Control of functional electrical stimulation with extended physiological proprioception

The use of functional electrical stimulation (FES) of muscle for paraplegic locomotion, or grasp augmentation in tetraplegia, is limited by the variability in muscle response to stimulation as a result of several external and internal factors. Previous approaches to this problem have used position-servo controllers, which have been shown to function satisfactorily in the laboratory. However, such systems will fail should obstacles be encountered or should the stimulation hardware develop a fault. To prevent such potentially dangerous failures some form of sensory feedback is required. This paper describes the first application of a technique known as extended physiological proprioception (EPP) to the control of FES to compensate for muscle response variability and provide proprioceptive feedback via the appropriate sensory pathways. In the experimental system described, a paraplegic subject controlled the extension of his paralysed knee by shoulder protraction. A Bowden cable linked the two joints, and a dynamometer in this cable was used to derive the control signal for a computer-controlled stimulator which delivered surface stimulation to the quadriceps muscle group. Modelling and parameter identification were performed by analysis of the step response, and the controller was designed from consideration of the root locus. The advantages of the system, in terms of improved proprioceptive feedback and reduced limb-positioning error were assessed in a test of joint positioning accuracy with vision occluded. The EPP system showed improvements over both open and closed-loop position-servo controllers.     

Predicting the voluntary arm forces in FES-assisted standing up using neural networks

 For individuals with paraplegia, standing up requires activation of paralyzed leg muscles by an artificial functional electrical stimulation (FES) controller and voluntary control of arm forces by the individual. Any knowledge of such voluntary control, particularly its prediction, could be used to design more effective FES controllers. Therefore, artificial neural network models were developed to predict voluntary arm forces from measured angular positions of the ankle, knee, and hip joints during FES-assisted standing up in paraplegia. The training data were collected from eight paraplegic subjects in repeated standing-up trials, and divided into two categories for training and validation. The predictions of the models closely followed both the training and validation data, showing good accuracy and generalization. The comparison of the models showed that, although there are striking similarities among the voluntary controls adopted by different subjects, each subject develops his/her own `personal strategy' to control the arm forces, which is consistent from trial to trial. The level of consistency was dependent on the experience in using FES, injury level, body weight, and other subject-specific parameters. Received: 5 January 1999 / Accepted in revised form: 29 January 2001     

Efficacy and stability performance of traditional versus motion sensor-assisted strategies for FES standing

Standing by means of functional electrical stimulation (FES) after spinal cord injury is a topic widely reported in the neurorehabilitation literature. This practice commonly uses surface stimulation over the quadriceps muscle to evoke knee extension. To date, most FES neuroprostheses still operate without any artificial feedback, meaning that after a fatigue-driven knee buckle event, the stimulation amplitude or pulse width must be increased manually via button presses to re-establish knee-lock. This is often referred to as ‘hand-controlled (HC) operation’. In an attempt to provide a safer, yet clinically practical approach, this study proposed two novel strategies to automate the control of knee extension based on the kinematic feedback of four miniaturised motion sensors. These strategies were compared to the traditional HC strategy on four individuals with complete paraplegia. The standing times observed over multiple trials were in general longer for the automated strategies when compared to HC (0.5–80%). With the automated strategies, three of the subjects tended to need less upper body support over a frame to maintain balance. A stability analysis based on centre of pressure (CoP) measurements also favoured the automated strategies. This analysis also revealed that although FES standing with the assistance of a frame was likely to be safe for the subjects, their stability was still inferior to that of able-bodied individuals. Overall, the unpredictability of knee buckle events could be more effectively controlled by automated FES strategies to re-establish knee-lock when compared to the traditional user-controlled approach, thus demonstrating the safety and clinical efficacy of an automated approach.     

Walking while using a sensory tactile feedback system: potential use with a functional electrical stimulation orthosis

A major limitation in the utilization of a functional electrical stimulation (FES) orthosis for routine, daily standing and walking of the spinal-cord-injured person is that visual monitoring is required to maintain balance and the walking pace. For standing and walking to be continuous and automatic with such an orthosis, a closed-loop sensory feedback system is proposed and evaluated; it provides vibrotactile feedback as a substitute to one's own visual sensation. Eight blindfolded experimental subjects were utilized as ‘imitators’ to interpret the footfalls of a second person (the pace setter). The experimental objective was to test the hypothesis that sufficient information could be transferred by way of the sensory (tactile) feedback system to the ‘imitator’ and to determine effectively foot position and anticipate the next step of the pacesetter. Quantitative analysis evaluated the effect of three different levels of training, under two different levels of cognitive load. The results disclosed a significant improvement in subject performance at the higher training levels compared with the ‘no training’ level (P = 0.01). Neither the cognitive load nor the interaction of training and cognitive load altered significantly the effect of training on subject performance. The experimental hypothesis is therefore satisfied that sufficient information was indeed transferred using the apparatus described. Such information (when utilized in conjunction with a thorough training programme) could be used in a practical sense by a paraplegic individual to interpret his own foot steps. Through continued use and training, it is likely that this information could become subconscious and automatic. Therefore, the spinal-cord-injured person would walk with an FES orthosis in various environments and minimize or remove their reliance on visual sensory information.     

Stabilization of human standing posture using functional neuromuscular stimulation

Functional neuromuscular stimulation (FNS)/functional electrical stimulation (FES) is a potential way to restore some functionality to the limbs of patients with spinal cord injury through direct/indirect stimulation of the motoneuron. One of the constraints for wider use of FNS on paraplegic patients is the lack of efficient control algorithm. Most of the published works on FNS/FES control are based on oversimplified models of human body dynamics. An innovative control strategy for stabilizing the standing posture of paraplegic patients is proposed here which is a combination of a proportional-plus-derivative controller for motions of the skeletal system and a control action prediction mechanism to produce musculotendon activation. The goal is to produce musculotendon torque which can approximate those demanded by the controller for the skeletal system. In computer simulations, using a detailed skeletal–musculotendon–muscle activation dynamics model of human body, this FNS/FES control approach can stabilize a paraplegic patient's standing posture with the minimum number of musculotendon groups. Also, it is found that this control strategy can maintain stability even in the presence of reasonable variations in the controller's musculotendon parameters.     

Control of constraint forces and trajectories in a rich sensory and actuation environment

A simple control strategy is proposed and applied to a class of non-linear systems that have abundant sensory and actuation channels as in living systems. The main objective is the independent control of constrained trajectories of motion, and control of the corresponding constraint forces. The peripheral controller is a proportional, derivative and integral (PID) controller. A central controller produces, via pattern generators, reference signals that are the desired constrained position and velocity trajectories, and the desired constraint forces. The basic tenet of the this hybrid control strategy is the use of two mechanisms: 1. linear state and force feedback, and 2. non-linear constraint velocity feedback - sliding mode feedback. The first mechanism can be envisioned as a high gain feedback systems. The high gain attribute imitates the agonist-antagonist co-activation in natural systems. The strategy is applied to the control of the force and trajectory of a two-segment thigh-leg planar biped leg with a mass-less foot cranking a pedal that is analogous to a bicycle pedal. Five computational experiments are presented to show the effectiveness of the strategy and the performance of the controller. The findings of this paper are applicable to the design of orthoses and prostheses to supplement functional electrical stimulation for support purposes in the spinally injured cases.     

Effects of 20 days horizontal bed rest on maintaining upright standing posture in young persons

The effects of 20 days horizontal bed rest (BR) on postural reflex were studied by measuring fluctuation of center of gravity in the body during two legs or one leg upright standing in 10 young volunteers. The fluctuation was decided as total moving distance of the center recorded during 60sec standing on a force plate. The stability was measured by the moved area. After BR, the moving distance increased during two legs standing with open eyes (p<0.05), but statistically unchanged with closed eyes. The moving area decreased during right one-leg standing with closed eyes (p<0.05), but unchanged during left one-leg standing. Despite with open eyes the increased distance suggested that postural reflexes to maintain upright position were probably decreased by increased unsuitable feedback informations from the visual receptor deconditioning during BR. The decreased area during right one-leg standing with closed eyes also suggested that the declined standing posture reflex was probably related to more rapidly lowered functions for maintaining standing position in the dominating leg than in the other.     

Advances in functional electrical stimulation (FES)

This review discusses the advancements that are needed to enhance the effects of electrical stimulation for restoring or assisting movement in humans with an injury/disease of the central nervous system. A complex model of the effects of electrical stimulation of peripheral systems is presented. The model indicates that both the motor and sensory systems are activated by electrical stimulation. We propose that a hierarchical hybrid controller may be suitable for functional electrical stimulation (FES) because this type of controller acts as a structural mimetic of its biological counterpart. Specific attention is given to the neural systems at the periphery with respect to the required electrodes and stimulators. Furthermore, we note that FES with surface electrodes is preferred for the therapy, although there is a definite advantage associated with implantable technology for life-long use. The last section of the review discusses the potential need to combine FES and robotic systems to provide assistance in some cases.     

A model of neuro-musculo-skeletal system for human locomotion under position constraint condition

The human locomotion was studied on the basis of the interaction of the musculo-skeletal system, the neural system and the environment. A mathematical model of human locomotion under position constraint condition was established. Besides the neural rhythm generator, the posture controller and the sensory system, the environment feedback controller and the stability controller were taken into account in the model. The environment feedback controller was proposed for two purposes, obstacle avoidance and target position control of the swing foot. The stability controller was proposed to imitate the self-balancing ability of a human body and improve the stability of the model. In the stability controller, the ankle torque was used to control the velocity of the body gravity center. A prediction control algorithm was applied to calculate the torque magnitude of the stability controller. As an example, human stairs climbing movement was simulated and the results were given. The simulation result proved that the mathematical modeling of the task was successful.     

Optimal combination of minimum degrees of freedom to be actuated in the lower limbs to facilitate arm-free paraplegic standing

Arm-free paraplegic standing via functional electrical stimulation (FES) has drawn much attention in the biomechanical field as it might allow a paraplegic to stand and simultaneously use both arms to perform daily activities. However, current FES systems for standing require that the individual actively regulates balance using one or both arms, thus limiting the practical use of these systems. The purpose of the present study was to show that actuating only six out of 12 degrees of freedom (12-DOFs) in the lower limbs to allow paraplegics to stand freely is theoretically feasible with respect to multibody stability and physiological torque limitations of the lower limb DOF. Specifically, the goal was to determine the optimal combination of the minimum DOF that can be realistically actuated using FES while ensuring stability and able-bodied kinematics during perturbed arm-free standing. The human body was represented by a three-dimensional dynamics model with 12-DOFs in the lower limbs. Nakamura's method (Nakamura, Y., and Ghodoussi, U., 1989, "Dynamics Computation of Closed-Link Robot Mechanisms With Nonredundant and Redundant Actuators," IEEE Trans. Rob. Autom., 5(3), pp. 294-302) was applied to estimate the joint torques of the system using experimental motion data from four healthy subjects. The torques were estimated by applying our previous finding that only 6 (6-DOFs) out of 12-DOFs in the lower limbs need to be actuated to facilitate stable standing. Furthermore, it was shown that six cases of 6-DOFs exist, which facilitate stable standing. In order to characterize each of these cases in terms of the torque generation patterns and to identify a potential optimal 6-DOF combination, the joint torques during perturbations in eight different directions were estimated for all six cases of 6-DOFs. The results suggest that the actuation of both ankle flexionextension, both knee flexionextension, one hip flexionextension, and one hip abductionadduction DOF will result in the minimum torque requirements to regulate balance during perturbed standing. To facilitate unsupported FES-assisted standing, it is sufficient to actuate only 6-DOFs. An optimal combination of 6-DOFs exists, for which this system can generate able-bodied kinematics while requiring lower limb joint torques that are producible using contemporary FES technology. These findings suggest that FES-assisted arm-free standing of paraplegics is theoretically feasible, even when limited by the fact that muscles actuating specific DOFs are often denervated or difficult to access.     

Thoracic leg motoneurons in the isolated CNS of adult Manduca produce patterned activity in response to pilocarpine,which is distinct from that produced in larvae

In the hawkmoth, Manduca sexta, thoracic leg motoneurons survive the degeneration of the larval leg muscles to innervate new muscles of the adult legs. The same motoneurons, therefore, participate in the very different modes of terrestrial locomotion that are used by larvae (crawling) and adults (walking). Consequently, changes in locomotor behavior may reflect changes in both the CNS and periphery. The present study was undertaken to determine whether motor patterns produced by the isolated CNS of adult Manduca, in the absence of sensory feedback, would resemble adult specific patterns of coordination. Pilocarpine, which evokes a fictive crawling motor pattern from the isolated larval CNS, also evoked robust patterned activity from leg motoneurons in the isolated adult CNS. As in the larva, levator and depressor motoneurons innervating the same leg were active in antiphase. Unlike fictive crawling, however, bursts of activity in levator or depressor motoneurons of one leg alternated with bursts in the homologous motoneurons innervating the opposite leg of the same segment and the leg on the same side in the adjacent segment. The most common mode of intersegmental activity generated by the isolated adult CNS resembled an alternating tripod gait, which is displayed, albeit infrequently, during walking in intact adult Manduca. A detailed analysis revealed specific differences between the patterned motor activity that is evoked from the isolated adult CNS and activity patterns observed during walking in intact animals, perhaps indicating an important role for sensory feedback. Nevertheless, the basic similarity to adult walking and clear distinctions from the larval fictive crawling pattern suggest that changes within the CNS contribute to alterations in locomotor activity during metamorphosis. Electronic Publication     

Reliability of System Identification Techniques to Assess Standing Balance in Healthy Elderly

Objectives

System identification techniques have the potential to assess the contribution of the underlying systems involved in standing balance by applying well-known disturbances. We investigated the reliability of standing balance parameters obtained with multivariate closed loop system identification techniques.

Methods

In twelve healthy elderly balance tests were performed twice a day during three days. Body sway was measured during two minutes of standing with eyes closed and the Balance test Room (BalRoom) was used to apply four disturbances simultaneously: two sensory disturbances, to the proprioceptive and the visual system, and two mechanical disturbances applied at the leg and trunk segment. Using system identification techniques, sensitivity functions of the sensory disturbances and the neuromuscular controller were estimated. Based on the generalizability theory (G theory), systematic errors and sources of variability were assessed using linear mixed models and reliability was assessed by computing indexes of dependability (ID), standard error of measurement (SEM) and minimal detectable change (MDC).

Results

A systematic error was found between the first and second trial in the sensitivity functions. No systematic error was found in the neuromuscular controller and body sway. The reliability of 15 of 25 parameters and body sway were moderate to excellent when the results of two trials on three days were averaged. To reach an excellent reliability on one day in 7 out of 25 parameters, it was predicted that at least seven trials must be averaged.

Conclusion

This study shows that system identification techniques are a promising method to assess the underlying systems involved in standing balance in elderly. However, most of the parameters do not appear to be reliable unless a large number of trials are collected across multiple days. To reach an excellent reliability in one third of the parameters, a training session for participants is needed and at least seven trials of two minutes must be performed on one day.     

A mathematical modeling study of inter-segmental coordination during stick insect walking

The biomechanical conditions for walking in the stick insect require a modeling approach that is based on the control of pairs of antagonistic motoneuron (MN) pools for each leg joint by independent central pattern generators (CPGs). Each CPG controls a pair of antagonistic MN pools. Furthermore, specific sensory feedback signals play an important role in the control of single leg movement and in the generation of inter-leg coordination or the interplay between both tasks. Currently, however, no mathematical model exists that provides a theoretical approach to understanding the generation of coordinated locomotion in such a multi-legged locomotor system. In the present study, I created such a theoretical model for the stick insect walking system, which describes the MN activity of a single forward stepping middle leg and helps to explain the neuronal mechanisms underlying coordinating information transfer between ipsilateral legs. In this model, CPGs that belong to the same leg, as well as those belonging to different legs, are connected by specific sensory feedback pathways that convey information about movements and forces generated during locomotion. The model emphasizes the importance of sensory feedback, which is used by the central nervous system to enhance weak excitatory and inhibitory synaptic connections from front to rear between the three thorax-coxa-joint CPGs. Thereby the sensory feedback activates caudal pattern generation networks and helps to coordinate leg movements by generating in-phase and out-of-phase thoracic MN activity.     

Modeling biological motor control for human locomotion with functional electrical stimulation

This paper develops a novel control system for functional electrical stimulation (FES) locomotion, which aims to generate normal locomotion for paraplegics via FES. It explores the possibility of applying ideas from biology to engineering. The neural control mechanism of the biological motor system, the central pattern generator, has been adopted in the control system design. Some artificial control techniques such as neural network control, fuzzy logic, control and impedance control are incorporated to refine the control performance. Several types of sensory feedback are integrated to endow this control system with an adaptive ability. A musculoskeletal model with 7 segments and 18 muscles is constructed for the simulation study. Satisfactory simulation results are achieved under this FES control system, which indicates a promising technique for the potential application of FES locomotion in future.     

Motor pattern selection by combinatorial code of interneuronal pathways

We use a modeling approach to examine ideas derived from physiological network analyses, pertaining to the switch of a motor control network between two opposite control modes. We studied the femur–tibia joint control system of the insect leg, and its switch between resistance reflex in posture control and “active reaction” in walking, both elicited by the same sensory input. The femur–tibia network was modeled by fitting the responses of model neurons to those obtained in animals. The strengths of 16 interneuronal pathways that integrate sensory input were then assigned three different values and varied independently, generating a database of more than 43 million network variants. We demonstrate that the same neural network can produce the two different behaviors, depending on the combinatorial code of interneuronal pathways. That is, a switch between behaviors, such as standing to walking, can be brought about by altering the strengths of selected sensory integration pathways. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Real-time gait assessment utilizing a new way of accelerometry

Real-time registration of body segment angles is essential in artificial body position control. A new method is presented for the real-time calculation of the lower extremity angles using data obtained from pairs of two one-dimensional accelerometers. It is shown that, assuming rigid-body dynamics and simple hinge joints, relative angles (i.e. angles between segments) can be calculated without integration, thereby solving the problem of integration drift normally associated with accelerometry. During the stance phase of walking, the relative angles can be transformed to absolute angles (i.e. relative to the gravitational field direction) for the different leg segments. The feasibility of relative angle calculation is demonstrated by calculation of the knee angle of a healthy subject. Stability and resolution were demonstrated with measurements during standing. Measurements during standing up, sitting down and walking showed that shock (heel-strike) and skin movements, due to movements of the underlying muscle tissue, are the main error sources. Additional signal processing, e.g. low-pass filtering, can be used to diminish this error. The accuracy of the knee angle found is shown to be high enough to be used in a feedback controller for functional electrostimulation of the lower extremities.     

Functional recovery following manipulation of muscles and sense organs in the stick insect leg

We studied functional recovery of leg posture and walking behaviour in the femur-tibia joint control system of stick insects. Leg extensions in resting animals and during walking are produced by different parts of a single extensor muscle. (a) Ablation of the muscle part responsible for fast movements prevented leg extension during the swing phase. Resting posture remained unaffected. Within a few post-operative days, extension movements recovered, provided that sensory feedback was available. Extension movements were now driven by the muscle part which in intact animals controls the resting posture only. (b) Selective ablation of this (slow) muscle part affected the resting posture, while walking was unaffected. The resting posture partly recovered during subsequent days. To test the range of functional recovery and underlying mechanisms, we additionally transected muscle motor innervation, or we inverted or ablated sensory feedback. We found that recovery was based on both muscular and neuronal mechanisms. The latter required appropriate sensory feedback for the process of recovery, but not for the maintenance of the recovered state. Our results thus indicate the existence of a sensory template that guides recovery. Recovery was limited to a behavioural range that occurs naturally in intact animals, though in different behavioural contexts.     

Contribution of sensory feedback to ongoing ankle extensor activity during the stance phase of walking

Numerous investigations over the past 15 years have demonstrated that sensory feedback plays a critical role in establishing the timing and magnitude of muscle activity during walking. Here we review recent studies reporting that sensory feedback makes a substantial contribution to the activation of extensor motoneurons during the stance phase. Quantitative analysis of the effects of loading and unloading ankle extensor muscles during walking on a horizontal surface has shown that sensory feedback can increase the activity of ankle extensor muscles by up to 60%. There is currently some uncertainty about which sensory receptors are responsible for this enhancement of extensor activity, but likely candidates are the secondary spindle endings in the ankle extensors of humans and the Golgi tendon organs in the ankle extensors of humans and cats. Two important issues arise from the finding that sensory feedback from the leg regulates the magnitude of extensor activity. The first is the extent to which differences in the magnitude of activity in extensor muscles during different locomotor tasks can be directly attributed to changes in the magnitude of sensory signals, and the second is whether the enhancement of extensor activity is determined primarily by feedback from a specific group of receptors or from numerous groups of receptors distributed throughout the leg. Limitations of current experimental strategies prevent a straightforward empirical resolution of these issues. A potentially fruitful approach in the immediate future is to develop models of the known and hypothesized neuronal networks controlling motoneuronal activity, and use these simulations to control forward dynamic models of the musculo-skeletal system. These simulations would help understand how sensory signals are modified with a change in locomotor task and, in conjunction with physiological experiments, establish the extent to which these modifications can account for changes in the magnitude of motoneuronal activity.     

Steadiness of Spinal Regions during Single-Leg Standing in Older Adults with and without Chronic Low Back Pain

The aims of this study were to compare the steadiness index of spinal regions during single-leg standing in older adults with and without chronic low back pain (LBP) and to correlate measurements of steadiness index with the performance of clinical balance tests. Thirteen community-dwelling older adults (aged 55 years or above) with chronic LBP and 13 age- and gender-matched asymptomatic volunteers participated in this study. Data collection was conducted in a university research laboratory. Measurements were steadiness index of spinal regions (trunk, thoracic spine, lumbar spine, and pelvis) during single-leg standing including relative holding time (RHT) and relative standstill time (RST), and clinical balance tests (timed up and go test and 5-repetition sit to stand test). The LBP group had a statistically significantly smaller RHT than the control group, regardless of one leg stance on the painful or non-painful sides. The RSTs on the painful side leg in the LBP group were not statistically significantly different from the average RSTs of both legs in the control group; however, the RSTs on the non-painful side leg in the LBP group were statistically significantly smaller than those in the control group for the trunk, thoracic spine, and lumbar spine. No statistically significant intra-group differences were found in the RHTs and RSTs between the painful and non-painful side legs in the LBP group. Measurements of clinical balance tests also showed insignificant weak to moderate correlations with steadiness index. In conclusion, older adults with chronic LBP demonstrated decreased spinal steadiness not only in the symptomatic lumbar spine but also in the other spinal regions within the kinetic chain of the spine. When treating older adults with chronic LBP, clinicians may also need to examine their balance performance and spinal steadiness during balance challenging tests.     

Feedback equilibrium control during human standing

Equilibrium maintenance during standing in humans was investigated with a 3-joint (ankle, knee and hip) sagittal model of body movement. The experimental paradigm consisted of sudden perturbations of humans in quiet stance by backward displacements of the support platform. Data analysis was performed using eigenvectors of motion equation. The results supported three conclusions. First, independent feedback control of movements along eigenvectors (eigenmovements) can adequately describe human postural responses to stance perturbations. This conclusion is consistent with previous observations (Alexandrov et al., 2001b) that these same eigenmovements are also independently controlled in a feed-forward manner during voluntary upper-trunk bending. Second, independent feedback control of each eigenmovement is sufficient to provide its stability. Third, the feedback loop in each eigenmovement can be modeled as a linear visco-elastic spring with delay. Visco-elastic parameters and time-delay values result from the combined contribution of passive visco-elastic mechanisms and sensory systems of different modalities