| | Effect of a Gravity-Compensating Orthosis on Reaching After Stroke: Evaluation of the Therapy Assistant WREXAbstract Iwamuro BT, Cruz EG, Connelly LL, Fischer HC, Kamper DG. Effect of a gravity-compensating orthosis on reaching after stroke: evaluation of the Therapy Assistant WREX. ObjectiveTo evaluate the effectiveness of an upper-extremity orthosis to improve the reaching workspace of stroke survivors. DesignWithin-subjects repeated-measures design evaluating reaching with and without the Therapy Assistant Wilmington Robotic Exoskeleton (WREX). ParticipantsStroke survivors (N=10) with chronic upper-extremity hemiparesis. InterventionsNot applicable. Main Outcome MeasuresArm movement kinematics (Optotrak Certus motion detection system), muscle activity for biceps, triceps, anterior deltoid, and brachioradialis muscles (bipolar surface electromyography). ResultsSignificant improvements of reaching distance occurred for all subjects across all targets (P<.001) when using the Therapy Assistant WREX. While the self-selected peak speed of hand movement during the reach decreased significantly with the Therapy Assistant WREX (P<.001), use of the Therapy Assistant WREX led to improved quality of movement as signified by a decrease in jerk (P<.001) and a shift in the timing of the peak speed to an earlier point in the movement (P<.001). Electromyographic muscle activity analysis showed that use of the Therapy Assistant WREX led to a reduction in biceps activity across all targets during the reach (P<.05), in conjunction with a marginally significant reduction in activity of the anterior deltoid (P<.055). No changes were observed in triceps (P=.47) or brachioradialis activity (P=.28). ConclusionsBy reducing requirements for shoulder activation, the Therapy Assistant WREX improved reaching performance among stroke survivors compared with free reaching, thereby potentially facilitating practice of functional tasks. CURRENT REHABILITATION practices after stroke primarily focus on adaptation and compensatory techniques to mitigate the impact of diminished motor control on independence at home and participation in the community. Upper-extremity impairments are treated within this context, and restoration of motor control is often a secondary goal unless it directly interferes with independence. These goals are understandable given the time and financial constraints inherent to rehabilitation, but a number of studies suggest that improvement of the hemiparetic limb may be possible. These studies emphasize the importance of repetitive practice to recovery. Indeed, numerous studies employing the constraint-induced technique, in which focus is placed on intensive practice with the impaired arm without using the less impaired arm, have shown improvement in hand capabilities.1, 2, 3, 4 This supports the observations in animal models of brain injury in which practice appears to be the primary factor leading to synaptogenesis and brain plasticity.5, 6, 7 Indeed, imaging performed during constraint-induced training studies has shown evidence of cortical plasticity after the training.8, 9 Unfortunately, many stroke survivors do not possess sufficient sensorimotor control to practice the desired movements. For these individuals, repetitive movement therapy with robots has shown promise.10, 11, 12, 13, 14 However, the robotic devices are quite expensive and are currently limited in terms of therapy routines. Typically, they have been used to provide training without a therapist being present. As an alternative, it may be possible to use a passive device to assist training of arm movement in conjunction with a therapist. It has been shown that simply by supporting the weight of the arm, the distance away from the body that a stroke survivor can reach is increased because of reductions in the required shoulder torques.15 The reduction in shoulder torques addresses both issues related to weakness after stroke16 and the existence of flexion synergies that make it difficult to produce shoulder torque and elbow extension concurrently.17, 18, 19 These distal areas away from the body are the regions of the workspace with the primary deficits after stroke.20 A gravity-compensating passive arm orthosis, termed the WREX, was developed to assist arm movement in children with muscular dystrophy.21 It is designed to compensate for the effects of gravity throughout the 3-dimensional workspace, thereby partially mitigating the weakness produced by muscular dystrophy. The counterbalancing forces are produced by elastic bands attached to the upper- and lower-arm sections of the exoskeleton. This device was then modified to accommodate adult stroke survivors and instrumented to interface with a computer such that it could be used as a stand-alone instrument for therapy.22 Movement of this Therapeutic WREX is translated into movement of a cursor on a computer screen such that the user is able to play computer games requiring controlled arm motion and hand grasp. Another application of the Therapeutic WREX, however, is as a direct assistant to the therapist. A simplified, noninstrumented version of the device could still prove beneficial during therapy sessions, essentially providing another set of helping hands. By improving the user's reaching capabilities, the device could enable the performance of task-specific training movements that may not be possible otherwise. In addition, through use of such a device, the therapist could focus on specific characteristics of the reach that may need correction, such as shoulder motion, without having to be as concerned about supporting the basic reaching movement itself. Thus, the purpose of this study was to modify the Therapeutic WREX further to suit the role better of an assistant to a therapist in the clinic. The extent to which this modified device, the Therapy Assistant WREX, improved reaching across the workspace was quantified in stroke survivors. While a preliminary study of reaching had been performed with the Therapeutic WREX, the parts of the workspace assessed and the kinematic analyses performed were limited.22 This study examined the kinematics and muscle activity associated with reaching movements performed by stroke survivors toward 12 different targets positioned throughout the arm workspace. Reaching performance with the Therapy Assistant WREX was compared with performance during free reaching with no device. Methods  Device Development Three modifications were introduced to the Therapeutic WREX in order to improve its capability for interaction with therapists. First, a simplified mounting system was developed to permit attachment directly to a wheelchair. By using commercially available clamps and 80/20a modular framing, the Therapy Assistant WREX can be easily mounted to standard wheelchairs and thus is quite portable (fig 1A). Translation adjustments in all 3 dimensions can still be made to position the first joint of the Therapy Assistant WREX over the glenohumeral joint of the shoulder. Second, another DOF was added to permit pronation/supination of the forearm and internal or external rotation of the shoulder when the elbow is fully extended. Supination and external rotation are often difficult movements for stroke survivors, especially when performed in combination with elbow extension.17 In the Therapy Assistant WREX, the forearm is secured to a splint that rides on carriages about 2 rings concentric with the long axis of the forearm (fig 1B). Elastic bands can be attached to permit preloading of this DOF. This allows for the possibility of biasing the user toward a certain posture, such as supination, while still permitting movement. Third, accessible mechanical locks were introduced at all Therapy Assistant WREX joints (fig 1C). These joints allow the therapist to lock certain DOFs in desired postures in order to focus on other DOF. In this manner, the Therapy Assistant WREX can assist the therapist in controlling certain body segments or motions. For example, shoulder horizontal abduction could be fixed so that the client could focus on elbow extension and forearm supination. The final device is shown in figure 2. Subjects Ten adult subjects with chronic hemiparesis subsequent to stroke participated in this study. The average duration of hemiparesis in the hand contralateral to the lesion site was 42±23 months, and the average age of participants was 58±14 years, with a range from 37 to 88 years (table 1). Subjects were recruited through fliers posted within the Rehabilitation Institute of Chicago, Chicago, IL, and from a registry of voluntary participants, both approved by the Institutional Review Board of Northwestern University, Chicago, IL. A research occupational therapist screened subjects based on the following inclusion and exclusion criteria. Inclusion criteria were (1) at least 3 months poststroke, (2) with 1 impaired upper extremity, characterized as stage 2 to stage 3 on the Chedoke-McMaster Stroke Assessment scale for the arm,23 and (3) not participating in a current rehabilitation therapy program. Exclusion criteria were shoulder pain, severe spasticity, severe cognitive limitations, and recent injections of botulinum toxin in the upper extremity. The protocol was approved by the Institutional Review Board of Northwestern University, and all subjects gave informed consent according to the Declaration of Helsinki. | | |  | Subject | Age (y) | Sex | Time Poststroke (mo) | Impaired Side | Dominant Arm | |
|---|
| 1 | 58 | F | 35 | R | R | | | 2 | 58 | F | 58 | R | R | | | 3 | 69 | F | 34 | R | R | | | 4 | 37 | M | 41 | R | L | | | 5 | 42 | M | 12 | R | R | | | 6 | 51 | F | 34 | L | R | | | 7 | 56 | F | 98 | L | R | | | 8 | 88 | M | 28 | L | R | | | 9 | 59 | M | 42 | L | R | | | 10 | 59 | M | 39 | L | R | | | Average ± SD | 58±14 | | 42±23 | | | | | | |
Protocol Subjects attended 2 sessions, during which they performed reaching tasks either with or without assistance from the Therapy Assistant WREX. Each session occurred on a separate day; 1 session consisted of free reaching and the other assisted reaching with the Therapy Assistant WREX. Session order was randomized across subjects. Participants were seated in a wheelchair to which the orthosis was attached. They were instructed to move their hand as close to the target as possible at a self-selected speed without leaning away from the backrest during the experiment. Subjects unable to maintain this trunk posture were restrained with a harness system. Twelve targets were positioned at the edge of the arm workspace for each subject. Target location was described using cylindrical coordinates scaled to each subject's anatomy. Three different target heights were employed. The lowest corresponded to the height of the subject's lateral epicondyle when the subject's arm remained at the side (0° of shoulder flexion/extension). The highest was located 15cm above the acromion. The middle height was centered between the lowest and highest target heights. Within each of the 3 horizontal planes, 4 targets were placed corresponding to rotations of 22.5° of horizontal adduction, 0°, and 22.5°, and 45° of horizontal abduction, with respect to the parasagittal plane through the glenohumeral joint (fig 3). The subject's arm was passively extended to its limit along each of these directions to determine the distance away from the body at which the target was placed (fig 3). Each subject performed 3 reaching trials with the impaired arm toward each target for a total of 36 trials a session. In 1 session, the subject was free to move without any connection to the Therapy Assistant WREX. In the other session, the arm was secured to the Therapy Assistant WREX. Namely, the impaired forearm was strapped to a thermoplastic trough connected to the Therapy Assistant WREX. Adjustments were made to the Therapy Assistant WREX to ensure correct positioning of the shoulder and elbow joints with respect to the corresponding joints of the orthosis. Elastic bands applied on the outside of the exoskeleton provided counterbalance for both the weight of the exoskeleton and of the limb until a resting arm position of 45° of shoulder flexion and approximately 90° of elbow flexion was achieved. A platform was positioned just in front of the umbilicus. This platform designated the starting hand position for each reach during both sessions. For the free-reaching trials, subjects did not need to activate their muscles to maintain their hand on the platform for the starting position. During the assisted trials, a few of the subjects had to activate their muscles minimally to maintain the starting position on the platform. Subjects were instructed to move as far toward the indicated target as possible and then return their hand to the platform, all at a self-selected pace. Because subjects had moderate to severe hand impairment, they were not asked to point to the target but rather were instructed to move the whole hand or fist toward the target. Arm movement kinematics was measured using the Optotrak Certusb motion detection system at a sampling rate of 30Hz. Four infrared light-emitting markers were placed on the proximal segments of the 4 fingers of the subject's hand. Two markers were also placed on each target. In addition, muscle activity was recorded at 500Hz using bipolar surface EMG electrodesc placed over 4 muscles in the upper extremity: biceps, triceps, anterior deltoid, and brachioradialis. Kinematic data collection was synchronized with EMG data collection using a trigger from custom software written in LabVIEW.d Data Analysis Position data were digitally low-pass–filtered forward and backward in time at 5Hz with a thirtieth-order finite impulse response filter to attenuate high-frequency noise without altering signal phase. Subsequent analyses used these filtered data. The data were numerically differentiated using a 5-point formula to obtain velocity, acceleration, and then jerk. Four kinematic parameters of reach were computed for each reach: the fraction of the full reach toward the target, the peak tangential speed, the timing of the peak speed within the movement, and the mean jerk during the movement. These kinematic parameters have been used in previous studies of reach both in neurologically intact individuals and in stroke survivors.14, 20, 24, 25, 26, 27, 28, 29, 30, 31 For this study, we focused on the movement toward each target. The beginning of each trial was determined by the time at which the hand velocity first surpassed 0.5cm/s and maintained that level for 5 successive samples. The end of the movement was defined as the time point when the subject became closest to the target. The fraction of reach was computed by comparing the minimum distance to the target achieved by the subject to the Euclidean distance between the starting hand posture and the target.20 Thus: where the subscripts T, S, and A refer to the target location, the starting location of the hand, and the actual position at which the hand was closest to the target, respectively. Peak speed was found by first computing the tangential speed from the norm of the velocity vector. Maximum tangential speed was defined as maximum speed during the movement toward the target. The time to peak speed within movement was found by normalizing the elapsed time from the start of the trial to the attainment of the peak speed by the total elapsed time for the entire movement. Finally, jerk was obtained by finding the average norm of the jerk across the movement. The EMG data were rectified and then digitally low-pass–filtered at 30Hz using a thirtieth-order finite impulse response filter to create envelopes for the rectified EMG signals. Envelope values for each muscle for each subject were normalized by the maximum envelope recorded across all trials (including attempted maximum voluntary contraction) for the corresponding muscle. Normalized EMG envelopes were averaged across the movement interval previously defined. Statistical tests were performed to compare reaching movements with and without the Therapy Assistant WREX. Namely, repeated-measures ANOVAs were run for each of the dependent variables: fraction of reach, maximum tangential speed, time to peak speed within movement, jerk, and EMG for biceps, triceps, anterior deltoid, and brachioradialis. The 3 independent factors of use of Therapy Assistant WREX/free reach, target height (3 levels), and target horizontal abduction location (4 levels) were tested for significance for each ANOVA. Results The kinematics and EMG characteristics for reaching toward targets placed throughout the distal edge of the arm workspace were quantified for a group of stroke survivors. The effect of use of the Therapy Assistant WREX on these characteristics was examined. Proximity to the target improved across all targets when subjects used the Therapy Assistant WREX. The fraction of reach improved by a value of .22±.21 (P<.001), from .41 to .63 (table 2). This denotes that, on average, subjects could move closer to the target by 22% of the overall distance to the target when using the Therapy Assistant WREX. This improvement was fairly uniform across all targets (fig 4A). There was no significant impact of target height or angular location with respect to the parasagittal plane on the difference in fraction of reach. | | | | | FOR | SPEED (cm/s) | TPS | Jerk | |
|---|
| With | 0.63±0.20 | 520.3±237.3 | 0.41±0.54 | 24.1±25.6 | | | Without | 0.41±0.27 | 599.5±308.2 | 0.54±0.23 | 40.3±41.1 | | | With - without | 0.22±0.21⁎ | −79.2±220.6⁎ | −0.13±0.27⁎ | −16.2±29.5⁎ | | | | |
The peak speed of the hand movement during the reach decreased significantly by 79.2±220.6cm/s when using the Therapy Assistant WREX (P<0.001), from a value of 599.5cm/s to 520.3cm/s (see table 2). Thus, the peak speed decreased on average by 13%. The location of the peak speed, however, shifted to an earlier point in the movement, from 54% of the movement duration to 41% of the movement duration (P<.001). No significant effect of target location on the change in time to peak speed within movement with and without the Therapy Assistant WREX was observed, although the smallest differences were observed for the targets requiring greatest horizontal abduction (fig 4B). In addition, the average jerk in the movement decreased when using the Therapy Assistant WREX. There was a 40% decrease in jerk from 40.3cm/s3 to 24.1cm/s3 (P<.001). The decrease in jerk occurred across target locations, because neither target height nor target location angle had a significant impact on the difference (fig 4C). Improved smoothness of the movement was evident in plots of the movement speed for individual subjects (see fig 4). EMG analysis showed that the use of the Therapy Assistant WREX led to a reduction in biceps activity during the reach (P<.05) (fig 5). The reduction occurred across targets. Target height tended to have a slight effect on biceps activity with greater activity for the higher targets (P=.06). Use of the Therapy Assistant WREX also led to a marginally significant reduction in activity of the anterior deltoid (P=.055). As might be expected, target height did have a significant effect on deltoid activity with or without the Therapy Assistant WREX (P<.001). Anterior deltoid EMG for the highest targets was significantly greater than that measured at the other 2 target heights, and anterior deltoid EMG at the middle target height was greater than at the lowest target height. Neither triceps activity (P=.47) nor brachioradialis activity (P=.28) exhibited significant changes when using the device (table 3). | | | | | Biceps | Triceps | Ant. Deltoid | Brachioradialis | |
|---|
| With | 0.10±0.06 | 0.11±0.08 | 0.13±0.06 | 0.08±0.05 | | | Without | 0.14±0.07 | 0.12±0.10 | 0.18±0.08 | 0.10±0.06 | | | With - without | −0.04±0.01⁎ | −0.02±0.02 | −0.05±0.02 | −0.02±0.01 | | | | |
Discussion Stroke survivors performed reaching tasks throughout the arm workspace either with or without the passive Therapy Assistant WREX. Reaching performance toward fixed targets, as measured with several kinematic variables, was improved with the use of the Therapy Assistant WREX compared with free reaching. Subjects were able to move closer to targets at the distal edge of the arm workspace, the portion of the workspace with the greatest deficits.32 The mean increase in distance toward the target was 22% of the total distance to the target. The improvement was seen across the workspace, in contrast with what was described previously,22 for targets at different heights and different locations with respect to the sagittal plane. This lack of dependence on target location is understandable given the placement of targets at the edge of the arm workspace. Each reaching trial required elbow extension in conjunction with shoulder abduction. As described in other studies,17, 18 activation of the shoulder abductors often leads to activation of the more distal elbow flexors in stroke survivors. This elbow flexion prevents attainment of the limits of the workspace. Indeed, excessive upper extremity flexor coactivation is an issue for stroke survivors with chronic hemiparesis.33, 34 In this study, both biceps and anterior deltoid EMG did increase with increasing target height. Use of the Therapy Assistant WREX led to reductions in biceps and anterior deltoid EMG, in accordance with a study examining reaching with gravity compensation in neurologically intact individuals.35 As expected, use of the antigravity orthosis led to a marginally significant (20%) decrease in the EMG activity of the shoulder muscles used to lift the arm. Thus, the observed decrease in biceps EMG activity may have resulted from the decrease in anterior deltoid activity. These muscles are commonly seen to be excessively coactivated in stroke survivors.36, 37 Conversely, there was no significant change in either triceps or brachioradialis EMG when using the device. While the reduction in deltoid activity may be disadvantageous in terms of strength training for the shoulder, which may be beneficial after stroke,16, 38 the corresponding increase in arm workspace could facilitate reaching practice. The amount of gravity compensation can be easily reduced as well, as the user progresses. In addition to reaching distance, reaching characteristics were altered by the use of the Therapy Assistant WREX. Peak speed was actually reduced by roughly 13%, although the movement speed was self-selected. Certainly, the presence of the Therapy Assistant WREX may have slowed movement. Alternatively, the maximum speed may have been reduced because the movement was more controlled with the Therapy Assistant WREX. Indeed, the time at which the peak speed occurred shifted to an earlier, more typical part of the movement, from 54% of the movement to 41% of the movement. Values for reaching in unimpaired individuals lie within the first 30% to 40% of the movement.39 Furthermore, previous research has shown that reaching deficits in speed are relatively minor compared with deficits in accuracy and efficiency for stroke survivors.27 Also, the smoothness of the reaching movement, as quantified by the mean jerk, improved. It should be noted that the improvement in smoothness may have been partly attributable to damping provided by the device. While the decrease in peak speed could have confounded the decrease in jerk, normalization of the jerk by the peak speed40 still resulted in a significant decrease in jerk when using the Therapy Assistant WREX (P<.001). Study Limitations While use of the Therapy Assistant WREX did improve the distance that stroke survivors could reach away from the body for the given targets, some limitations regarding the study and the device should be considered. The Therapy Assistant WREX itself limited the workspace at extreme shoulder extension and shoulder abduction. Perhaps finer adjustments of the device segment lengths would minimize this interference. Also, because of the bulk of the exoskeleton, collisions with the body may be an issue when working in regions close to the body. Active range of motion was improved, but function in these more distal regions of the workspace was not assessed. It is possible that the functional workspace was not improved because of lingering hand impairment. Conclusions Thus, it appears as though the assistance provided by the Therapy Assistant WREX enables stroke survivors to control their arm movement better and to explore distal regions of the arm workspace that are normally inaccessible for this population. By increasing the available workspace, clients are able to use the arm in a more functional manner, and therapists can thereby incorporate task-specific training more easily into their rehabilitation programs. This is especially important for stroke survivors who have more severe limitations in upper-extremity performance. In addition, the assistance allows the user and the therapist to focus fully on distal movement to complete a specific task rather than on creating movement throughout the arm. Furthermore, retraining of certain movement patterns, such as trying to keep the elbow down rather than up during the reach, can be attempted. To facilitate this type of training further, each joint of the Therapy Assistant WREX can be individually locked. For example, the shoulder DOF could be locked to enable the use to focus on elbow extension and forearm supination without having to worry about stabilizing the shoulder until the user is ready to so do. The WREX devices have advantages over other gravity compensation devices such as a suspension arm sling or a mobile arm support, including an increase in available workspace, which allows for task-specific practice. The other devices do not permit movement above 90° of shoulder flexion or abduction; mobile arm supports do not allow full elbow extension. In addition, while mobile arm supports allow limited shoulder rotation and forearm supination, suspension arm slings allow neither. Furthermore, mobile arm supports and suspension arm slings were designed to work in a horizontal plane and movement below that plane is difficult, whereas the Therapy Assistant WREX allows ease of movement throughout the 3-dimensional workspace. Examples of functional tasks possible with the Therapy Assistant WREX include ability to reach the head to comb hair or assist with donning shoes and socks with the leg crossed over the knee. Furthermore, with the Therapy Assistant WREX, compensation can be adjusted for different DOF and can assist or resist nongravitational forces. For example, in this study, assistance for the upper arm section of the Therapy Assistant WREX was adjusted until 45° of shoulder flexion was achieved and the lower arm section was adjusted until 90° of elbow flexion was attained. In addition, tension could be adjusted such as to bias joint rotation toward the weaker direction. For example, stroke survivors typically have difficulty creating forearm supination. The pronation/supination DOF of the Therapy Assistant WREX may be biased toward supination as the resting posture or lift the shoulder into extension. In this manner, the Therapy Assistant WREX can facilitate the incorporation of practicing the control of hand orientation in combination with proper positioning of the hand in space. In the future, a passive glove, biased toward finger extension, will be combined with the Therapy Assistant WREX to facilitate the practice of reach-to-grasp. Suppliers Acknowledgments We thank Tariq Rahman, PhD, David Reinkensmeyer, PhD, and Robert Sanchez, PhD, for graciously sharing their mechanical designs for the WREX and Therapeutic-WREX. References 1. 1Wolf SL, Blanton S, Baer H, Breshears J, Butler AJ. Repetitive task practice: a critical review of constraint-induced movement therapy in stroke. Neurologist. 2002;8:325–338. MEDLINE |
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Supported by the National Institute on Disability and Rehabilitation Research, Rehabilitation Research and Training Center for Stroke (grant no. H133B031127), and by the Coleman Foundation. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated. Reprints are not available from the author. PII: S0003-9993(08)00794-6 doi:10.1016/j.apmr.2008.04.022 © 2008 American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation. Published by Elsevier Inc. All rights reserved. | |
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