Design and Analysis of a Compliant Bimanual Rehabilitation Device

Samuel McAmis and Kyle B. ReedDepartment of Mechanical Engineering

University of South FloridaTampa, Florida, 33620

Email: smcamis@mail.usf.edu, kylereed@usf.edu

Abstract—Bimanual rehabilitation allows an individual withhemiparesis to use their sound arm to help rehabilitate theirimpaired arm and shows promise as a means of low cost home userehabilitation. However, few studies have attempted to determinethe best combination of bimanual symmetry modes and couplingstiffnesses. To examine the effects of stiffness and symmetry type,we have developed a compliant bimanual rehabilitation device(CBRD) that allows the two hands to be coupled in a variety ofsymmetry modes and coupling stiffnesses so that their efficacymay be tested. Preliminary analysis has shown that the CBRDcan couple the motions of two healthy individuals, which we areusing to simulate an individual with a hemiparesis, and improvesbimanual task performance of a single individual. The results alsoshow that a higher coupling stiffness reduces the coupling errorand that some tasks were easier to complete in visual symmetry.

I. INTRODUCTIONBimanual rehabilitation allows an individual to self-

rehabilitate by guiding his paretic arm with his sound armusing an external physical coupling. This coupling allowsthe individual to move his impaired hand through motionshe would not otherwise be able to make while still givinghim complete control over the motion generated, somethingthat a physical therapist or robot would not be able do. Thismethod also allows for upper-limb rehabilitation devices thatare significantly lower in cost than robotic systems since muchof the required force could be provided by the patient’s healthylimb instead of the larger motors included on many currentupper-limb rehabilitation robots. This would result in a lowercost and safer rehabilitation method that could be used athome, increasing access to rehabilitation. Here, we present adevice that allows the hands to be coupled in several commonsymmetry modes and with a selectable coupling stiffnesses.The device was tested with healthy subjects in tasks that mimicaspects of hemiparesis as well as standard bimanual tasks.

II. BACKGROUNDThe goal of upper-limb rehabilitation following a stroke

is to enable a person to use both hands in activities ofdaily living. Of the new rehabilitation methods proposed andtested in recent years, many show positive results, but there isno method that clearly shows better results than traditionalmethods [1][2]. A common thread among the successfulstudies is that the amount of time spent training the affectedarm plays an important role in improving the functional abilityof the affected arm. As it is difficult for therapists to devoteas much time as is needed, researchers have looked to roboticand other methods to supplement rehabilitation.

A. Traditional and Robotic Rehabilitation TechniquesConventional stroke rehabilitation therapies, such as

the Bobath method [3] and proprioceptive neuromuscularfacilitation [4] have been used for decades. However, these

methods are time-consuming and require significant effortfrom physical therapists. Forced use [5] and the more recentlydeveloped Constraint-Induced Movement Therapy [6] bind thesound arm and force the individual to use only the paretic limb,but is only viable for small to moderate impairment.

In recent years, robotic technologies have been usedto provide rehabilitation to individuals, allowing access torehabilitation for longer and more frequent periods of time.However, recent review papers have stated that it is unclearwhether robotic methods have the potential to produce greaterbenefits than conventional techniques when practiced for thesame amount of time [1][2].

To allow patients greater access to rehabilitative training,several methods have been developed to allow patients torehabilitate at home [7][8]. However, many of these home-based methods use a home computer with limited accessoriesthat cannot provide assistance forces and can only operate overa small workspace. These methods are able to provide somebenefit, but the rehabilitation effect is limited to people whohave relatively high motor function.

B. Bimanual RehabilitationThe idea behind bimanual rehabilitation is that an

individual uses their healthy arm to assist their ownimpaired arm through simultaneous bimanual motions. Thekey mechanism of rehabilitation is that the same neural signalis sent to the arms, which results in the same proprioceptivefeedback from each limb since the arms are constrained tomove together. Sending the same efferent signals to each limbwill result in similar afferent signals from the limbs, which willhelp retrain the motor pathways to the impaired side [9][10].Several research groups have studied certain aspects of coupledand uncoupled bimanual rehabilitation [11][12][13][14][15],but few studies to date have examined what the ideal physicalparameters for bimanual interaction should be.

The above studies either did not physically connect thehands or coupled the hands rigidly, and few studies haveanalyzed the effect of the coupling stiffness. The ideal couplingis likely an intermediate stiffness, since a soft coupling wouldprevent severely impaired individuals from using this trainingmethod and, with a completely rigid connection, the individualis likely to apply minimal force in their impaired hand sincethe healthy side will dictate all the motions [1][16].

It is not currently known which types of symmetrymodes are most effective for bimanual rehabilitation. Mirrormotions have been the most commonly used in bimanualrehabilitation studies to date. However, most daily tasks occurin a visual reference frame where the hands move in the samedirection. Three common reference frames used in bimanualrehabliltation are Mirror or Joint Space Symmetry (JSS),Visual Symmetry (VS), and Point Mirror Symmetry (PMS),as shown in Fig. 1 [17][18].

2013 IEEE International Conference on Rehabilitation Robotics June 24-26, 2013 Seattle, Washington USA

978-1-4673-6024-1/13/$31.00 ©2013 IEEE

Joint space symmetry

Visual symmetry(task space)

Point mirror symmetry(task space)

Fig. 1. Common bimanual symmetry modes include Joint Space Symmetry(JSS) where the joint angles are mirrored, Visual Symmetry (VS) where thehands move through the same visual path, and Point Mirror Symmetry (PMS)where the hand motions are mirrored about a point in space.

Preliminary studies of bimanual symmetric motions onhealthy participants have shown that it is easier to followand recreate motions in VS and JSS than in PMS [19] andthat a coupling stiffness of 200 N/m or greater resulted inbetter path following and motion coupling. These studies wereperformed on a pair of Phantom Omni force feedback devices.The research presented here details the design and preliminaryanalysis of a device that will allow testing the efficacy ofdifferent coupling stiffnesses and symmetry modes in bimanualrehabilitation.

III. COMPLIANT BIMANUAL REHABILITIATION DEVICEWe developed a device that physically couples two handles

in a selected symmetry with an adjustable coupling stiffness,shown in Fig. 2. It is divided into two sub-assemblies: thecoupling system connects the handle in a desired symmetrymode; the compliant handle assemblies allow the handles to bemoved away from the correct symmetric positions, but providea spring force back towards the symmetric positions. The totalmaterial cost to produce the prototype, including sensors andinterface hardware, was approximately $700.

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Fig. 2. CBRD components: (1) Base, (2) Carrier Assembly, (3) UpperAssembly, (4) Right Handle Slide, (5) Left Handle Slide, (6) Compliant HandleAssembly, (7) Lock for JSS and VS modes, (8) Lock for PMS mode, (9) RotaryEncoder, (10) Spring Stack, (11) Load Cell.

A. Coupling SystemThe coupling system consists of a four jointed mechanism

with three prismatic joints and one revolute joint. The firstjoint, hereafter referred to as the Y-axis joint, is prismatic andconnects the base to a captive carrier assembly that supportsthe rest of the device, allowing for motion towards or awayfrom the participant for both JSS and VS modes. A bolt witha captive nut is used to remove this degree of freedom forPMS. The second joint, in the center of the carrier assembly,is revolute and connects the carrier assembly to the upperassembly and allows the latter to rotate for PMS. This joint isreferred to as the Z-axis joint. A locking plate removes thisdegree of freedom for JSS and VS symmetry modes.

The remaining two joints allow for lateral motion of bothhandle slides in JSS and VS and for radial motion in PMS. Themotion of these X-axis joints is monitored by optical encoderswith a resolution of 0.25◦. The encoders contact the handleslides with friction wheels of radius 2.38 mm, resulting in alinear resolution of 0.10 mm.

The motions of the last two joints are coupled by cable runson the back side of the upper assembly. As shown in Fig. 3,altering the cable path changes the coupling. If the cables looparound the pulleys an odd number of times, the motions of thehandle slides will be mirrored, as necessary for JSS and PMS.If the cable loops around the pulleys an even number of times,the handle slides will move in the same absolute direction asrequired for VS.

In JSS and VS, each handle has a workspace 330 mm deepand 431 mm wide, starting 124 mm from the centerline. InVS the distance between the handles is 679 mm, so that themaximum extension for one handle is the minimum extensionfor the other. In PMS the workspace is a disk with an innerradius of 124 mm and an outer radius of 555 mm. At fullextension in JSS or PMS, the handles are 1110 mm apart.

The stiction in the base to carrier assembly joint isapproximately 4-20 N, though typically less than 10 N,dependent on the extension of the handle slides and theresultant torque applied to the joint. The resistance in thecarrier plate-upper assembly joint is negligible. The stiction inthe upper assembly to handle slide joint is approximately 10-15 N. The total mass of the carrier and all moving componentsis 6.9 kg. Future versions will reduce the stiction and weight.

B. Compliant Handle AssemblyEach handle is connected to the coupling system by

a compliant assembly that forces the handle towards thecorrect position, but allows the handle to deviate from it. Thecompliant handle assembly consists of three links, connectedby two pins, and a stack of custom torsion springs on each pin.These springs consist of an L-shaped piece of acetal plastic,51 mm per leg, with a hole for the connecting pin where the

a)

b)

Fig. 3. Diagram of cable layouts as viewed from the rear: a) Joint SpaceSymmetry and Point Mirror Symmetry, b) Visual Symmetry

legs meet. We use custom springs because standard torsionsprings are designed for larger deflections than we are using,and to achieve the same stiffness, standard springs require morematerial, substantially increasing the size and weight. Thesecustom springs also allow for more control over the stiffnessesimplemented. The performance of the custom springs wasconfirmed to be linear over the range used.

In the assembly, the second and third links make up thehypotenuse and one leg, respectively of a 45◦- 45◦- 90◦

triangle, with the handle at the 90◦ corner. This results in thetorques about the pins producing a symmetric stiffness ellipseat the handle, for small deflections, although large deflectionswill result in distorted stiffness ellipse. Future versions willoptimize the shape of the stiffness elipse.

The compliant handle assembly is designed for a maximumdeflection of 75 mm in any direction. For this deflection, themaximum width of the springs is 6 mm, hence a stack of6.4 mm thick springs is used to achieve higher stiffnesses.Each spring adds 110 N/m to the stiffness of the connectionbetween the handle and the handle slide, however, since bothhandles are connected in this way, the overall coupling stiffnessadded by each set of springs is 55 N/m, and the maximumcombined deflection from correct coupled positions is 150 mm.The stiffness ellipse for one handle with two springs is shownin Fig. 4.

The forces in the links are monitored by shear load cells.From the load cell readings, the force on the handle can becalculated, and given a known joint stiffness, based on thenumber of springs used, the joint deflection can be calculated,along with the handle position.

C. Display and Interaction GameThe workspace of the device is visually represented on a

display located above and slightly behind the device to allowusers to interact with visually displayed targets. The displayedworkspace was scaled down by a factor of 2.5:1, resulting ina visual workspace area 132mm tall and 442mm wide. Forconsistency, unless otherwise noted, all dimensions given arefor the physical workspace. Desired positions of the handleswere presented as red circles 40 mm (16 mm displayed) indiameter. The right and left handles were displayed as greenand blue circles, respectively, both 40 mm in diameter.

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Fig. 4. Diagram of cable layouts as viewed from the rear: a) Joint SpaceSymmetry and Point Mirror Symmetry, b) Visual Symmetry

For all of the studies presented here, the task thatparticipants were asked to complete consisted of matchingthe handle position(s) with the desired position(s). Each trialconsisted of a series of eighteen segments, beginning with thedisplay of randomly generated desired positions. The segmentwould end and, after a brief delay, the desired position wouldshift to a new position once the handle position was within5mm of the desired position, or 15 seconds had elapsed sincethe desired position was first displayed.

IV. PROCEDURETo evaluate the effectiveness of the device at coupling hand

motions, a series of studies were conducted. The eventual goalis stroke rehabilitation; here, we are using two people to mimicthe lack of bimanual coordination that occurs in individualswith stroke. This is a harsher test since the two participantsare completely uncoupled neurally whereas an individual withstroke can couple the motions, but cannot fully control one ofthe arms. Thus, we evaluated the device in both a dual andsingle participant study.

A. Two Participant StudyIn the dual-participant study, two participants stood in front

of the device and each grasped a handle. The participant on theright held the right handle and the participant on the left heldthe left handle, mimicking the way that it would be held by aperson with a stroke during rehabilitation. For each trial, oneparticipant was designated as the guiding participant and theother participant was considered the following participant. Thedesired positions and handle positions were only displayed tothe guiding participant and the following participant was askedto close their eyes or use a blindfold. A curtain separated theparticipants so that the guiding participant could only see theirside of the device and the computer screen. The purpose of thetwo participant study was to quantify the performance of thedevice when one hand applies minimal input to the system.

The participants were asked to complete two types oftasks in different coupling symmetry modes and with differentcoupling stiffnesses. The symmetry modes tested were JSSand VS; PMS was omitted because it has been shown to bemore difficult to coordinate bimanual motions in [19] and tolimit the total study time to 1 hr to reduce the possibilityof participant fatigue. The coupling stiffnesses tested were110 N/m and 380 N/m. The lower stiffness was selected tobe between 50 N/m and 200 N/m since this was shown tobe an area of transition in path perception accuracy [19].The 380 N/m stiffness was selected as the highest possiblestiffness without reducing the compliant workspace area belowthe maximum diameter of 300 mm.

In one task, hereafter referred to as Two Person-GuidingVisible (2P-GV), only the guiding participant’s desired andhandle position were displayed. For this task, the guidingparticipant was asked to match their handle position withthe desired position as quickly as possible. In the other task,hereafter referred to as Two Person-Following Visible (2P-FV),the following participant’s desired position and both handlepositions were displayed. For this task, the guiding participantwas asked to match the following participant’s handle positionwith the desired position.

Both participants completed all combinations of symmetrymode, stiffness and task type twice, once as the guide andonce as the follower. The overall order of symmetry mode,

stiffness, task and guiding participant was randomized for eachpair of participants. However, to avoid confusion, and reducedelay time from switching configurations, the trials for eachcoupling stiffness were presented together. Similarly, for eachcoupling stiffness, all of the trials for one symmetry mode werepresented before changing the symmetry mode, and for eachsymmetry mode, one guiding participant completed both tasksbefore the guiding participant was changed. Ten participantsperformed this study with IRB approval: eight were male, allwere right handed, age 21-61 years old.

B. Single Participant StudyIn this study, a single participant stood in front of the device

and held both handles. The participants were asked to completethree types of tasks in different coupling symmetry modesand with the handles of the device in one of two couplingconditions: either physically coupled in the desired symmetrymode, or uncoupled. The symmetry modes tested were thesame as those tested in the two participant study. When thehandles were coupled, a coupling stiffness of 380 N/m wasused for consistency with the two participant study.

For the physically coupled trials, the device was lockedin the desired symmetry mode. To uncouple the handles,neither the Y nor Z-axis joints were locked, allowing thehandles to be positioned independently, anywhere in the deviceworkspace, however, they were dynamically coupled by inertiaand friction, and the handles would still twist by the sameangle about the the Z-axis. In the uncoupled trials, participantswere instructed to couple their hand motions in the desiredsymmetry mode.

One task was identical to that of the two participant study.In this task, referred to as One Person-Single Visible (1P-SV),participants were asked to match one handle position to adesired position as quickly as possible, while moving both oftheir hands together in the desired symmetry mode. In anothertask, referred to as One Person-Both Visible (1P-BV), both leftand right handle and desired positions were displayed in thecurrent symmetry mode, and participants were asked to matchboth handle positions to the desired positions. The purpose ofthese tasks was to analyze the effect of the CBRD on assistinga healthy paricipant in coordinating their hand motions.

In the third task, referred to as One Person-DistortedPositions (1P-DP), both left and right handle and desiredpositions were displayed, but their positions from the zeroposition for the symmetry mode were distorted by a factor of1:1.5, and participants were, again, asked to match both handlepositions to the desired positions. The purpose of this task wasto mimic the decreased perceptional ability of individuals withstroke and test the device’s ability to transmit forces.

Participants completed all combinations of symmetrymode, coupling condition and task twice; 1P-SV wascompleted once with the left visible and once with the rightvisible, and similarly 1P-DP was completed once with thedistortion on the left and once with the distortion on the right.The 1P-BV condition was simply completed twice under thesame conditions.

The overall order of symmetry mode, coupling condition,task, and left or right display/distortion was randomized.However, to avoid confusion, and reduce delay time fromswitching configurations, the trials for each symmetry modewere presented together. Similarly, for each symmetry mode,all of the trials for one coupling condition were presented

before changing the coupling condition. If the first trial thata participant would conduct in a new symmetry mode wasuncoupled, and only one desired position displayed, i.e. theywould have neither visual nor haptic indication of how tocouple their hand motions, they were permitted to practicemoving in the desired symmetry mode until they understoodthe correct way to couple their motions. Six participantsperformed this study with IRB approval, five were male, allwere right handed, age 21-25.

C. AnalysisTo quantify performance during a trial, the average

completion time and the average coupled position errorwere analyzed. The average completion time for a trial wasdetermined by calculating the average segment time, from thedisplay of a desired position or positions to the matchingof the handle position(s) with the desired position(s), andaveraging these segment times for each trial. The averagecoupled position error was the average, for a trial, of thedistance between the right handle position and the projectedsymmetric position of the left handle at the end of eachsegment. The projected symmetric position of the left handlewas determined by mirroring the position of the handle forJSS mode or adding 679 mm to the left handle position forVS mode.

For statistical analysis, we conducted an analysis ofvariance (ANOVA) to analyze the effects of symmetry mode,coupling stiffness or condition, task type and guiding side onthe average completion time and average coupling positionerror. When the ANOVA yielded significant results, we usedTukey’s honestly significant difference test. We used an alphaof 0.05 for all statistical tests

V. RESULTSA. Two Participant Study

Since the two types of tasks in the dual-participant studyare inherently different: moving a handle directly vs. movinga handle through the coupling of the device, we performed ouranalysis with both task types together, and for each task typeindividually.

For both tasks, an analysis of the average com-pletion time showed statistically significant results be-tween symmetry modes (F1,79 = 9.31,p = 0.003), cou-pling stiffnesses (F1, 79 = 4.69,p = 0.03) and task types(F1, 79 = 131.2,p < 0.001). Post hoc analysis showed that thecompletion time was lower for VS mode, for the 110 N/mstiffness, and for the 2P-GV task. The completion times forthe symmetry modes and tasks are shown in Fig. 5. Theaverage completion time for 2P-GV was 2.7 s, and the averagecompletion time for 2P-FV was 5.7 s.

For the 2P-GV task, analysis of the average completiontime did not show statistically significant results betweensymmetry modes or coupling stiffnesses. For the 2P-FVtask, analysis of the average completion time showedstatistically significant results between symmetry modes(F1, 39 = 9.45,p = 0.004). Post hoc analysis showed that theaverage completion time was lower for VS than for JSS.

For both tasks, analysis of the average coupled positionerror showed statistically significant results between symmetrymodes (F1, 79 = 4.90,p = 0.03) and coupling stiffnesses(F1, 79 = 265.48,p < 0.001). Post hoc analysis showed thatthe the error was smaller for JSS than VS, 51 mm and 56 mm,

respectively, and that the error was lower for the 380 N/mstiffness than for the 110 N/m stiffness.

For the 2P-GV task, analysis of the average coupledposition error showed statistically significant results betweencoupling stiffnesses (F1, 39 = 140.53,p < 0.001). For the2P-FV task, analysis of the coupled position error showedstatistically significant results between coupling stiffnesses(F1, 39 = 117.97,p < 0.001). Post hoc analysis showed thatthe average error was lower for the 380 N/m stiffness and wascomparable to the average for both tasks.

B. Single Participant StudyFor the single participant study, the analysis was performed

both with the data from the three tasks combined as well as forthe data of the tasks individually. The coupled position errorwas only analyzed for the 1P-SV task because in the othertasks, the correct final position for both handles was displayed

For all three tasks and both coupling conditions, analysisof the average completion time showed statistically significantresults between the task types (F2, 143 = 40.17,p < 0.001).Post hoc analysis showed that 1P-SV was completed fasterthan 1P-BV, which, in turn, was completed faster than 1P-DP.The average completion times for 1P-SV, 1P-BV, and 1P-DPwere 2.2 s, 2.8 s and 3.3 s, respectively.

For the 1P-SV task and both coupling conditions,analysis of the average completion time showed sta-tistically significant results between coupling conditions(F1, 47 = 40.17,p = 0.003). Post hoc analysis showed that thetask was completed faster with the handles coupled (Fig. 6).

For the coupled 1P-SV task, analysis of the averagecompletion time showed statistically significant resultsbetween symmetry modes (F1, 23 = 7.14,p = 0.05). Post hocanalysis showed that the task was completed faster in VS thanin JSS. For the uncoupled 1P-SV task, analysis of the averagecompletion time did not show statistically significant results.

For the 1P-BV task and both coupling conditions,analysis of the average completion time showed sta-tistically significant results between coupling conditions(F1, 47 = 34.13,p = 0.001). Post hoc analysis showed thatthe task was completed faster when the handles were coupled(Fig. 6).

JSS VS0

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Fig. 5. Results of average completion time analysis for Two ParticipantStudy. Error bars represent 95 % confidence interval.

For the 1P-DP task and both coupling conditions,analysis of the average completion time showed sta-tistically significant results between coupling conditions(F1, 47 = 11.24,p = 0.002). Post hoc analysis showed that thetask was completed faster when the handles were uncoupled(Fig. 6). Analysis of the completion time for the uncoupled1P-DP task showed statistically significant differences betweensymmetry modes (F1, 23 = 15.34,p = 0.001). Post hocanalysis showed that the task was completed faster in JSS thanin VS. Analysis of the completion time for the coupled 1P-DPtask did not show statistically significant results.

For the 1P-SV task and both coupling conditions, analysisof the coupled position error showed statistically significantresults between symmetry modes (F1, 47 = 8.7,p = 0.005)and coupling conditions (F1, 47 = 32.2,p < 0.001). Post hocanalysis showed that the error was smaller in JSS than in VS,and when the handles were coupled.

For the coupled 1P-SV task, analysis of the coupledposition error showed statistically significant results betweensymmetry modes (F1, 23 = 45.54,p < 0.001). Post hocanalysis showed that the error was smaller for JSS than VS. Forthe uncoupled 1P-SV task, the error did not show statisticallysignificant results between symmetry modes.

VI. DISCUSSIONThe two participant study showed that both the 380 N/m

stiffness and VS mode results in faster completion times. Thehigher stiffness may improve completion time due to betterhaptic communication with the following partner, but may alsobe attributable to better control over the dynamic motion of thesystem. The fact that 2P-FV task is completed faster in VSthan in JSS, as shown in Fig. 5, makes sense because in JSSthe guiding participant must account for the mirrored motionof the handle that he is attempting to move to the desiredposition, while in VS the following handle moves in the samedirection as the handle that he is controlling directly does. Thisindicates that for bimanual rehabilitation tasks in JSS mode,it may be beneficial to display the desired position of bothhandles so that an individual may focus on generating bothmotions together rather than on the motion of the healthy armrequired to assist the impaired arm in the correct direction.

1P−SV 1P−BV 1P−DP0

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Fig. 6. Results of average completion time analysis for Single ParticipantStudy. Error bars represent 95 % confidence interval.

The two participant study also showed that the coupledposition error is smaller for the 380 N/m stiffness thanthe 110 N/m stiffness at approximately 30 mm, and 75 mm,respectively, corresponding to forces applied of 11.4 N and8.25 N, respectively, which is consistent with the friction inthe coupling system as stated in Section III-A.

The coupled position error showed a difference betweensymmetry modes, indicating that there may be a difference inperformance in coupling modes, although the difference is onthe order of 10% of the coupled position error.

The 1P-SV task with the handles coupled showed thatthe average completion time was lower for VS than for JSS.This is consistent with the idea that many VS tasks, suchas moving a large object, are done with the hands coupledtogether, and may be a more natural symmetry mode if onlythe desired position of one handle is displayed. However, ourpreliminary studies [19] show that uncoupled non-harmonicmotions should also be faster in VS than in JSS. The differencemay be attributable to friction and inertial forces slowing themotions enough to mask the differences in completion time.Therfore, further coupled bimanual studies on a device withlower impedance should be conducted, and an effort should bemade to reduce the impedance of the CBRD.

For the 1P-DP task, the average completion time was lowerwhen the handles were uncoupled. This makes sense becausewhen the handles are coupled for this task, the participant mustfight against the device to move the handles to the distorteddesired positions. The forces required to reach the desiredpositions ranged from 0-45 N.

The single participant study also showed that for the 1P-SVand 1P-BV tasks, when the handles were coupled in the desiredsymmetry mode, the average completion time was lower, asshown in Fig. 6. The figure also shows that the averagecompletion time for 1P-SV uncoupled is comparable to 1P-BV coupled, demonstrating that coupling motions through theCBRD can reduce the difficulty of matching two visuallydisplayed positions to that of matching only one. These resultsshow that coupling the hand motions through the CBRDimproves performance of a healthy subject at completingbimanual tasks, indicating that it should be implemented inbimanual rehabilitation studies to test its efficacy.

VII. CONCLUSIONS AND FUTURE WORKOur results show that the CBRD effectively couples

the bimanual motions of healthy subjects in JSS and VSmodes, and that a higher coupling stiffness results in betterperformance in two participant bimanual tasks simulatinghemiparesis. This two participant study also showed that whenonly the desired position of the following participant wasdisplayed, the trials were completed faster in VS than JSS,and that displaying both desired positions in a JSS bimanualrehabilitation task may be beneficial.

Our future work includes making improvements to theCBRD to improve coupling performance and make it suitablefor testing with individuals with stroke. These improvementsinclude reducing the friction in the prismatic joints, optimizingthe stiffness ellipse of the compliant handle assembly, andadding a dynamic arm rest with a means to secure anindividual’s impaired arm to the compliant handle assembly.

REFERENCES[1] L. Marchal-Crespo and D. Reinkensmeyer, “Review of control strategies

for robotic movement training after neurologic injury,” Journal ofNeuroEngineering and Rehabilitation, vol. 6, no. 1, p. 20, 2009.

[2] G. Kwakkel, B. J. Kollen, and H. I. Krebs, “Effects of Robot-AssistedTherapy on Upper Limb Recovery After Stroke: A Systematic Review,”Neurorehabil Neural Repair, vol. 22, no. 2, pp. 111–121, 2008.

[3] B. Bobath, Adult hemiplegia: Evaluation and treatment. London, UK:Heinemann Medical Books Ltd., 1970.

[4] M. Knott and D. Voss, Proprioceptive Neuromuscular Facilitation:Patterns and Techniques, 2ed, 2nd ed. New York, NY: Harper &Row Publishers Inc., 1968.

[5] R. Oden, “Systematic therapeutic exercises in the management of theparalyses in hemiplegia,” JAMA, vol. 23, pp. 828–833, 1918.

[6] E. Taub, G. Uswatte, and R. Pidikiti, “Constraint-induced movementtherapy: A new family of techniques with broad application to physicalrehabilitation–a clinical review,” Journal of Rehabilitation Res, vol. 36,no. 3, pp. 237–251, 1999.

[7] M. Johnson, X. Feng, L. Johnson, and J. Winters, “Potential of asuite of robot/computer-assisted motivating systems for personalized,home-based, stroke rehabilitation,” Journal of NeuroEngineering andRehabilitation, vol. 4, no. 1, p. 6, 2007.

[8] D. J. Reinkensmeyer, C. T. Pang, J. A. Nessler, and C. C. Painter, “Javatherapy: Web-based robotic rehabilitation,” Integration of AssistiveTechnology in the Information Age, vol. 9, pp. 66–71, 2001.

[9] C. Burgar, P. Lum, P. Shor, and H. Van der Loos, “Developmentof robots for rehabilitation therapy: The Palo Alto VA/Stanfordexperience,” J. of Rehab Research and Development, vol. 37, pp. 663–674, 2000.

[10] S. L. Wolf, D. E. LeCraw, and L. A. Barton, “Comparison of MotorCopy and Targeted Biofeedback Training Techniques for Restitution ofUpper Extremity Function Among Patients with Neurologic Disorders,”Physical Therapy, vol. 69, no. 9, pp. 719–735, 1989.

[11] P. Lum, D. Reinkensmeyer, R. Mahoney, W. Z. Rymer, and C. Burgar,“Robotic devices for movement therapy after stroke: Current status andchallenges to clinical acceptance,” Topics in Stroke Rehab, vol. 8, pp.40–53, 2002.

[12] S. Hesse, G. Schulte-Tigges, M. Konrad, A. Bardeleben, and C. Werner,“Robot-assisted arm trainer for the passive and active practice ofbilateral forearm and wrist movements in hemiparetic subjects,”Archives of Physical Medicine and Rehab, vol. 84, no. 6, pp. 915 –920, 2003.

[13] J. Whitall, S. Waller, K. Silver, and R. Macko, “Repetitive Bilateral ArmTraining With Rhythmic Auditory Cueing Improves Motor Function inChronic Hemiparetic Stroke,” Stroke, vol. 31, no. 10, pp. 2390–2395,2000.

[14] K. Jordan, M. Sampson, J. Hijmans, M. King, and L. Hale, “Imablesystem for upper limb stroke rehabilitation,” in Virtual Rehabilitation(ICVR), 2011 International Conference on, june 2011, pp. 1 –2.

[15] S. Hesse, C. Werner, M. Pohl, J. Mehrholz, U. Puzich, and H. I. Krebs,“Mechanical arm trainer for the treatment of the severely affected armafter a stroke,” Am J Phys Med Rehabil, vol. 87, pp. 779–788, 2008.

[16] R. A. Schmidt and R. A. Bjork, “New conceptualizations of practice:Common principles in three paradigms suggest new concepts fortraining,” Psychological Science, vol. 3, no. 4, pp. 207–217, 1992.

[17] H. G. Malabet, R. A. Robles, and K. B. Reed, “Symmetric motions forbimanual rehabilitation,” in Proc. IEEE/RSJ Int Intelligent Robots andSystems (IROS) Conf, 2010, pp. 5133–5138.

[18] S. McAmis and K. B. Reed, “Symmetry modes and stiffnessesfor bimanual rehabilitation,” in Proc. IEEE Int. Conf. RehabilitationRobotics, 2011, pp. 1106–1111.

[19] S. H. L. McAmis and K. B. Reed, “Simultaneous perception of forcesand motions using bimanual interactions,” Haptics, IEEE Transactionson, vol. 5, no. 3, pp. 220–230, 2012.