muscles in an amputated arm can be used to control powered electronic hands, wrists and elbows. Sensory signals such as knee angle and ankle torque can be used to control the visco-elastic properties of a knee joint for above-knee amputees. Finally, percutaneous electrodes can be used to stimulate paralyzed muscles to replace hand function in quadriplegics and leg function in paraplegics. This article summarizes recent progress in each of these areas.