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.

Efficient FES triggering applying Kalman filter during sensory supported treadmill walking.

In this paper an algorithm for a functional electrical stimulation (FES) gait re-education system for incomplete spinal cord injured persons, providing efficient stimulation triggering, is presented. During neurological impaired gait FES was provided as motor augmentation support. Simultaneously the gait kinematics were recorded using the proposed sensory system, which is equipped with a dual-axial accelerometer and a gyroscope. The sensory device was placed at the shank of the paretic leg. The data assessed were input into a mathematical algorithm applied for shank angle estimation. The algorithm is based on the Kalman filter, estimating the angle error and correcting the actual measurement. Furthermore the information was combined with other kinematic data for the purpose of efficient and reliable stimulation triggering. The algorithm was tested with preliminary measurements on several neurologically intact persons during even terrain and treadmill walking. Trial measurements were verified with a contactless optical measurement system, with FES only simulated on controller output. Later on a treadmill training in combination with FES triggering was carried out. The outcome of the measurements shows that the use of sensory integration may successfully solve the problem of data assessment in dynamic movement where an inclinometer does not provide sufficient information for efficient control of FES.

A quantitative gait assessment method based on energy exchange analysis during walking: a normal gait study.

In this paper a gait efficiency assessment method, Gait Energy Efficiency Index (GEEI), which can be used in evaluation of the progress of the rehabilitation process in disabled persons, is proposed and described. The method is based on calculation of cross correlation between normalized time courses of kinetic and potential energy of the body's centre of mass (COM). We hypothesized that GEEI in energetically optimal normal walking should be high and invariable of gait speed. The method was tested on twelve healthy subjects walking at three different speeds and contrasted to five established gait energy consumption assessment methods. The results showed that GEEI in normal walking is close to 1 regardless of walking speed.

Standing-up robot: an assistive rehabilitative device for training and assessment.

In this paper a robotic assistive device is presented, aimed at assisting physically impaired individuals when rising from a sitting to a standing position. The robotic device is designed as a three degrees of freedom (3-DOF) mechanism supporting the subject under the buttocks. The device is driven by an electrohydraulic servosystem capable of operating in multiple control modes. It is instrumented with a sensory system providing information about the standing-up parameters. Evaluation of the standing up assistive device was accomplished in robot-supported rising trials of a paraplegic subject. The experiments demonstrated that stable risings in different standing - up manoeuvres were achieved. The measurement results revealed the role of the arm support and the support of the artificially evoked moments in the paralysed lower extremities during rising. The results show that the device can be used efficiently for training and evaluation of standing up manoeuvres.

The effect of age on the grip force control in lateral grip.

In the paper we present the grip force tracking system for the evaluation of grip force control. We developed a grip measuring device which can be used for the computer assisted measurements of the grip force in real time. The device was used as an input to a force-tracking task where the subject applied the grip force according to the visual feedback from the computer screen. The performance of the task was evaluated by the tracking error between the target signal and the measured force. We assessed the grip force control in the groups of 10-year old children, 25- to 35-year old adults and 50- to 60-year old adults. The subjects performed a sinus tracking task which required periodic muscle activation to produce the desired output. The results of the average tracking errors show significant differences in grip force control among the three tested groups. The largest variability among subjects was observed in the group of children and older adults. No significant difference in force control was found between the dominant and non-dominant hand. The grip force tracking system presented is aimed to be used for the evaluation of grip force control in patients with different sensory-motor impairments.

Paraplegics standing up using multichannel FES and arm support.

The suitability of multichannel functional electrical stimulation (FES) during the standing-up manoeuvre for therapeutic home use was investigated. Two spinal cord-injured subjects (SCI) participated in the study. Ankle plantar flexors, knee extensors and hip extensors were stimulated. The amplitude of the stimulation pulses depended on the current phase of raising. The sit-to-stand process was divided into three phases by detecting characteristic events in the vertical handle reaction force. It was found that the multichannel FES did not contribute to the decrease of the arm support force when compared with stimulation of knee extensors only. However, stimulation of the hip extensors could speed up the raising process. Increased repeatability and faster standing up were observed when the stimulation began before the start of raising.

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.

Stability analysis of four-point walking.

The aim of the experiment reported here was to determine the static and dynamic stability of two-point stance phases when walking on hands and knees at different speeds. In addition, we defined the methods and predicted the consequences of including two-point stance phases into crutch assisted functional electrical stimulation (FES) walking. Crawling on hands and knees was performed at three speeds by five healthy male persons. With twelve joint-position markers placed on the subject, we determined two stability indices for every instant of gait. We analysed the peak values of these two indices during the two-point stance phases. The results indicate that we have to ensure the proper position of the centre of gravity to increase the speed of walking. To reach speeds, lower than 0.6 m/s, it is not necessary to include statically unstable phases. The shift of the centre of gravity towards and across the leading stability edge can result in getting into the dynamically unstable state. Considering the results we can effectively introduce two-point stance phases into crutch assisted FES walking and therefore increase the speed and energy effectiveness of walking

Postural activity of constrained subject in response to disturbance in sagittal plane.

This study examines the postural activity of partially constrained subjects for three different initial standing postures in response to disturbances in the sagittal plane. When the ankle strategy suffices for disturbance rejection in response to anterior disturbances, a mostly linear relationship between the ankle torque and ankle angle was observed, resulting in a constant stiffness at the ankles. However, when the ankle torque saturation was reached, a combined ankle-trunk postural strategy was utilized mainly in response to posterior perturbations due to the properties of the base-of-support. This caused a nonlinear scaling of ankle responses, thereby increasing variability of ankle stiffness. Distinctions in the ankle responses were also observed for different initial standing postures. The anterior initial stance generally increases the overall postural stability and renders the utilization of ankle strategy feasible, even for the rejection of posterior disturbances. Therefore, a linear torque-angle relationship at the ankles was observed for the anterior initial stance, regardless of the perturbation parameters.

Use of telekinesthetic feedback in walking assisted by functional electrical stimulation.

A telekinesthetic feedback implemented into functional electrical stimulation (FES) orthosis is described. Single channel FES is used to provoke ankle dorsiflexion during walking. FES is controlled manually by a special lever, built into the handle of the crutch. The angular position of the lever defines the intensity of stimulation and thus the magnitude of the ankle dorsiflexion. The measured joint angle provides the feedback information about the ankle joint position, which is presented to the user as a force feedback applied to the control lever. As the first step in the development of a complex micromechatronic device, a simulated testing environment was prepared. A computer model, comprising dynamic foot characteristics, as well as agonistic and antagonistic muscle groups, substitutes the ankle joint. The model also includes fatiguing of the electrically stimulated muscles. For experimental purposes an actuated control lever was built. The efficacy of the telekinesthetic feedback was evaluated in a group of six healthy persons.

Development and evaluation of a two-dimensional electrocutaneous cognitive feedback system for use in paraplegic standing.

Fatigue of electrically activated paralysed muscles is a major factor limiting the duration of functional electrical stimulation (FES) supported paraplegic standing. Fatigue can be significantly delayed by changing the posture. Since paralysed individuals are deprived of proprio- and exteroception from the lower limbs they are not aware of the posture and loading of their paralysed legs. If suitable cognitive feedback (CF) information about posture in the sagittal and frontal planes is provided, they might be able to successfully exercise posture switching. A two-dimensional electrocutaneous CF system was developed. Relative limb loading and the location of the weighted centre of pressure were selected as informational variables. Discrete encoding schemes in the form of spatial and frequency codes were employed and the informational signals were divided into three sub-regions. The ability to correctly interpret the CF was investigated using one- and two-dimension tracking tests in three paralysed subjects, each of whom were studied over five consecutive days. All three subjects were able to use the CF in one-dimension tests. Two subjects could do the same also in two-dimension tests. The encoding scheme which was developed to communicate the selected biomechanical variables proved to be easily understood and thus appropriate for use in paraplegic standing.

FES Rehabilitative Systems for Re-Education of Walking in Incomplete Spinal Cord Injured Persons.

Objective. The aim of the paper is to present various relatively simple functional electrical stimulation (FES) systems that affect neural circuits and reflex behavior by providing necessary peripheral input to the lower extremities of incomplete spinal cord injured (SCI) persons. Methods. The proposed FES re-education walking systems make use of feedback information that is transmitted from the paralyzed limb to the nonparalyzed part of the patient's body. A single gait variable can be analogously transmitted to the walking subject in a form of sensory stimulation. The information about several gait variables can be first integrated and afterwards delivered to the walking subject as a single command. Conclusions. Significant improvements in the duration of the double support phase, metabolic energy expenditure, and physiologic cost index were observed when using FES-assisted training of walking in incomplete SCI persons.

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.

Use of functional electrical stimulation in the lower extremities of incomplete spinal cord injured patients.

After a program of therapeutic electrical stimulation, 3 groups of incomplete spinal cord injured (SCI) patients were identified, those in whom an improvement of both voluntary and stimulated muscle force was observed, those with an increase in stimulation response only, and patients in whom no effect of electrical stimulation training could be recorded. As it is difficult to predict the outcome of the electrical stimulation rehabilitation process, a diagnostic procedure was developed to predict soon after accidents which incomplete SCI patients are candidates for permanent use of a functional electrical stimulation (FES) orthotic aid. The candidates for chronic use of FES are patients with weak ankle dorsiflexors and sufficiently strong knee extensors. These patients are equipped with a single channel peroneal stimulator augmenting dorsiflexion and knee and hip flexion in a total lower limb flexion response. By applying FES to the ankle plantar flexors, the swing phase of walking can be significantly shortened and faster walking obtained.

Functional electrical stimulation and arm supported sit-to-stand transfer after paraplegia: a study of kinetic parameters.

The sit-to-stand transfer of paraplegic patients using functional electrical stimulation (FES) of the knee extensors and arm support was analyzed in the study. In a group of 8 completely paralyzed subjects who were trained FES users, kinematic and dynamic parameters were recorded during standing up trials. A contactless optical system was used to assess the human body motion. The forces acting on the human body were measured by multi-axis force transducers. On the basis of recursive Newton-Euler inverse dynamic analysis, the forces and torques acting on the body joints were calculated. The joint moments in the lower and upper extremities during the sit-to stand task are presented in this paper. The influences of the patient's strength, FES training duration, and rising strategy on the joint loading are discussed.

Postural activity of constrained subject in response to disturbance in sagittal plane.

The study examines postural activity of a constrained subject in response to various disturbances in the sagittal plane. Three different initial standing postures were examined. Each response to a perturbation was divided into three consecutive phases according to the intensity of the muscular activity. The relation of the ankle joint torque versus the ankle joint angle was studied. A linear relationship, resulting in constant ankle stiffness, was observed in each phase of the response. Only negligible differences in the stiffness values were observed among different phases. The results indicate an ankle stiffness value of 10 N m/o for the majority of initial stance postures and perturbation intensities.

Comparison of position repeatability of a human operator and an industrial manipulating robot.

Robot performance criteria of position repeatability are studied. Weight-to-payload ratio is in manipulating robots significantly higher than in human operators. Bracing strategy improving the robot performances is introduced in the paper. The strategy copies human behavior during fine motion operations. A comparison is made between the robot and the human operator performing approximately the same manipulating task. Contactless measurements of position repeatability were accomplished with the OPTOTRAK motion analysis system. The results of tests demonstrate considerable improvement of robot and human operator's position repeatability when using bracing.

Arm-free paraplegic standing--Part I: Control model synthesis and simulation.

The following paper is the first part of our investigation into the feasibility of arm-free paraplegic standing. A novel control strategy for unsupported paraplegic standing which utilizes the residual sensory and motor abilities of the thoracic spinal cord injured subjects is proposed. The strategy is based on voluntary and reflex activity of the paraplegic person's upper body and artificially controlled stiffness in the ankles. The knees and hips are maintained in an extended position by functional electrical stimulation (FES). The analysis of a linearized double inverted pendulum model revealed that with properly selected ankle stiffness the system can be easily stabilized. We developed a closed-loop double inverted pendulum model including a neural system delay, trunk muscles dynamics, body segmental dynamics and linear quadratic regulator (LQR) optimal controller. Through simulations of the closed-loop model two different strategies for disturbance rejection were explained. We investigated the capability of the closed-loop model to reject disturbances, imposed at the ankle joint (in anterior and posterior directions) for various stiffness levels and neural system delays in the presence of biomechanical constraints. By limiting permissible excursions of the center of pressure, we found out that the length of the foot is the most important constraint, while the strength of the trunk muscles is not of major importance for successful balancing. An ankle stiffness of approximately 10 Nm/degree suffices for arm-free standing of paraplegic subjects.

Arm-free paraplegic standing--Part II: Experimental results.

In Part I, we proposed an approach for restoring unsupported standing to thoracic-level paraplegics. The theoretical analysis and simulation of an underactuated double inverted pendulum, representing the standing subject, showed that arm-free standing might be achieved. Here in Part II, we present the mechanical apparatus which we used in our experiments and experimental results from tests of the balance-control strategy. We demonstrate that an intact and a paraplegic subject could perform quiet standing with the ankle stiffness set to 8 Nm/degree or even less (the intact subject). Both were also able to recover from disturbances, imposed by the artificial ankle joint of the apparatus. Introducing cognitive auditory feedback greatly improved the standing abilities of both subjects.

Control of FES in paraplegia: modeling voluntary arm forces.

Skilled behavior is difficult or impossible to articulate explicitly by the performers. Likewise biomechanical models of skilled motor actions are often limited by the lack of knowledge of the underlying mechanisms. A 'behavioral cloning' technique is described, based on a trained artificial neural network (ANN), that precisely mimics an individual's learned skill. In this paper the motor skill considered is that of paraplegics using their upper limbs whilst standing-up with FES. In a group of eight paraplegics with complete spinal injuries, it was possible to develop clones that followed closely the observed behavior of the subjects. Each subject used a unique and consistent voluntary control strategy. Subjects with more experience in using FES were more consistent in the use of their arms from trial to trial. Comparison of the clones revealed features suggestive of some common underlying voluntary control strategies.