Assessing the Cognitive Demand of Hand Controlled Exoskeleton Walking.

Individuals with spinal cord injury have motor and sensory deficits leading to ambulatory problems. Our current research is focused on developing innovative control mechanisms for wearable robotic exoskeletons to provide such users with complete control of their gait while allowing them to perform other activities (such as conversing, etc.). In this study, we evaluated the cognitive load due to using the user's hand movement to control the gait of a robot using a dual-task paradigm. The results show that there was no difference in symmetry and duty cycle between with and without a competing cognitive task, and the number of cognitive responses was similar to healthy controls walking on the treadmill. There was also no difference in obstacle navigation with and without the cognitive task. Results of this study suggest that using our control mechanisms is intuitive, easy to learn, and requires cognitive attention that is similar to normal human walking. Clinical Relevance-Initial evidence to understand the effects of the novel control mechanism on cognitive load over that of typical walking.

Evaluation of admittance control as an alternative to passive arm supports to increase upper extremity function for individuals with Duchenne muscular dystrophy.

The degree of upper extremity active range of motion provided by an admittance control robot compared with a commercially available passive arm support for individuals with DMD who have limited arm function was investigated in this study. The reachable workspace evaluation was used to assess active range of motion provided by both devices. A visual analog scale was also used to secure participant-reported outcome measures. The admittance control robot significantly increased reachable surface area scores compared with the passive arm support for the dominant arm (Wilcoxon T = 5, P = .022, r2 = 0.263) and for the nondominant arm (paired-samples t test, t(9) = 4.66, P = .001, r2 = 0.71). The admittance control robot also significantly decreased participant-reported exertion compared with the passive arm support. Results of this study substantiated the benefits of admittance control for individuals with DMD compared with a commercially available passive arm support.

A Novel User Control for Lower Extremity Rehabilitation Exoskeletons.

Lower extremity exoskeletons offer the potential to restore ambulation to individuals with paraplegia due to spinal cord injury. However, they often rely on preprogrammed gait, initiated by switches, sensors, and/or EEG triggers. Users can exercise only limited independent control over the trajectory of the feet, the speed of walking, and the placement of feet to avoid obstacles. In this paper, we introduce and evaluate a novel approach that naturally decodes a neuromuscular surrogate for a user's neutrally planned foot control, uses the exoskeleton's motors to move the user's legs in real-time, and provides sensory feedback to the user allowing real-time sensation and path correction resulting in gait similar to biological ambulation. Users express their desired gait by applying Cartesian forces via their hands to rigid trekking poles that are connected to the exoskeleton feet through multi-axis force sensors. Using admittance control, the forces applied by the hands are converted into desired foot positions, every 10 milliseconds (ms), to which the exoskeleton is moved by its motors. As the trekking poles reflect the resulting foot movement, users receive sensory feedback of foot kinematics and ground contact that allows on-the-fly force corrections to maintain the desired foot behavior. We present preliminary results showing that our novel control can allow users to produce biologically similar exoskeleton gait.

The Importance of Haptics in Generating Exoskeleton Gait Trajectory Using Alternate Motor Inputs.

Human gait requires both haptic and visual feedback to generate and control rhythmic movements, and navigate environmental obstacles. Current lower extremity wearable exoskeletons that restore gait to individuals with paraplegia due to spinal cord injury rely completely on visual feedback to generate limited pre-programmed gait variations, and generally provide little control by the user over the gait cycle. As an alternative to this limitation, we propose user control of gait in real time using healthy upper extremities. This paper evaluates the feedback conditions required for the hands to generate complex rhythmic trajectories that resemble gait trajectories. This paper involved 18 subjects who performed a virtual locomotor task, where contralateral hand movements were mapped to control virtual feet in three feedback conditions: haptic only, visual only, and haptic and visual. The results indicate that haptic feedback in addition to visual feedback is required to produce rhythmic hand trajectories similar to gait trajectories.

Motor control investigation of dystonic cerebral palsy: A pilot study of passive knee trajectory.

The purpose of this study is to better understand dystonia in CP and be able to objectively distinguish between individuals who experience spasticity, dystonia, or a combination of these conditions while evaluating the effect of 2Hz vestibular stimulation. Selected outcome measures included knee ROM, angular velocity and acceleration and all measures increased post vestibular stimulation; these results are indications of a possible reduction in the level of disability. The current investigation also identified an unexpected and unique behavior of the knee in children with dystonic cerebral palsy (CP) that was noticed while administering the Pendulum Knee Drop test (PKD) at approximately 0.4 rad (a mid-angle between full extension and zero vertical). There was a catch-like phenomenon at the described mid-angle in dystonic individuals. These results may suggest that dystonia is not a velocity dependent hypersensitivity of reflexes, but may include position dependent muscle reflexes and co-contractions. This reinforces the need for a more precise objective measure or perhaps a modified measure such as a mid-angle PKD test. Furthermore, based on the results obtained through the modified technique, beneficial alterations can be made to the form of treatment such as: robotic therapy or physical therapy that specifically accommodates the unique motor control disorder in individuals with dystonic CP.

Encoding of forelimb forces by corticospinal tract activity in the rat.

In search of a solution to the long standing problems encountered in traditional brain computer interfaces (BCI), the lateral descending tracts of the spinal cord present an alternative site for taping into the volitional motor signals. Due to the convergence of the cortical outputs into a final common pathway in the descending tracts of the spinal cord, neural interfaces with the spinal cord can potentially acquire signals richer with volitional information in a smaller anatomical region. The main objective of this study was to evaluate the feasibility of extracting motor control signals from the corticospinal tract (CST) of the rat spinal cord. Flexible substrate, multi-electrode arrays (MEA) were implanted in the CST of rats trained for a lever pressing task. This novel use of flexible substrate MEAs allowed recording of CST activity in behaving animals for up to three weeks with the current implantation technique. Time-frequency and principal component analyses (PCA) were applied to the neural signals to reconstruct isometric forelimb forces. Computed regression coefficients were then used to predict isometric forces in additional trials. The correlation between measured and predicted forces in the vertical direction averaged across six animals was 0.67 and R (2) value was 0.44. Force regression in the horizontal directions was less successful, possibly due to the small amplitude of forces. Neural signals above and near the high gamma band made the largest contributions to prediction of forces. The results of this study support the feasibility of a spinal cord computer interface (SCCI) for generation of command signals in paralyzed individuals.

Estimation of intrinsic joint impedance using quasi-static passive and dynamic methods in individuals with and without Cerebral Palsy.

Modeling the passive behavior of the knee in subjects with spasticity involves the applied external torques (e.g. gravitational torque), the intrinsic moments due to tissue properties, as well as active, neurally defined moments resulting from the hypersensitivity of reflexes introduced by disability. In order to provide estimates of the necessary intrinsic terms in the equation of motion, the push-pull and Wartenberg Pendulum Knee Drop (PKD) tests were administered. Four subjects without disability and two subjects with Cerebral Palsy (CP) were evaluated for their active and intrinsic knee stiffness parameters. Separation of these two terms requires an additional stiffness term be added to the traditional equation of motion. This holds true for subjects with and without neurological disability. Very interestingly, the optimized non-disabled PKD produced lumped stiffness (K) that is similar to the push-pull passive stiffness (KI) for both populations. On the other hand the optimized K value in the PKD test for subjects with disability was approximately 19 times larger than the KI value found graphically from the push-pull test. This leads us to the conclusion that we can partition our lumped K as the sum of a neurally generated stiffness (Ka) and KI to complete the trajectory model. Therefore, this study shows that spasticity is a velocity dependent, that would not appear in disabled individuals unless the examined limb has a non-zero velocity.

Corticospinal signals recorded with MEAs can predict the volitional forearm forces in rats.

We set out to investigate if volitional components in the descending tracts of the spinal cord white matter can be accessed with multi-electrode array (MEA) recording technique. Rats were trained to press a lever connected to a haptic device with force feedback to receive sugar pellets. A flexible-substrate multi-electrode array was chronically implanted into the dorsal column of the cervical spinal cord. Field potentials and multi-unit activities were recorded from the descending axons of the corticospinal tract while the rat performed a lever pressing task. Forelimb forces, recorded with the sensor attached to the lever, were reconstructed using the hand position data and the neural signals through multiple trials over three weeks. The regression coefficients found from the trial set were cross-validated on the other trials recorded on same day. Approximately 30 trials of at least 2 seconds were required for accurate model estimation. The maximum correlation coefficient between the actual and predicted force was 0.7 in the test set. Positional information and its interaction with neural signals improved the correlation coefficient by 0.1 to 0.15. These results suggest that the volitional information contained in the corticospinal tract can be extracted with multi-channel neural recordings made with parenchymal electrodes.

Piecewise parametric interpolation for temporal compression of multijoint movement trajectories.

Computer animation of sign language used by deaf individuals has been produced from 51 time-varying trajectories of the fingertips, the centers of rotation of the joints of the hands and arms, and facial landmarks. These trajectories are sampled at 1/5th the National Television Systems Committee (NTSC) video frame rate and interpolated using piecewise sequences of cubic Bezier splines. The resulting trajectories are used to control sparse, stick figure animations of sign language resulting in considerable spatiotemporal compression with intelligibility in the range of 90%. This method introduces an additional 5:1 compression in the temporal domain that has not been previously exploited, and is potentially useful in sign language telecommunication, multimedia presentations, and gesture recognition.

Neuromuscular modeling of spasticity in cerebral palsy.

Data from the pendulum knee test has been used to develop two active models that use external torques to closely match the experimental knee trajectories of subjects with spasticity due to cerebral palsy. These data were collected from three subjects who are identical triplets; two of whom have clinically measurable spasticity. A passive model that accurately describes the knee trajectory of the nonspastic subject serves as the passive plant for two active models. One of these models allows direct application of external torques, and the second provides additional torque as the result of velocity feedback. Both active models and the passive model use separate parameters of stiffness and damping for the agonist and antagonist muscles.

Biomechanical and perceptual constraints on the bandwidth requirements of sign language.

Access to telecommunication systems by deaf users of sign language can be greatly enhanced with the incorporation of video conferencing in addition to text-based adaptations. However, the communication channel bandwidth is often challenged by the spatial requirements to represent the image in each frame and temporal demands to preserve the movement trajectory with a sufficiently high frame rate. Effective systems must balance the portion of a limited channel bandwidth devoted to the quality of the individual frames and the frame rate in order to meet their intended needs. Conventional video conferencing technology generally addresses the limitations of channel capacity by drastically reducing the frame rate, while preserving image quality. This produces a jerky image that disturbs the trajectories of the hands and arms, which are essential in sign language. In contrast, a sign language communication system must provide a frame rate that is capable of representing the kinematic bandwidth of human movement. Prototype sign language communication systems often attempt to maintain a high frame rate by reducing the quality of the image with lossy spatial compression. Unfortunately, this still requires a combined spatial and temporal data rate, which exceeds the limited channel of residential and wireless telephony. While spatial compression techniques have been effective in reducing the data, there has been no comparable compression of sign language in the temporal domain. Even modest reductions in the frame rate introduce perceptually disturbing flicker that decreases intelligibility. This paper introduces a method through which temporal compression on the order of 5:1 can be achieved. This is accomplished by decoupling the biomechanical or kinematic bandwidth necessary to represent continuous movements in sign language from the perceptually determined critical flicker frequency.

Robust region of interest coding for improved sign language telecommunication.

More than 500,000 deaf people in North America use American Sign Language or a similar signed system as a first language. Long-distance communication of this visually based medium is hampered by its incompatibility with audio and text telecommunication systems. Movements associated with signed languages require a more consistent and higher frame rate than is available with residential video telephony. New video compression standards (JPEG 2000 and MPEG-4) allow optional region of interest coding in which areas within a frame can be assigned different levels of compression. This paper presents a novel skin color segmentation approach that identifies the hands and face each video frame. This method is robust in terms of variations in skin pigmentation in a single subject, in skin pigmentation across a population of potential users, subject clothing, and image background. Specifying these critical regions of interest to the compression algorithm maintains high visual quality in the regions of the hands and face, while allowing very lossy, high compression of the remainder of the video frame. This reduces the coded representation of each frame, and offers a potential increase in the frame rate for telecommunication.