Design of Spiral-Cable Forearm Exoskeleton to Assist Supination forHemiparetic Stroke Subjects

Ava Chen1, Lauren Winterbottom2, Katherine O’Reilly1, Sangwoo Park1, Dawn Nilsen2,3,Joel Stein2,3, and Matei Ciocarlie1,3

Abstract— We present the development of a cable-basedpassive forearm exoskeleton that is designed to assist supina-tion for hemiparetic stroke survivors. Our device uniquelyprovides torque sufficient for counteracting spasticity withina below-elbow apparatus. The mechanism consists of a spiralsingle-tendon routing embedded in a rigid forearm brace andterminated at the hand and upper-forearm. A spool withan internal releasable-ratchet mechanism allows the user tomanually retract the tendon and rotate the hand to counteractinvoluntary pronation synergies due to stroke. We characterizethe mechanism with benchtop testing and five healthy subjects,and perform a preliminary assessment of the exoskeleton witha single chronic stroke subject having no volitional supinationcapacity. Our cable-based mechanism also allows for futureintegration into an active hand-opening orthosis, aiming to addsupination support for grasping tasks.

I. INTRODUCTION

Wearable assistive devices have established themselvesas promising tools for supplementing an impaired hand’sfunction and encouraging its use in everyday life [1]. Forexample, we [2], [3] and other groups [4] have introduceda number of robotic devices were introduced to assisthemiparetic stroke survivors with grasping tasks. However,assisting only grasping when a user lacks active forearmrotation severely limits the functional workspace of thehand and makes many daily tasks difficult or impossible [5].We informally observed this phenomenon in our own workwith chronic stroke survivors, where several subjects wouldattempt to grasp cups or bottles from a pronated handposition and had difficulty supinating the forearm enoughto reliably grasp and use these objects. Flexor synergiesfrom squeezing objects in the hand or lifting the arm fur-ther exacerbated unwanted pronation. These patient-drivenobservations motivated the development of a lightweightexoskeleton specifically designed to assist supination thatfeatured a small footprint feasible for integration in existingbelow-elbow exoskeletons.

Although many instrumented benchtop devices andworkstation-mounted exoskeletons have been developed toprovide rehabilitative therapy in supination [6]–[9], there

This work was supported in part by the National Institute of NeurologicalDisorders and Stroke under grant R01NS115652

1Department of Mechanical Engineering, Columbia University, NewYork, NY 10027, USA. {ava.chen, k.oreilly, sp3287,matei.ciocarlie}@columbia.edu

2Department of Rehabilitation and Regenerative Medicine, ColumbiaUniversity, New York, NY 10032, USA. {lbw2136, dmn12,js1165}@cumc.columbia.edu

3Co-Principal Investigators

Fig. 1. Supination exoskeleton worn as a standalone device (Top) andintegrated with a robotic hand orthosis (Bottom).

are few devices that provide rotational assistance in afully-wearable form. SpringWear, developed by Chen andLum [10], uses a shoulder-anchored forearm brace with anelbow linkage mechanism. Gasser et al. [11] implementedforearm rotation adjustment with a lockable ball-detentmechanism as part of a rigid elbow-joint module within ahand-arm orthosis. Myomo’s MyoPro Motion G is a com-mercial device that combines these concepts by using a ball-detent system within a C-shaped cuff at the wrist [12]. Butthese all require precise positioning about several joints; inparticular, obstruction of shoulder and elbow movement formany stroke survivors encourages maladaptive compensationfrom the trunk to perform reaching movements and managehand pose [13]. Commercial spiral-supinator elastic bandsanchor around the bicep and thumb to provide constantpassive resistance against pronation, with minimal constraintto elbow flexion. However, elastic bands couple supinationwith arm extension and cannot be easily adjusted whenworn underneath a hand orthosis. Soft-actuator versions ofthe spiral concept [14], including one contained within abelow-elbow sleeve [15], use inflatable bladders envelopingthe entire forearm that similarly make integration difficult.

This article describes the design of a forearm exoskeleton(Fig. 1) and embedded spiral cable route (Fig. 2) thatconverts torque about a hand-turned dial to supination torqueabout the forearm, enabling adjustment of forearm rotationangle. The dial incorporates a ratcheting mechanism to lockthe device at the desired position; tension in the cable

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counteracts spasticity and stroke-imposed flexor synergies.Our device is specifically intended for individuals with uni-directional deficit in supination; thus, we focus on providingunidirectional assistance and use a release mechanism toenable unrestricted, body-powered pronation.

To our knowledge, we are the first to present a physi-cal wearable device featuring a cable-based mechanism foradjusting forearm rotation. This implementation is uniquelyable to leave the elbow unencumbered, instead using differ-ential rotation (Fig. 3) along the length of the forearm toapply torque. Our device is designed to assist repositioningfrom a fully-pronated to vertical hand position, doing so withan easily adjustable, unobtrusive package that can be inte-grated in an existing robotic hand orthosis. We characterizedevice angle and force–torque metrics with testing on able-bodied subjects, then conduct a preliminary feasibility testof the device’s potential to assist maintenance of a neutralarm position in a case study with one chronic stroke subjectwith minimal supination ability.

II. SUPINATION FOREARM MECHANISM

The robotic hand exoskeleton shown in Fig. 1 consists oftwo independent, integrated mechanisms mounted to a rigid,aluminum forearm splint: a two-actuator finger-thumb exten-sion device connected to 3D-printed components strappedto the distal links of the digits, and a manually-adjustablesupination device strapped around the hand and forearm(shown alone in Fig. 1, Top). Design of the digit extensiondevice used in this study was previously described in [2], [3]whereas the supination exoskeleton has not been previouslydescribed. For our hemiparetic population of interest, lackof supination function is usually accompanied by handimpairment; thus, our supination exoskeleton assumes usagein conjunction with passive or robotic hand orthoses thattypically feature straps encircling the forearm but confineequipment to the dorsal side.

A. Spiral-cable Actuation Principle

Our design is inspired by the supinator (supinator brevis)muscle, which occupies the posterior compartment of theforearm and curves around the upper third of the radiusto connect to the ulna. Muscle contraction pulls the radiuslaterally, turning the arm about its longitudinal axis. Theexoskeleton likewise rotates the wrist in supination by pullinga cable wrapped around the forearm, (highlighted red inFig. 2). The cable is anchored at the hand and at theposterior area of the forearm. This design relies on the hand-wrist component of the exoskeleton having sufficient rotationrelative to the proximal region of the forearm. Because thereis a lack of literature on torque requirements needed to resistpronation and spasticity in a hemiparetic stroke population,we set an initial goal of achieving 0.4–0.5 Nm supinationtorque (approx. 12–15 N cable tension) and 60◦ range ofmotion (ROM) based on reported actuator performance ofrelated devices [10], [14].

Because the device rigidly splints the wrist, the hand-armsystem acts similarly to a cantilever beam in torsion. When

Fig. 2. Diagram highlighting spiral exotendon-routing concept. The tendon(red) is anchored to an orthosis on the dorsal side of the hand, and is guidedin a spiral path around the arm via a sheath embedded in a thermoplasticbrace (cyan) strapped to the forearm. This cable can be retracted within ahand-turned winch (blue) mounted at the base of the forearm; cable tensionimposes supination torques about the wrist.

Fig. 3. Diagram illustrating supination as a distributed-rotation motionalong the forearm. The proposed spiral cable mechanism generates relativetorque between the hand and proximal region of the forearm (sectionshighlighted green). Use of the forearm itself as a reference plane enablesthe device to be wholly contained below the elbow.

the arm is subjected to an axial moment, each cross-sectiontwists about its torsional center. The angle of twist decays asaxial distance from the applied torque increases, generatinga distributed rotation as illustrated in Fig. 3. We maximizeeffective distance by placing the cable’s distal terminationpoint on the orthosis at the base of the index finger.

Any external twist will impose substantial shear and warp-ing stresses on the arm. To prevent potential discomfort ortorque losses from cable constriction, the exoskeletal housingfor the cable must be sufficiently rigid in compression toprevent transmitting undesired pressures to the soft tissue.Additionally, the exoskeleton must prevent collapse of thespiral routing when tension is applied to the cable. Weaccomplish these criteria by routing the cable through asheath embedded within a rigid thermoplastic brace.

The supination exotendon’s proximal termination point ismounted to a mechanism strapped to the bulk soft tissue ofthe forearm, which is a compliant structure that will deformunder cable tension. We note that our design requirementfor sufficient relative rotation between hand and forearm canaccommodate twisting losses due to soft tissue deformationso long as the proximal anchor does not slip against the skin.

B. Forearm Brace

Practical considerations for the design of an exoskeletonto house a cable tightly wrapped around the arm includepreventing the cable from constricting around soft tissue,mitigating risk of the cable chafing against skin, and aidingease of donning/doffing while transmitting sufficient torques.

The proposed solution is a two-part brace, for which onepart is a mid-forearm open-C brace secured with fabricstraps shared with the hand-opening orthosis and the other

part is an armband strapped around the proximal forearm(Fig. 4, Left). The brace geometry is designed to unclaspwith the same motions used to doff the hand-opening brace,adding only one operation to unstrap the proximal armband.The supination cable exits a spool mounted dorsally on thearmband, is guided circumferentially along the forearm overa plastic skid plate, then crosses an elevated gap above theforearm to enter the spiral teflon sheath embedded withinthe distal brace. Both components of the supination braceare made from self-bonding thermoplastic molded to matchthe shape of the forearm. Altogether, the supination modulehas a mass of 161.1 g, for a total mass of 445.7 g whenintegrated with the hand-opening orthosis.

Soft high-density foam padding is positioned in the interiorof the brace and underneath the armband’s skid plate toelevate the exoskeleton where the cable crosses the opengap between components, mitigating risk of the plastic platespinching against the skin when the cable is retracted. A fabricpad attached around the cable at this gap serves to furtherminimize risk of skin pinching or chafing, although duringnormal usage the cable does not contact the skin even at fullretraction. We also use foam padding to cushion the bonyprominences at the wrist, so that we can more tightly securethe brace around the arm.

The fabric strap securing the armband to the forearmincludes a layer of high-friction Dycem fabric sewn on theunderside, such that the device moves with the skin insteadof slipping. The armband is positioned on the user at theposterior compartment immediately below the elbow, butexact positioning is not required for function of the device.

Cable retraction is accomplished by rotating a dial attachedto a ratchet assembly, shown in Fig. 4. Twisting a dial locatedat the top of the device directly turns the spool to wind thecable and rotate the arm, as demonstrated in Fig. 5. Theinternal gear mechanism enables one-way progress captureand prevents backward rotation and force one-way progresscapture. The user can release the cable by pressing a buttonat the center of the dial to disengage the ratchet mechanism,allowing the wrist to freely rotate in pronation. The dial-ratchet mechanism was empirically found to apply up to 40Ntension before failure, well beyond the point where a humanarm would twist and slip within the brace as discussed below.

III. METHODS

For initial validation of the design, we characterized howour device withstands applied forces from the cable trans-mission and its ability to help a user to resist pronation in aseries of experiments with a set of five able-bodied subjectsand with a single stroke subject having minimal volitionalsupination. Testing occurred at Morningside Campus (able-bodied) and at NewYork-Presbyterian Hospital (stroke) inaccordance with the protocol (IRB-AAAS8104) approved bythe Columbia University Medical Center Institutional ReviewBoard. Experiments with the stroke subject were performedunder supervision of an occupational therapist.

Fig. 4. Left: Supination brace and the hand orthosis used for this study.Right: assembly and internals of hand-turned cable adjustment mechanism.

A. Benchtop Mechanical Tests

We first conducted a characterization of the ratchetingwinch assembly in isolation to test the locking mechanism.An experimenter used a handheld force gauge (OmegaDFG31-200) to apply forces against the mechanism, whichwas fixed in a vise. We measured compression forces re-quired to release the mechanism when under tension, andconducted tensile testing to determine ultimate strength ofthe ratchet system.

To simulate unlocking the device under tension, we tight-ened the dial until approximately 15–20 N tension on thecable was achieved. We then pressed the force gauge againstthe button until the cable was released. Mean compressionforce after four trials was (8.3 ± 1.5) N.

We determined ultimate holding strength by progressivelyincreasing cable tension until device failure. We then disas-sembled the mechanism to determine cause. Ultimate tensileforce after three trials was (115.3 ± 4.0) N, and in all casesfailure was due to fracture or material wear at the pawl.

B. Able-Bodied Characterization

Because evaluating feasibility requires studying deviceoperation in conjunction with body-powered movements,we obtained the relationship between cable pulling forceand forearm angles in a resistive task with five able-bodiedsubjects. We note that a healthy population still presents widevariance in hand-arm sizes and strengths, muscle tone, skintightness, joint flexibility, and user comfort levels. We usethis population to test the distributed-rotation concept giveninherent compliance of soft-tissue, without the additionalcomplexity from spasticity or impaired motor synergies.

Subjects were instructed to position the hand at a neutral0◦ angle, then twisted the dial to tighten the cable mecha-nism until snug. Subjects applied a user-defined “moderateamount of effort” to pronate against device resistance, thenreturned to the neutral position after the device preventedfurther movement. This task was repeated four more timeswithout adjusting the mechanism. After each set of five

Fig. 5. Image series of stroke subject demonstrating usage of cable adjustment mechanism. From left to right: (1) subject wearing device in unlockedstate; (2) reaching to turn adjustment dial; (3) tightening mechanism to supinate arm; (4) device fully-engaged, with arm fixed in a near-neutral orientation.

trials, subjects were asked if they experienced any discom-fort and were allowed a short break to retighten the dial.Three runs of the task were performed for a total of 15repetitions per subject. Throughout the experiment, subjectswere instructed to hold the elbow in-place to minimizeshoulder activation. Cable tension was recorded using a loadcell (Futek LSB200-FSH00097) that was mounted inlinewith the mechanism, and end-effector angle was recordedusing a handheld two-axis gyroscope (STMicroelectronicsL2G2IS) with inclinometer display (Syleo Apps, “SimpleInclinometer”). Sensor data were synced over video (30 fps),for an overall sampling rate of 10Hz.

Results of the able-bodied characterization are shown inFig. 6. We found that the proximal forearm anchor wasgenerally able to resist pronation torques, as indicated bythe increasing tension with hand angle. The effective ro-tational stiffness of the device varied between individuals,as we allowed users to self-select cable tightness and bothmagnitude and rate of applied pronation torque. For allsubjects, compliance in the exoskeleton’s straps and thechange in forearm shape as the user strains against the deviceeventually allowed the arm to rotate within the brace tobypass the cable mechanism. This incremental slippage canbe seen in the slight loss of performance over repeated cyclesbefore the user retightens the cable. However, the device wasable to meet our target goal of 15N throughout the ROM evenaccounting for device slippage. Our exoskeleton achievescomparable able-bodied ROM and torque performance todata reported by Realmuto and Sanger [15] while requiringa smaller device footprint. Subjects reported zero instancesof discomfort during testing.

C. Feasibility Testing with a Hemiparetic Stroke Subject

We evaluated the device on a single subject with right-sided hemiparesis and moderate spasticity in hand and arm;specifically, having scores of 2 for finger/elbow flexorsand 1+ for forearm pronators on the Modified AshworthScale. The subject had no ability to rotate the forearmpast a neutral position. The subject was able to activelyrotate the forearm 15◦ out of a fully-pronated position(Table I), and had sufficient arm strength to lift the handto touch the face or extend the elbow. This subject was aparticipant in our previous studies with the hand-openingorthosis; however, involuntary pronation when lifting the

Fig. 6. Top: able-bodied resistive loading results reported as mean andstandard deviation aggregated across all five subjects and trials (n = 70).Five trials were excluded from analysis due to camera occlusion. Bottom:results for Trials 1, 3, and 5 combining experiment runs and subjects(n = 15, 15, 11). Holding stiffness of the device slightly degrades withrepeated cyclic strain.

arm prevented the subject from completing many of thefunctional tasks requiring grasping cylindrical objects with aneutral hand pose. We first determined the subject’s forearmpronation–supination (PS) ROM, validating our inclinometermethod with standardized goniometer measurements. Wethen qualitatively evaluated the subject’s functional abilityto grasp and maneuver cylindrical objects with and withoutexoskeletal assistance. Our final set of tasks evaluated thedevice’s ability to assist maintaining the hand in a neutralorientation when reaching. Because effects of spasticity varydaily, the tasks were conducted over the course of three45-minute sessions, spaced three weeks apart. The subjectreported zero instances of discomfort during the sessions.

1) Forearm PS, elbow bent: To study forearm rotationin isolation, we instructed the subject to sit with theirshoulder adducted and elbow flexed at a right angle. With theinclinometer placed in their hand and with the arm restingon the chair’s armrest, the subject was instructed to slightly

lift the arm above the armrest (to prevent assistance from thechair) while attempting to keep the inclinometer in a verticalposition. The subject performed this task without wearingthe exoskeleton (baseline condition), while wearing the ex-oskeleton but without engaging the supination mechanism,and after fully retracting the mechanism (assisted condition).To measure active pronation, we asked the subject to pronateas much as comfortably possible, then return to the baselineorientation. This task mimics the standard clinical procedurefor measuring forearm PS ROM; we validated inclinometerreadings by also taking goniometer measurements, whichall agreed within 5◦. Results are shown in Table I. Withassistance, the subject was able to achieve a more-neutralposition, but remained slightly pronated.

TABLE IFOREARM PS ROM, ELBOW BENT. INCLINOMETER ANGULAR

DISTANCES FROM 0◦ NEUTRAL STARTING POSITION ARE REPORTED.

Measurement (deg) Baseline With Device

Passive-ROM pronation 0 —- 60 ——-—Passive-ROM supination 0 —- −35 ——-—Total Active ROM 45 —- 60 25 —- 50

2) Hand Orientation, elbow extended: The following setof reaching tasks focused on tracking total orientation of thehand instead of strictly measuring forearm PS rotation in iso-lation, since end-effector pose is most relevant for functionalgrasping. We note that hand orientation involves forearm PSas well as shoulder rotation, which we do not assist; however,we are interested in studying whether assisting forearm PSalone can enhance arm function. Overall, we expect the handto pronate further when the subject reaches forward dueto flexor-pronator synergies with the shoulder, along withforearm pronation. However, we attempted to limit effects ofshoulder movement during these tasks by asking the subjectto consciously keep their elbow tucked close to the bodywhen performing the forward reach. This reduced habitualabduction and internal rotation of the shoulder.

Fig. 7 shows a qualitative comparison between reachingforward while wearing a commercial elastic supinator band(McKie Supination Strap and thumb splint) and while wear-ing our device. Although we do not assist shoulder rotation,we found the resulting end-effector angles to be visuallysimilar to those from a device that anchors around the bicep.

We then conducted a guided exercise of grasping a bottleand maneuvering it around the table surface, with and with-out the exoskeleton, to qualitatively evaluate ease of reach-to-grasp along with smoothness of arm motion (Fig. 8). Videosof the subject performing the bottle-maneuvering exerciseare included in the supplemental material associated withthis article. Without device assistance, the subject could onlygrasp the top of the bottle from a pronated position. By usingthe device, the subject could grasp the bottle from the sideand could lift it to their face as if to drink.

We quantitatively evaluated device assistance by measur-ing end-effector angle while the subject attempted to reach

Fig. 7. Photos of stroke subject reaching forward while wearing an elasticsupinator band (Left) and while wearing the proposed device (Right). Ourdevice assists supination independent of arm extension.

Fig. 8. Photos of stroke subject reaching for (Top) and grasping (Bottom)a bottle during the on-table maneuvering task. Device assistance enables afunctional grasp for drinking.

forward while maintaining the inclinometer in a vertical ori-entation. As with the elbow-bent assessment, this test beganwith the subject’s arm placed on the armrest with elbowbent and tucked next to the body. The subject was instructedto slowly lift their arm and reach forward, extending theelbow. When the arm was fully straightened, the subjectattempted to hold the inclinometer as vertical as possible fora 30-second duration. Angle results for the straightened-armportion of the exercise are shown in Fig. 9.

Fig. 9 shows the maximum and minimum extents ofinclinometer readings recorded when the subject attempted tokeep the hand in a vertical orientation after reaching forward.Although the subject’s arm could be passively rotated intosupination, the subject lacked the volitional ability to rotateout of a horizontal hand pose. Overall, most angle deviationsoccured from the arm moving in space as the subject exertedeffort to both rotate their hand and hold their arm in theair. When wearing the device, the rigid brace provides aslight bias towards vertical hand orientation; although therange of angles between No Device and Device Unlockedare similar, the latter is shifted about 15◦ towards neutral.Fully engaging the device further rotated the hand, achievinga total PS ROM gain of about 60◦. The locking mechanismalso reduced effort required to maintain this position. Therange of end-effector angles recorded with the device fully-engaged is narrower than with the other conditions, partiallybecause cable tension resisted forearm pronation and par-tially because the elbow shook less during the attempt. The

TABLE IIHAND-ORIENTATION RANGE WHEN ATTEMPTING TO

MAINTAIN NEUTRAL 0◦ POSITION, ELBOW EXTENDED

Condition Angle Extent (deg)

No Device 55 —- 150Device Disengaged 45 —- 125

Device Engaged 15 —- 45

Fig. 9. Table II and Left: total extent of inclinometer readings when stroke subject attempts to maintain a vertical (0◦) end-effector orientation whileperforming forward-reach task. Right, Top: photos of stroke subject performing experiment without device assistance (left) and with device fully engaged(right). Without device assistance, the subject has some control over end-effector orientation but cannot bring it out of a fully-pronated position. Engagingthe cable mechanism both rotates the hand into a more vertical orientation and allows the subject to better maintain this angle.

subject provided feedback that they felt the exoskeleton madeperforming this task easier; one possibility is that the deviceassistance allowed the subject to only exert the minimal effortrequired to lift the arm, minimizing spasticity.

These results are only for a single subject, but indicatethat for this individual, the exoskeleton was comfortableand able to effectively supinate the forearm in order toachieve and maintain a near-neutral hand position. For thisindividual, our device improved PS ROM by about 60◦

and visually matches performance of a commercial passivesplint, all while retaining a below-elbow footprint. Althoughwe do not achieve a truly neutral hand orientation, we dofind that reorienting the hand to a 15–30◦ pose provided anoticeable assistive impact for some functional tasks. Ourfindings are congruent with those reported by Chen andLum [10], who also found that even an incomplete PS ROMgain (38.8± 32.2 deg) was sufficient to aid functional tasks.

IV. CONCLUSIONS

In this paper, we present a novel cable-based exoskeletonto assist supination for hemiparetic stroke. We demonstratefeasibility of an approach that leaves the elbow unencum-bered; characterization tests suggest that the device canwithstand the torques required to adjust forearm rotationeven given soft-tissue compliance. We successfully integrateour proposed supination exoskeleton with an existing roboticorthosis and demonstrate that our device does not interferewith hand-opening. Preliminary testing on a stroke subjectwith minimal volitional supination suggests functional utility.

Positioning the forearm in a more neutral position enablesstroke survivors to approach vertically oriented objects with amore functional hand position during forward reach. This hasthe potential to increase ease of performance during commonbimanual tasks such as grasping a bottle to open it, holdinga cup when pouring, or opening the refrigerator to retrievefood. Although internal rotation of the shoulder also con-tributes to downward orientation of the palm during forwardreach for stroke survivors, this device has demonstrated thepotential to address part of the problem.

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