Leonard E. Kahn

Robot-assisted reaching exercise promotes arm movement recovery in chronic hemiparetic stroke: a randomized controlled pilot study.

Background and purposeProviding active assistance to complete desired arm movements is a common technique in upper extremity rehabilitation after stroke. Such active assistance may improve recovery by affecting somatosensory input, motor planning, spasticity or soft tissue properties, but it is labor intensive and has not been validated in controlled trials. The purpose of this study was to investigate the effects of robotically administered active-assistive exercise and compare those with free reaching voluntary exercise in improving arm movement ability after chronic stroke.MethodsNineteen individuals at least one year post-stroke were randomized into one of two groups. One group performed 24 sessions of active-assistive reaching exercise with a simple robotic device, while a second group performed a task-matched amount of unassisted reaching. The main outcome measures were range and speed of supported arm movement, range, straightness and smoothness of unsupported reaching, and the Rancho Los Amigos Functional Test of Upper Extremity Function.Results and discussionThere were significant improvements with training for range of motion and velocity of supported reaching, straightness of unsupported reaching, and functional movement ability. These improvements were not significantly different between the two training groups. The group that performed unassisted reaching exercise improved the smoothness of their reaching movements more than the robot-assisted group.ConclusionImprovements with both forms of exercise confirmed that repeated, task-related voluntary activation of the damaged motor system is a key stimulus to motor recovery following chronic stroke. Robotically assisting in reaching successfully improved arm movement ability, although it did not provide any detectable, additional value beyond the movement practice that occurred concurrently with it. The inability to detect any additional value of robot-assisted reaching may have been due to this pilot studys limited sample size, the specific diagnoses of the participants, or the inclusion of only individuals with chronic stroke.

Adaptive assistance for guided force training in chronic stroke

This work describes a novel form of robotic therapy for the upper extremity in chronic stroke. Based on previous results, we hypothesized that a training task that encourages subjects to consciously guide endpoint forces generated by the hemiparetic arm will result in significant gains in functional ability of the arm, superior to more conventional methods of therapy. In addition, since stroke survivors present with varying degrees of arm movement ability, we developed an adaptive algorithm that tailors the amount of assistance provided in completing the guided force training task. The algorithm adapts a coefficient for velocity-dependent assistance based on measured movement speed, on a trial-to-trial basis. The training algorithm has been implemented with a simple linear robotic device called the ARM Guide. One participant completed a two month training program with the adaptive algorithm, resulting in significant improvements in the performance of functional tasks.

Design of robot assistance for arm movement therapy following stroke

This paper describes the mechanical and control design of a robotic device for providing therapeutic assistance to arm movement following stroke. The device uses a single motor and a passively oriented linear constraint to allow patients to reach across their workspace. Experimental evaluation of two controllers for assisting in reaching is presented.

Directional control of reaching is preserved following mild/moderate stroke and stochastically constrained following severe stroke

Recent evidence suggests that brain injury can impair the ability to independently activate shoulder and elbow muscles. We hypothesized that if muscle activation patterns are constrained, then brain-injured subjects should not be able to accurately grade initial hand movement direction during reaching toward a broad range of target directions. To test this hypothesis, we measured hand trajectories during reaching in three-space by 16 hemiparetic stroke subjects to an array of 75 targets distributed throughout the workspace. Contrary to our hypothesis, we found that the ability to grade movement direction was largely preserved following mild and moderate stroke. However, the most severely impaired subjects exhibited a degradation of directional control consistent with a loss of independent muscle control. Initial and final hand movement directions for these subjects were grouped roughly in two opposing directions, in a plane parallel with the coronal plane of the body, rather than distributed across the normal range. Selection between the two movement directions appeared partially random, in that subjects initiated over 50% of movements in the direction generally opposite the intended target, for targets to one side of the body. These results suggest that individuals with severe stroke are constrained to use only two gross, stereotypical muscle coactivation patterns for reaching control, and that selection between these patterns is stochastically influenced as the actual direction of motion is not strictly predictable given the desired direction.

Modeling reaching impairment after stroke using a population vector model of movement control that incorporates neural firing-rate variability

The directional control of reaching after stroke was simulated by including cell death and firing-rate noise in a population vector model of movement control. In this model, cortical activity was assumed to cause the hand to move in the direction of a population vector, defined by a summation of responses from neurons with cosine directional tuning. Two types of directional error were analyzed: the between-target variability, defined as the standard deviation of the directional error across a wide range of target directions, and the within-target variability, defined as the standard deviation of the directional error for many reaches to a single target. Both between and within-target variability increased with increasing cell death. The increase in between-target variability arose because cell death caused a nonuniform distribution of preferred directions. The increase in within-target variability arose because the magnitude of the population vector decreased more quickly than its standard deviation for increasing cell death, provided appropriate levels of firing-rate noise were present. Comparisons to reaching data from 29 stroke subjects revealed similar increases in between and within-target variability as clinical impairment severity increased. Relationships between simulated cell death and impairment severity were derived using the between and within-target variability results. For both relationships, impairment severity increased similarly with decreasing percentage of surviving cells, consistent with results from previous imaging studies. These results demonstrate that a population vector model of movement control that incorporates cosine tuning, linear summation of unitary responses, firing-rate noise, and random cell death can account for some features of impaired arm movement after stroke.

The Effect of Vastus Medialis Forces on Patello-femoral Contact: A Model-based Study

A mathematical model of the patello-femoral joint was introduced to investigate the impact of the vastus medialis (longus, obliquus) forces on the lateral contact force levels. In the model, the quadriceps were represented as five separate forces: vastus lateralis, vastus intermedius, rectus femoris, vastus medialis longus (VML), and obliquus (VMO). By varying the relative force generation ratios of the quadriceps heads, the patello-femoral contact forces were estimated. We sought to analytically determine the range of forces in the VMO and VML that cause a reduction or an increase of lateral contact forces, often the cause of patello-femoral pain. Our results indicated that increased contact forces are more dependent on combinations of muscle forces than solely VMO weakness. Moreover, our simulation data showed that the contact force levels are also highly dependent on the knee flexion angle. These findings suggest that training targeted to reduce contact forces through certain joint angles could actually result in a significant increase of the contact forces through other joint angles.

Shoulder support for children with subluxation: A case study

Children with brachial plexus injury from birth may present with varying degrees of muscle imbalance, as well as a subluxation of the glenohumeral joint. Shoulder subluxation occurs when the muscles of the shoulder girdle are weak or flaccid. The deltoid and the rotator cuff musculatures are unable to position the humerus appropriately to the glenoid fossa, and there is concurrent stretching of the glenohumeral joint capsule, ligaments, and nonactive muscles. Treatment for reducing the subluxation and positioning the arm typically has involved use of an appropriate sling or humeral cuff support. There are commercially available slings for children but no child-size shoulder support. The purpose of this case study was to design a custom-fitted shoulder support for children that reduces subluxation and maintains alignment through extended periods of the day. A validated radiographic method was used to quantify the subluxation before application of the shoulder support, immediately after applying the shoulder support, and after 3 hours of wear. A motion tracking system objectively quantified active shoulder and elbow movements in the presence and absence of the shoulder support. This case study suggests that the custom-designed child support significantly reduced the subluxation, maintained alignment through extended periods of the day, and maintained the active range in elbow flexion.

Mechanical actions of individual muscles at the knee joint with varus/valgus malalignment

The mechanical actions of individual muscles crossing the knee joint were studied in the frontal plane. Through electrical stimulation, individual knee muscles were selectively activated and the resulting three-dimensional torques at the joint were measured. The isometric experiment was repeated at neutral, 5/spl deg/ varus, 5/spl deg/ valgus positions in the frontal plane and full knee extension to examine the effect of varus/valgus alignment on the muscle actions. In all muscles studied, the varus and valgus malalignment caused the muscles to exert a considerably larger torque in the direction opposite to that of the malalignment.