FES gait restoration and balance control in spinal cord-injured patients.

The status of gait restoration in spinal cord-injured patients by means of FES is reviewed and the main aspects are discussed. This introduction highlights the issues of balance control, stimulation sequence synthesis, and control of enhanced gait modes containing unbalancing. The use of statically unstable dynamic weight-transfer phases is important for enhanced gait modes. To show how this phase can be employed the mode of static balance currently used for FES-assisted four-point gait in paraplegic patients is discussed, and how this mode of gait can be converted to a semi-dynamic gait mode is described. The possibilities and consequences of such an approach are briefly discussed.

Setup and procedure for online identification of electrically stimulated muscle with Matlab Simulink.

This paper first describes a laboratory setup for biomechanical experiments that runs within the universal simulation environment Matlab Simulink. The overall system comprises a personal computer, two AMTI (Advanced Mechanical Technology, Inc., Watertown, MA 02472) force plates, Parotec force-sensor shoe insoles, Optotrak system for noncontact three-dimensional (3-D)-position measuring, and a computer-controlled four-channel electrical stimulator. Conceptually, the most important application is implementation of closed-loop electrical stimulation of intact and paralyzed subjects in the laboratory. Second, the system was tested in real-time muscle model identification procedure during a standing experiment. The plantarflexors of three nonimpaired subjects were excited with pseudorandom binary sequences (PRBSs) with small deviations around selected operating points. Electrically stimulated muscles were presented with a linear local dynamic block that was identified with a recursive least-square method (RARX). RARX block was designed with fundamental Matlab Simulink blocks that support real-time operation. Introduced was online estimation of model output, which offers a great manner of instant model validation. Two modes of operation with online validation were tested. In the first mode, the operating point for selected excitation level was identified online. In the second mode, the operating point was measured in preceding experiments. Both procedures resulted in satisfying second-order models that will be used in the adaptive controller design.

Modelling muscle activity in standing with considerations for bone safety.

The functional use of electrical stimulation (FES) for the restoration of movement to paraplegics has been improved in the last decade but questions about the mechanical effect of stimulation on the skeleton have arisen. In intact people, neuromuscular activity not only controls movement, but also minimizes bone and joint tissue loading. Current FES systems do not use feedback and do not even use average natural patterns of muscle activation. FES systems would be safer if muscle activation patterns were synthesized so as to minimize bending in the long bones. By modelling, we have verified that appropriate muscular activity reduces bone bending stresses, an approach we named active unloading of the skeleton. Using this criterion for control is novel. The muscle activation was calculated using measurements from intact people in different postures, and later modelling of the musculoskeletal system. The two-dimensional model of the lower limb includes 23 muscles relevant primarily for movement in the sagittal plane. The muscle model for constraint calculation is divided into first-order activation dynamics and first-order contraction dynamics. Optimization, which includes minimization of net bending moment calculated along the long bones, is static because changes in the observed postures are slow. In the calculated muscle activity patterns, muscle coactivation and cocontraction yield very uniform and low bone loading. Net bending moment values were fairly stable as the posture varies. The moment distribution in the femur was found to be U-shaped, while in the tibia it is sometimes V-shaped. The bones are naturally thicker at the points of peak moment.

Analysis of standing up and sitting down in humans: definitions and normative data presentation.

A formal definition of human standing up and sitting down movements based on sagittal plane goniometric and force plate data from 20 normal subjects is presented. This definition is comparable to the established gait cycle diagram, and consists of defined characteristic events and relative time intervals between them. The characteristic events are selected primarily on changes in ground reaction forces. The terminology proposed may be valuable for introducing more formalized and standardized reporting of both qualitative and quantitative studies in both normals and in patients. This presentation is directed toward the process of defining generally acceptable standards for human standing up and sitting down movements.

Investigation of the Hammerstein hypothesis in the modeling of electrically stimulated muscle.

To restore functional use of paralyzed muscles by automatically controlled stimulation, an accurate quantitative model of the stimulated muscles is desirable. The most commonly used model for isometric muscle has had a Hammerstein structure, in which a linear dynamic block is preceded by a static nonlinear function. To investigate the accuracy of the Hammerstein model, the responses to a pseudo-random binary sequence (PRBS) excitation of normal human plantarflexors, stimulated with surface electrodes, were used to identify a Hammerstein model but also four local models which describe the responses to small signals at different mean levels of activation. Comparison of the local models with the linearized Hammerstein model showed that the Hammerstein model concealed a fivefold variation in the speed of response. Also, the small-signal gain of the Hammerstein model was in error by factors up to three. We conclude that, despite the past widespread use of the Hammerstein model, it is not an accurate representation of isometric muscle. On the other hand, local models, which are more accurate predictors, can be identified from the responses to short PRBS sequences. The utility of local models for controller design is discussed.

Apparatus to measure simultaneously 14 isometric leg joint moments. Part 1: Design and calibration of six-axis transducers for the forces and moments at the ankle.

An apparatus has been developed for making isometric measurements of the joint moments corresponding to the 14 degrees of freedom of the legs, in postures ranging between sitting and near full extension. The apparatus is called the multi-moment chair system (MMCS) and is described in the companion paper. This paper describes the most critical components of the MMCS, which are the six-axis transducers for measuring the force and moment components on the plantar-flexion axis of each ankle while the feet are laced into fixed shoes. The transducers are made of steel bars, on which strain gauges are mounted, joined by clamps. The design of the transducer and methods of calibration and error estimation are described. The RMS errors are less than 2 N for the forces and 1 Nm for the moments, but these may be correlated. A method for error reduction that compensates for the finite compliance of the transducer does not reduce the measured errors.

Implantable telemeter for long-term electroneurographic recordings in animals and humans.

A system is described that amplifies an electroneurographic signal (ENG) from a tripolar electrode nerve cuff and transmits it from the implanted amplifier to an external drive box. The output was raw ENG, bandpass filtered from 800 to 8000 Hz. The implant was powered by radio-frequency induction and operated for coil-to-coil separations up to 30 mm. The testing and performance of the system is described. The input-referred noise was never more than 1 microV RMS, and, at some positions of the radio-frequency field, was 0.7 microV, close to the expected value for the amplifier used. The common-mode rejection ratio (CMRR) depended on the impedance imbalance from the cuff and the length of input cable. Devices with a short cable and low source impedance had CMRR of 84 dB, but, with 31 cm of cable and a real cuff, the CMRR fell to 66 dB. Recovery from a stimulus artifact took 5 ms. The responses of the cuff to external potential gradients and to common-mode signals are described theoretically or by simulation. The devices are available for use in neuroprosthetic or neurophysiological research.

Variation of recruitment nonlinearity and dynamic response of ankle plantarflexors.

The ankle plantarflexor muscles of paraplegics may be trained to provide balance without support from the hands (in the laboratory environment) with the controller based on a two-block Hammerstein muscle model. This paper presents data on the variations of the recruitment curve block and linear dynamics block with electrode position, among various individuals and with fatigue. The tests were conducted in six groups: 'a' tests of a neurologically-intact subject were repeated on one day several times to record the effect of muscle fatigue; 'b' the same individual kept electrodes attached for a week and the muscle was identified every day; 'c' the same subject attached electrodes at marked positions every day for a week prior to identification; 'd' another normal attached electrodes at notionally the same positions over a period of one week; 'e' three normals and 'f' two paraplegics. Measurements were made with the Wobbler apparatus, in which the subject is supported upright in a standing posture. When comparing tests of fresh muscle every day, little difference was found between the nonlinear recruitment curves and linear dynamics of groups 'b' and 'c'. In fatigued muscle the dynamics were slower. When the electrode position was not carefully reproduced, and over a longer period, significant differences in nonlinearity appear in the curve shapes (group 'd') and a similar amount of variation occurs between normals, between paraplegics, and from normals to paraplegics. The paraplegic curves show wider deadbands. The effect of prolonged stimulation on normals is slight but on paraplegics it is significant.

Apparatus and methods for studying artificial feedback-control of the plantarflexors in paraplegics without interference from the brain.

Apparatus has been built to explore the practical feasibility of using automatic control with electrical stimulation of paralysed legs to restore function. The experiments are performed with paraplegics with the aim of achieving a realistic postural task: to see whether the body may be maintained upright by stimulation of the plantarflexors when the other joints are braced. Significantly, the intact upper body, under natural control of the brain, cannot interfere with the automatic control. The "Wobbler" apparatus allows measurement of the ankle muscle properties in isometric conditions or in sinusoidal motion. Using the biomechanical properties of the body, which are also measured, controllers for stabilising the body can be designed. Controllers can be dynamically tested, imitating anterior-posterior sway, while the body is held upright, before "actual standing" is attempted.

Human energy - optimal control of disturbance rejection during constrained standing.

An optimal control system that enables a subject to stand without hand support in the sagittal plane was designed. The subject was considered as a double inverted pendulum structure with a voluntarily controlled degree of freedom in the upper trunk and artificially controlled degree of freedom in the ankle joints. The control system design was based on a minimization of cost function that estimated the effort of the ankle joint muscles through observation of the ground reaction force position relative to the ankle joint axis. By maintaining the centre of pressure close to the ankle joint axis the objective of the upright stance is fulfilled with minimal ankle muscle energy cost. The performance of the developed controller was evaluated in a simulation-based study. The results were compared with the responses of an unimpaired subject to different disturbances in the sagittal plane. The proposed cost function was shown to produce a reasonable approximation of human natural behaviour.

Voluntary telemetry control of functional electrical stimulators.

With the assistance of crutches and functional electrical stimulation (FES) we are able to restore standing and simple gait in some spinal cord injured (SCI) patients. In the present rehabilitative systems the patient divides the gait cycle into 'stance' and 'swing' phase by using pushbuttons mounted in the handles of the crutches. These are then hard wired to the functional electrical stimulator. We present the development and evaluation of a surface mount technology based telemetry system that provides reliable and interference resistant wireless control of FES assisted walking. The system makes use of radio frequency carriers operating at a frequency of 40 MHz. Crutch pushbutton signals are coded and transferred from the transmitter placed in the crutch to the receiver which is firmly attached to the patient's waist and connected to the stimulator. The telemetry system was found to be of special importance for both complete and incomplete SCI subjects and is currently in use at the Rehabilitation Institute of the Republic of Slovenia.

Problems associated with FES-standing in paraplegia.

Prolonged immobilization, such as occurs after the spinal cord injury (SCI), results in several physiological problems. It has been demonstrated that the standing posture can ameliorate many of these problems. Standing exercise can be efficiently performed by the help of functional electrical stimulation (FES). The first application of FES to a paraplegic patient was reported by Kantrowitz in 1963. It was later shown by our group that standing for therapeutic purposes can be achieved by a minimum of two channels of FES delivered to both knee extensors. The properties of the stimulated knee extensors (maximal isometric joint torque, fatiguing, and spasticity) were not found as sufficient conditions for efficient standing exercise. According to our studies, the ankle joint torque during standing is the only parameter which is well correlated to the duration of FES assisted standing. For good standing low values of the ankle joint torque are required. To minimize the ankle joint torque the lever belonging to the vertical reaction force must be decreased. Adequate alignment of the posture appears to be the prerequisite for efficient FES assisted and arm supported standing exercise. Some patients are able to assume such posture by themselves, while many must be aided by additional measures. At present, surface stimulation of knee extensors combined with some appropriately "compliant shoes" looks to be adequate choice.

Kinematics based sensory fusion for wearable motion assessment in human walking.

Measuring the kinematic parameters in unconstrained human motion is becoming crucial for providing feedback information in wearable robotics and sports monitoring. This paper presents a novel sensory fusion algorithm for assessing the orientations of human body segments in long-term human walking based on signals from wearable sensors. The basic idea of the proposed algorithm is to constantly fuse the measured segment's angular velocity and linear acceleration via known kinematic relations between segments. The wearable sensory system incorporates seven inertial measurement units attached to the human body segments and two instrumented shoe insoles. The proposed system was experimentally validated in a long-term walking on a treadmill and on a polygon with stairs simulating different activities in everyday life. The outputs were compared to the reference parameters measured by a stationary optical system. Results show accurate joint angle measurements (error median below 5°) in all evaluated walking conditions with no expressed drift over time.

Development of gait segmentation methods for wearable foot pressure sensors.

We present an automated segmentation method based on the analysis of plantar pressure signals recorded from two synchronized wireless foot insoles. Given the strict limits on computational power and power consumption typical of wearable electronic components, our aim is to investigate the capability of a Hidden Markov Model machine-learning method, to detect gait phases with different levels of complexity in the processing of the wearable pressure sensors signals. Therefore three different datasets are developed: raw voltage values, calibrated sensor signals and a calibrated estimation of total ground reaction force and position of the plantar center of pressure. The method is tested on a pool of 5 healthy subjects, through a leave-one-out cross validation. The results show high classification performances achieved using estimated biomechanical variables, being on average the 96%. Calibrated signals and raw voltage values show higher delays and dispersions in phase transition detection, suggesting a lower reliability for online applications.

Comparison of visual and haptic feedback during training of lower extremities.

We compared the effects of visual and haptic modalities on the adaptation capabilities of healthy subjects to the virtual environment. The visual cueing (only the reference motion is presented) and visual feedback (the reference motion as well as the current tracking deviation are presented) were provided by a real-time visualization of a virtual teacher and a virtual self - avatar, using optical measurements. The subjects had to track the virtual teacher during stepping-in-place movements. The haptic feedback was provided by the actuated gait orthosis Lokomat programmed with the same stepping movements employing an impedance control algorithm. Both setups included auditory cueing. The stepping task was performed by engaging different modalities separately as well as combined. The results showed that (1) visual feedback alone yielded better tracking of the virtual teacher than visual cueing alone, (2) haptic feedback alone yielded better tracking than any visual modality alone, (3) haptic feedback and visual feedback combined yielded better tracking than haptic feedback alone, and (4) haptic feedback combined with visual cueing did not improve tracking performance compared to haptic feedback alone. In general, we observed a better task performance with the haptic modality compared to visual modality.

Current status and future prospects for upper and lower extremity motor system neuroprostheses.

This paper reviews the current state of the art and identifies the major challenges facing the future development and clinical application of neuroprostheses to provide limb movement. It gives insight into the current status of functional electrical stimulation (FES) for motor control, identifies problems, and proposes possible directions of development in cervical cord injury, thoracic spinal cord injury, and stroke. For upper extremity function, existing clinical applications are covered, major problems are identified, and possible future trends are highlighted. The discussion on lower extremity applications describes current and possible future solutions of the major impediments to the development of FES systems for individuals with paraplegia after spinal cord injury and surface and implantable setups for stroke survivors with hemiplegia. Particular attention is given to sensor issues and requirements for walking with FES after stroke.

Bending moments in lower extremity bones for two standing postures.

The goal of this paper is to study how external gravitational forces stress the lower extremity bones and to ascertain and study how muscle activation compensates for the external load. For these purposes relatively accurate anatomical and biomechanical modelling is necessary. For a comparison of the calculated results to the naturally occurring muscular activity, seven-channel surface EMG activity was recorded. For simplicity a two-dimensional model was developed for the sagittal plane including 19 lower extremity muscles relevant to human standing and walking. In the calculation procedure of muscle forces an optimization procedure is also included. The results give rise to the expected assumption that muscle action is covered by two main requirements: first, to stabilize the joint actively (moment equilibrium) and, second, to compensate efficiently for bending moments produced by gravitational and external forces.

Feedback control of unsupported standing in paraplegia--part I: optimal control approach.

This is the first of a pair of papers which describe an investigation into the feasibility of providing artificial balance to paraplegics using electrical stimulation of the paralyzed muscles. By bracing the body above the shanks, only stimulation of the plantarflexors is necessary. This arrangement prevents any influence from the intact neuromuscular system above the spinal cord lesion. In this paper, we extend the design of the controllers to a nested-loop LQG (linear quadratic Gaussian) stimulation controller which has ankle moment feedback (inner loops) and inverted pendulum angle feedback (outer loop). Each control loop is tuned by two parameters, the control weighting and an observer rise-time, which together determine the behavior. The nested structure was chosen because it is robust, despite changes in the muscle properties (fatigue) and interference from spasticity.

Dermatome electrical stimulation as a therapeutic ambulatory aid for incomplete spinal cord injured patients.

Electrical stimulation of the L-3,4 dermatome during treadmill walking is proposed as a gait training modality in incomplete spinal cord injured patients. The dermatome stimulation proved to be efficient in diminishing the extensor tone occurring after loading of the paralyzed limb during the stance phase of walking and resulting in improved flexion of the leg during the swing phase.