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Journal of Rehabilitation Research andDevelopment Vol . 33 No. 3, July 1996Pages 290-304

Development of test methodologies for determining the safetyof wheelchair headrest systems during vehicle transport

Patricia Karg, MS and Stephen Sprigle, PhDDepartment of Rehabilitation Science and Technology, University of Pittsburgh, Pittsburgh, PA 15238 ; Center forRehabilitation Technology, Helen Hayes Hospital, West Haverstraw, NY 10993

Abstract—For wheelchair users unable to transfer to avehicle seat, the wheelchair serves as a means of mobility andpostural support during activities of daily living and as aseating support in a vehicle . The performance of commer-cially available adaptive seating components in a dynamic orimpact situation, as well as their effect on the safety of theuser, is unknown and should be determined . The mainobjective of the project was to develop a test methodology tostatically determine the crashworthiness of wheelchair head-rest systems and show the efficacy of that methodology byapplying it to several commercially available headrest sys-tems . The methodology was based on Federal Motor VehicleSafety Standard (FMVSS) test conditions, which use statictest procedures to ensure that vehicle head restraints willperform adequately during actual crash conditions.

The procedure developed to evaluate the headrests gaveinformative and repeatable results . The tests performedcalculated the energy associated with a critical deformation ofthe headrest under quasi-static conditions . The results wereused to determine the level of safety provided by the devicesand to recommend design improvements . The headrests testedexhibited similar modes of deformation due to bending of avertical adjustment bar, and several devices were determinedto be capable of providing adequate restraint in an impactsituation.

Key words : physical disability, postural support, vehicletransport, wheelchair.

This material is based upon work supported by the National Institute onDisability and Rehabilitation Research.Address all correspondence and requests for reprints to : Patricia Karg.University of Pittsburgh-SHRS, Forbes Tower, Room 5044, Pittsburgh . PA15260 ; email : tkarg@pitt .edu .

INTRODUCTION

For several reasons, wheelchair users are beingtransported more frequently than in past years whileseated in their wheelchairs . Of an estimated 1 .1 millionwheelchair users, approximately two-thirds require spe-cial transportation services (1) . The Department ofTransportation (DOT) has estimated that approximately72,000 children are transported in their wheelchairs toand from school (2) . A better understanding of thesafety characteristics of devices involved in the trans-portation of wheelchair users is needed to ensure anacceptable level of safety for everyone.

Individuals transported in their wheelchairs areoften supported by pads, cushions, and restraintsdesigned for postural support, not transportation safety.Headrests are one type of device used by somewheelchair users to support and align the head and neckin a stable, functional position . An integral part of theseating system, headrests are typically utilized through-out the day and, therefore, remain on the wheelchairwhile the user rides in or drives a vehicle . New andproposed regulations mandate that wheelchair users betransported in a forward or rear facing direction (3-5).These orientations increase the importance of headreststo prevent whiplash and other neck injuries.

Schneider recommended provision of a head re-straint for wheelchair occupants in a vehicle, suggestingthat without head support the individual is moresusceptible to whiplash or head injury (6) . Seeger et al.specified the use of a padded head restraint as acrashworthiness design principle for transporting per-

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KARG and SPRIGLE : Wheelchair Headrest Transport Safety

sons in their wheelchairs in buses (7) . The authorsreported that head restraints are not provided duringtransport as a rule, and that a removable restraint ofsufficient strength is needed . These researchers wereadvocating that an add-on headrest be designed to attachto those wheelchairs without headrests ; however, indi-viduals who need head support throughout the day mustrely on the integrity of their current headrest duringtransportation in a vehicle.

The National Highway Traffic Safety Administra-tion (NHTSA) regulates head restraints that are part ofstandard vehicle seating systems (8) . The regulationdoes not apply to headrests attached to wheelchairs orother mobility devices . However, it can serve as anadequate benchmark when assessing the transportationsafety of adaptive seating components.

ObjectiveThe objective of this project was to develop a test

methodology to aid in determining the transportationsafety of the wheelchair-seated vehicle occupant using acurrent headrest system, as well as for testing futureheadrest designs . The goal was to evaluate the headrestsaccording to current clinical use and practice.

The first goal was to develop static testing proce-dures to evaluate the headrests' strength and usefulnessin the prevention of whiplash injury . A static testingprotocol is necessitated by the vast number of commer-cially available devices and the high cost of dynamic testprocedures . At minimum, the level of safety should beequal to that intended for the occupant in a vehicle seatthat has complied with Federal Motor Vehicle SafetyStandards (FMVSS). FMVSS are enforced for alloccupants seated in an original equipment manufacturer(OEM) seat, whether the individual has a disability ornot. The intent of this study is to assess the headrestsystems for their ability to provide an equivalent level ofsafety to the individual transported while seated in his orher mobility aid . The second goal was then to subjectcommercially available headrests to the developed pro-cedures to evaluate the efficacy of the proceduresapplied, as well as the performance of the devices.

METHOD

Review of Standards and ResearchTo determine the level of risk or safety provided

by the devices, a standard method of evaluation isrequired. FMVSS 202, which specifies safety require-

ments for head restraints, was studied along with otherpertinent research, and the test procedures were modi-fied and performance criteria established to evaluatewheelchair-mounted headrests (8).

FMVSS 202, "Head Restraints," specifies require-ments for head restraints to reduce the frequency andseverity of neck injuries in rear-end and other collisions.The head restraint may satisfy the regulation with adynamic or static test procedure applied to the restraintin its fully extended position. If the static test procedureis used for compliance, the restraint must also meet sizerequirements.

Dynamic Test Procedure

The head restraint must limit rearward angulardisplacement of the head to 45° with respect to the torsoduring an 8 g forward acceleration applied to the seatsupporting structure.

Static Test Procedure

This procedure is designed for headrests integratedinto vehicle seats . A headform is used to apply arearward load 6.35 cm below the top of the headrestraint perpendicular to the torso centerline . The loadis such that it will produce a 373 N-m torque about aseating reference point defined in the standard . Thiscorresponds to an approximate headrest load of 587 N.The head restraint must limit horizontal (anterior-posterior) displacement of the headform to 10 cm fromthe torso centerline and must withstand increasing thisload to 890 N or until seat failure.

Size Requirements

The top of the head restraint must not be less than70 cm above the seating reference point, measuredparallel to the torso centerline . When measured at apoint either 6.35 cm below the top of the head restraintor 63.5 cm above the seating reference point, the widthof the head restraint must be 25 .4 cm for a bench seatand 17 cm for bucket seats.

A search for references supporting FMVSS 202resulted in minimal documentation on the developmentof the standard . However, extensive research has beendone on injuries associated with rear-end collisions andthe effectiveness of head restraints in preventing theseinjuries . A few studies have identified safe anatomicalneck loads and strength criteria for head restraints.These studies, however, do not indicate that any of the

subjects had a disability and no data were found onpersons with disabilities .

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Journal of Rehabilitation Research and Development Vol . 33 No . 3 1996

Mertz and Patrick investigated the kinematics andkinetics of whiplash, determining the mechanics ofwhiplash injury mechanisms and the effectiveness ofhead support in reducing whiplash injury (9) . Thekinematics of actual car-to-car rear-end collisions werereproduced on a crash simulator using anthropomorphicdummies, human cadavers, and a volunteer . An indexwas developed based on voluntary limits of human

tolerance to statically applied head loads and was usedto define the severity of the simulations in casesinvolving an unsupported head . The index was based onthe maximum tolerable static limits determined frommaximum resistive forces and moments to static loadingof the volunteer's head . The tests showed that headtorque at the occipital condyles, rather than neck shearor axial forces at the base of the skull, is the majorfactor causing whiplash injury . The severity of thewhiplash simulation and the effectiveness of the safetydevices were evaluated by determination of neckreactions and head restraint loads . With the headinitially supported by a flat, padded head restraint and arigid seat back, the volunteer withstood a simulation ofa 70.4 km/h rear-end collision with only slight discom-fort . The maximum head restraint load seen at a 10 g,36.8 km/h simulation was approximately 667 N.

In a subsequent study, Mertz and Patrick presenteddata relative to the static strength of the neck in flexionand extension (10) . They also presented dynamicresponse and strength data for the human neck inflexion and extension . Finally, they recommendednoninjurious tolerance values for hyperextension andflexion of the neck. In their study, human volunteerswere subjected to static and dynamic environmentswhich produced noninjurious responses of the neck.Cadavers were then used to extend the data into theinjury region . The data revealed that the equivalentmoment at the occipital condyles is the critical injuryparameter in flexion and extension . Resultant shear andaxial forces at the base of the skull were well belowtolerable levels and were not considered critical param-eters by the authors . A maximum dynamic value of 47 .5N-m in extension was reached without injury in a 50thpercentile male cadaver . This torque level occurred at arotation of the neck of approximately 80° . The authorsconcluded that neck extension should be limited to 80°,and preferably 60°, to avoid injury.

More current research by Foret-Bruno et al . studiedthe effects of seat stiffness on the effectiveness of headand torso restraints (11) . An accidentology study wasperformed on the frequency and severity of injuries to

occupants in rear impact accidents . The laboratoryanalyzed data involving 8,000 vehicles for impactseverity, occupants' conditions, and types of injury . Aconclusion from this study was that head restraints areeffective in reducing the risk of cervical injury by 30percent for men and women.

The Foret-Bruno research also included a dynamicstudy. Using Hybrid III dummies, testing manipulatedhorizontal head-to-restraint distance, head restraint stiff-ness, seat back elasticity, and impact speed to studytheir influence on neck load measurements . The testssupported the observed effectiveness of head restraintsin actual rear impacts, and also indicated that in orderfor the restraint to be as effective as possible, it shouldstay in contact with the head and not deflect easilyunder the force of the head . As the distance between therestraint and the head increases, the shearing force atvertebra C1, and the rotation of the head in relation tothe thorax, also increases . These investigators concludedthat the head restraint, when placed in contact with theback of the head, should be able to withstand thefollowing loads "without any change in position:"longitudinal force of 750 N, a vertical force of 250 N,and a torque of 70 N-m.

Performance CriteriaTo evaluate wheelchair headrest effectiveness,

performance criteria were established . In general, head-rests should be designed to limit displacement of thehead to within a noninjurious range and to withstand theloads applied.

The dynamic test of FMVSS 202 requires that thehead be limited to a 45° angular displacement withrespect to the torso . As mentioned earlier, Mertz andPatrick reported the maximum extension that could bereached without injury was 80° ; therefore, the rotationof the head should be limited to 80 and preferably 60°(9) . Due to the lower tolerance levels that may occur foran individual with disabilities and the desire to providethe same level of safety as FMVSS 202, a moreconservative limit of 45° was chosen as our criterion.

A critical linear displacement of the head wasdefined for testing by calculation of the linear displace-ment that would correspond to a 45° angular displace-ment. This measurement was based on anatomical andanthropomorphic data . The center of rotation for theneck of a Hybrid II 50th percentile male Anthropomor-phic Test Device was used to calculate the chordaldisplacement of the center of gravity of the head for a45° rotation of the head relative to the thorax (Figure

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1) . The chord displacement was calculated to beapproximately 9 .0 cm and is close to the 10 cm limit ofthe horizontal displacement of the headform specified inthe static procedure of FMVSS 202.

A restraint must also sustain loads applied by thehead during an acceleration . Based on crash test results,FMVSS 202 established that the restraint must limit thedisplacement of the head to 10 cm when a load ofapproximately 587 N is applied and maintain itsintegrity up to a 890 N load. Mertz and Patrick found amaximum headrest load of 667 N during a 10 g, 36.8km/h simulation, supporting the load values of FMVSS(9). We used the FMVSS 202 load requirement of 587N and the displacement of 10 cm to calculate the workrequired to deform the restraint assuming an elasticdeformation . Therefore, a critical energy of 29 .8 J wasdefined as the criterion used to determine whether therestraint would prevent neck injury . Thus, the criteriaset to determine whether a restraint system was suitableto provide protection during vehicle transportation werethat within a 9.0 cm displacement of the head, theapplied load must reach a minimum of 587 N and theassociated energy must be greater than 29 .8 J . Inaddition, the restraint must withstand loads up to 890 N.

Products TestedThe headrest systems tested were commercially

available and included:

• Adaptive Engineering Lab's (AEL) AdjustableBracket• Dan Mar Mini• Dan Mar Sweep• Miller's Fixed Occipital• Miller's Swingaway Occipital• Otto Bock Single-Axis Offset• Otto Bock Multi-Axis Offset• Techni Seat Contoured Assembly.

Most systems included : attachment hardware, verti-cal adjuster, anterior/posterior or horizontal adjuster,and a headrest pad (Figure 2) . The attachment hardwarefor all the headrests tested was designed to mount on asolid seat back and usually consisted of a receptaclesecured with bolts and T-nuts . The vertical adjusterallows the user to raise and lower the pad to theappropriate height . The horizontal adjuster also allowsthe restraint to be adjusted based on the needs of theuser. This allows the headrest pad to be placed atvarious distances anterior to the seat back. The padusually attaches to the end of the horizontal adjuster and

Figure 1.Calculation of critical displacement of the head ; d = chordaldisplacement for 45° rotation.

Figure 2.Example of a typical headrest and components.

may include additional adjustability by allowing variousangles of tilt. The pads contain an inner structure ofmetal or plastic surrounded by foam padding containedin a cover . The specific characteristics of the headreststested are described in Table 1.

horizontal adjuster

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Journal of Rehabilitation Research and Development Vol . 33 No. 3 1996

Table I.Headrest characteristics .

HorizontalHeadrest

Vertical Adjuster

AdjusterAttachment

Pad, Width & Height

Receptacle

AEL Aluminum Solid Square

Aluminum1 .5

0.5 x 2 .5Curved 17 .8 x 7 .6

Steel

Dan Mar Mini

Mild Steel 0.3 x 3 .2

None

Curved 30 .5 x 12 .7

None*

Dan Mar Sweep

T5052 Aluminum

None0.6 x 2 .5

Miller's

Mild Steel Solid Round

Same as Vertical1 cm Diameter

Adjuster

Otto Bock (All Types)

Mild Steel Square Tubing

Same as Vertical1 .3 cm O.D .

Adjuster

Techni Seat

Stainless Steel 0.3 x 3 .8

Same as VerticalAdjuster

Soft Foam Occipital Support

None*35 .6 x 20 .3

5 cm high Occipital "Fixed"-Steel"Swingaway: "Wood/Steel

Curved 24.8 x 9

Aluminum

32 .4 x 12 .7

Stainless SteelLateral Support

All measurements in cm . *Vertical adjuster(s) bolted directly to seat back.

Test SetupFor the modified procedure, a headfoml was used

to load the headrests to simulate the loading from theocciput during a collision . The headform (Figure 3)used to load the restraint was a wooden half cylinderwith a 16.5 cm diameter and a 15 .25 cm profile. Theheadform is the same as that defined by FMVSS 202,and the dimensions represent those of a 50th percentilemale . The headform was mounted by a ball joint to aload cell on the crosshead of an 1122 Instron UniversalTesting Instrument (Instron Corporation, Canton, MA),the accuracy of which is ±0 .5 percent of the indicatedload (Figure 4). The system was configured in itscompression mode to measure and record the displace-ment of the crosshead and the load applied to theheadrest system. The linear displacement of theheadform, along with the freedom of movement pro-vided by the ball joint, was chosen to represent thetranslation and rotation of the head while allowing for aconsistent method of calculating the energy associatedwith the displacement of the headrest being tested.

A support structure was designed for attaching aplywood mount unit to which headrest assemblies weresecured (Figure 4) . A stiff support structure was neededthat permitted deflection of the headrest linkages duringloading and permitted adjustment of the headrests intoproper alignment with the headform for testing . Theplywood unit was mounted on the top of the support

structure so that the plywood, simulating the wheelchairseat back, was horizontal . The long axis of thehalf-cylinder headform was oriented along the verticalhead axis . The curved side of the headform was loweredto meet the pad of the headrest mounted on theplywood . The amount of vertical clearance required fordeformation of the restraint depends on the maximumlinear displacement of the headform and the additionalclearance for the headrest components . Horizontalclearance was determined based on the maximum widthof the headrests tested . In addition, the support structureand the means used to secure the plywood to thestructure must remain rigid relative to the test unit toisolate the behavior of the headrest during loading andto minimalize the movement of the plywood withrespect to the support structure.

Testing Protocol

Each headrest was tested according to the follow-ing protocol:

1 . The attachment hardware of all the headrests wasmounted to 1 .3 cm (half-inch) thick plywood,based on the manufacturer's directions and usingall provided hardware . Plywood was chosen tosimulate current clinical practice and manufacturerrecommendations with regard to headrest andbackrest attachment : foam-covered plywood of 1 .3

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KARG and SPRIGLE: Wheelchair Headrest Transport Safety

cm to 1 .5 cm thickness is commonly used as asolid seat back insert for wheelchairs . The range ofvertical height adjustment was determined for eachdevice, and two tests were performed at themaximum and minimum height settings (Table 2).

2. The horizontal adjustment, when available, was setsuch that the anterior surface of the headrest padwas located 4 cm anterior to the seat back surface.This setting was based on the standard uprightseating position of an anthropomorphic test device.In the upright position, there is an approximatedistance of 4 cm between the anterior plane of theseat back and the rear portion of the head . Thissetting also corresponded to the average mid-rangevalue of the headrests tested.

3. The adjustment hardware for the horizontal andvertical adjusters was tightened using a torquewrench to approximately 2 .25—3 .0 N-m. This wasdone for consistency between tests and to preventhorizontal slip from occurring. The restraints weremarked prior to the test to determine if anyhorizontal slip occurred during loading.

4. The headrest/plywood unit was fixed to the Instronbase, oriented so that the base of the headform waslocated flush to the bottom edge of the headrestpad. This orientation gives optimum headformcontact throughout any rotation of the head orheadrest. The headform was adjusted so that itscenterline was parallel to the centerline of theheadrest pad. A preload of 5—10 N was applied todefine surface contact and the zero point . AnInstron crosshead speed of 100 mm/min was usedto apply the load. The load was applied to theheadrest through displacement of the headformattached to the Instron crosshead . Load-deformation information was collected continu-ously as the headform was displaced 14 cm.Permanent deformation was recorded photographi-cally, and the modes of failure were recordedduring testing.

5. Load-displacement information was recorded on achart recorder and by data acquisition software at9.1 samples/sec . Values obtained from the ac-quired data included : maximum load, displacementat maximum load, partial energy, energy to yield,displacement at yield, and yield load . The yieldwas found using the slope-threshold method,which determines the slope in the initial linearregion of the curve and then finds the point on thecurve where the slope decreases to a fraction of

Ntr)

8

Figure 3.Dimensions of headform used for loading restraint.

Figure 4.Test setup.

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Table 2.

Headrest height settings.

Height AELDan MarMini

Dan MarSweep

Miller'sFixed

Miller'sSwingaway

Otto BockSingle Axis

Otto BockMulti Axis

TechniSeat

Minimum 10 .8 11 .4 20 .3 10 .2 11 .4 16 .2 10.8 11 .4

Maximum 20 .9 26 .6 30 .5 30 .5 36 .8 32 .3 35 .5 15 .5

All measurements in cm.

0.6 of the initial slope. The energy functioncalculated the energy under the load-displacementcurve between specified limits . Partial energy wascalculated for a headfottn displacement of 9 .0 cm,which corresponds to a 45° angular displacementof the head.

The partial energy and maximum load were used todetermine whether the unit met the criteria for theperformance of the headrest system.

RESULTS

The load-deflection data recorded are displayed inFigures 5 through 12 for the eight headrest configura-tions tested . Each figure displays four curves represent-ing the two tests each at the minimum and maximumvertical adjustment heights . As expected, the curvesshow greater strength of the headrests when positionedat their minimum height . The similarity of the curves ateach height adjustment demonstrates repeatable results.The characteristics of each curve, including localmaxima and minima, result from the mode of deforma-tion or failure of the headrest. From the data, informa-tion on the yield points, maximum loads achieved, andthe energy absorbed during deformation can be ob-tained. Each figure also includes a box which definesthe acceptable performance criteria of reaching a load of587 N within 9 cm of deflection.

Three major deformation modes were observed inthe course of testing : 1) the plastic deformation of thevertical adjuster, 2) the plastic deformation and/orfailure of the attachment receptacle, and 3) the yieldingof the plywood board . Combinations of the differenttypes of deformation were also observed.

The AEL headrest (Figure 5) experienced noyielding of the vertical adjuster . Failure occurred in theattachment receptacle and in the plywood . At the

a .

AEL

Figure 5.AEL: a) load-displacement curves ; b) mode of deformation.

minimum height setting, extensive board failure oc-curred as the T-nuts were pulled out of the plywood . Nohorizontal slip resulted in any of the tests, and only aslight downward rotation of the headrest pad wasobserved.

Tests on the Dan Mar Mini headrest (Figure 6)resulted in the bending of the single vertical adjuster

0

4 .5

6 .0

7 .5

9 .0 10.5 12 .0 1 3 .5 15 .0Deflection(cm)

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the bar . The spikes observed in the figure are a result ofthe headform pivoting on the headrest pad as deforma-tion occurred . The minima are caused by a series ofslips of the headform on the headrest pad . The largedrop in load experienced in one of the minimum heighttests was due to a slight yielding of the plywood.

Figure 7 displays the tests on the Dan Mar Sweepheadrest . The Sweep restraint contains two vertical barsbolted directly to the plywood. For all test cases, the

a .

DanMarSweep

a. DanMarMini

Minimum Height— —

Maximum Height

Figure 6.Dan Mar Mini : a) load-deflection curves ; b) mode of deformation.

3 .0

4 .5

6 .0

7 .5

9 .0 10.5 12 .0 1 3 .5 1 5 .0Deflection (cm)

Minimum Height- - - Maximum Height

6 .0

7 .5

9 .0

10 .5 12 .0 13 .5Deflection(cm).

bar. The vertical bar of the Mini headrest is directlybolted to the plywood, and bending occurred at the topattachment point . The minimum height adjustmentresulted in a shorter moment arm and greater bending of

Figure 7.Dan Mar Sweep : a) load-deflection curves ; b) mode of deformation .

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z000J

vertical bars bent at the top point of attachment to theplywood. No yielding of the plywood was observed inany procedure.

Miller's Fixed headrest exhibited failure of thereceptacle and board (Figure 8) . At the maximumheight setting, deformation of the receptacle and yield-ing of the vertical adjuster occurred at relatively lowloads . At the minimum height setting, the receptaclerotated and deformed between the bolt attachments,

MillerFixed

pulling away from the plywood . During one test, theT-nuts pulled out at approximately 1,200 N . Tests at themaximum height adjustment also resulted in a deforma-tion and slight rotation of the receptacle . The verticaladjuster also exhibited elastic yielding and there wasslight yielding of the plywood . No horizontal slipoccurred during testing, and the headrest pad rotateddownward approximately 45° during each test.

Figure 9 displays the results of tests performed ona Miller's Swingaway headrest . The minimum heightadjustment shows much greater strength than themaximum setting . At the minimum adjustment, nodeformation of the vertical adjuster or the receptacle

a .

MillerSwingaway

Minimum HeightMaximum Height

ZagJ 648

486

Minimum Height- - - - Maximum Height

5

3 .0

4 .5

6 .0

7 .5

9 .0 10 .5 12.0 13 .5 1 5 .0Deflection(cm)

a.

1620

1 458

1296

1134

972

810

324

0 .0

1 .5 .3 .0

4 .5 •6 .0

7 .5

9 .0

10 .5 12 .0 13 .5 15 .0Deflection(cm)

162

0

Figure 8.Miller's Fixed : a) load-deflection curves ; b) mode of deformation .

Figure 9.Miller's Swingaway : a) load-deflection curves ; b) mode of deforma-tion .

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was observed . The failure occurred as the board beganto yield at loads around 1,500 N. At the maximumheight setting, the receptacles remained intact and noboard yielding was noted . Deformation took placethrough the bending of the receptacle rod that containedthe mount for the vertical adjuster and through slightelastic yielding of the vertical adjuster . No horizontalslip occurred in any of the tests performed.

The Otto Bock Single-Axis Offset headrest con-tains a 15° bend in its vertical adjuster to offset theheadrest pad. In all cases, plastic deformation occurredat this offset angle . Figure 10 displays the results of thetests and shows different curve characteristics for thetwo height settings . Deformation at the minimum settinginvolved only bending at the offset, since this positioncoincided with the point that the maximum moment wasapplied. At the maximum setting, bending of theadjuster occurred at the offset and at the point at whichthe bar was clamped, where the bending moment wasmaximum. No horizontal slip and no receptacle defor-mation or board yield was observed in any of the cases.

Figure 11 shows the outcome of tests performedon the Otto Bock Multi-Axis Offset . In this headrest,the minimum setting also exhibited much greaterstrength than the maximum. At the minimum heightadjustment, failure occurred at the point on the verticaladjuster that was initially curved for an offset of theheadrest pad. Slight yielding of the board at the higherloads resulted as the restraint further deformed . Thevertical adjuster bent at the point of attachment to thereceptacle at the maximum height settings . No recep-tacle deformation or horizontal slip occurred in any ofthe tests.

The Techni Seat headrest test results are shown inFigure 12. The same mode of failure occurred duringall test conditions, as the vertical adjuster bent at the topof the attachment receptacle. No deformation of thereceptacle nor yielding of the plywood was noticeable.

A summary of the mechanical properties deter-mined from the testing is presented in Table 3. Twocriteria were for the device to reach a minimum load of587 N and require a minimum energy of 29 .8 J for a 9cm displacement of the headform. The headrests thatmet these criteria at minimum height were : AEL,Miller's Fixed, Miller's Swingaway, Otto Bock Single-Axis, and the Otto Bock Multi-Axis . The AEL was theonly headrest that also met the criteria at maximumheight.

The Dan Mar Mini and Sweep headrests meet theenergy criterion at their minimum height, but failed to

a.

OttoBockSingle—Axis

b.

Figure 10.Otto Bock Single-Axis: a) load-deflection curves ; b) mode ofdeformation.

reach the critical load within a 9 cm displacement (SeeFigures 6 and 7) . Except for AEL, the headrests did notmeet the criteria at their maximum height setting . TheTechni Seat restraint failed for its full range ofadjustment.

Another criterion was for the headrests to maintaintheir functional integrity up to a load of 890 N . The

300


a. Otto BockMulti–Axis

a. Techni Seat

7201 620

Minimum HeightMaximum Height

6481 458

5761296

5041134

z-432972z

0 360-0

810 0

28864821 6486144324 i'

721 6200

12 .0

13 .5

15 .0

0 .0

1 .5

3 .0

4 .5

6 .0

7 .5

9 .0Deflection(cm)0 .0

1

5

3 .0

4 .5

6 .0

7 .5

9 .0

10 .5=flection(cm)

b.

10 .5 12 .0 13.5 15 .0

Figure 11.Otto Bock Multi-Axis : a) load-deflection curves ; b) mode ofdeformation.

restraints that reached this load at minimum heightincluded: AEL, Miller's Fixed, Miller's Swingaway,and the Otto Bock Multi-Axis . These restraints did notfail or lose their integrity until the plywood boardyielded and the T-nuts were pulled out at loads from1,000 to 1,200 N .

Figure 12.Techni Seat : a) load-deflection curves ; b) mode of deformation.

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Table 3.Static test results.

Headrest Maximum Displacement at EnergyHeight Setting Load (N) Maximum (cm) (J)

AELMinimum 1962 .08 7 .89 117 .62Maximum 718 .66 10.05 37 .90

Dan Mar-MiniMinimum 615 .24 12 .41 30 .63

Maximum 212 .55 11 .21 10 .53

Dan Mar-SweepMinimum 595 .62 11 .44 33 .95Maximum 326 .41 10 .54 18 .78

Miller's-FixedMinimum 1268 .30 11 .54 72 .49Maximum 229 .21 11 .99 12 .08

Miller' s-SwingawayMinimum 1601 .59 6 .26 96 .09Maximum 354.72 11 .98 18 .65

Otto Bock-SingleMinimum 636 .28 3 .68 44.13Maximum 325 .16 11 .49 16 .89

Otto Bock-MultiMinimum 1038 .57 4 .81 72 .55Maximum 277 .82 12 .03 13 .77

Techni SeatMinimum 418 .25 10 .10 26 .72Maximum 268 .75 9 .71 17 .78

These results are an average of two tests.

DISCUSSION

Determination of the test procedure and criteriawas based on several assumptions . An assumption wasmade that the wheelchair will remain intact during acrash, including the frame and upholstery or seat back.The assumption was valid because the study is aimed atdeveloping a methodology to assess the crashworthinessof the positioning devices . The wheelchair must beassessed separately . A second presumption was that theheadrests were all attached to the seat back with T-nutsand secured with quality hardware . All the modelstested were designed to install onto a rigid seat backusing T-nuts for mounting . The adjustments were alltightly secured with a torque wrench within a presettorque range to reduce the variability of testing . Thestudy was not intended to evaluate cases with weak orimproper installation and securement.

The mechanisms of injury from rear collision varyaccording to anatomical factors . However, studies oncervical injury from rear impact are performed using


cadavers and test dummies for obvious reasons : thesestudies accept the anatomical differences between ve-hicle occupants and cadavers in order to investigate thecervical injury region. This study acknowledges that theneck musculature of wheelchair users who requireheadrests might differ from a nondisabled subjectpopulation, depending on disability . However, theassumption was made that neck musculature shouldhave no effect on protocols developed to test headrestintegrity . This assumption was supported by the ac-cepted use of cadaver spine and test dummies duringcrash situations and by Seletz, who stated that activemuscle forces have no effect on head motion in rearcollision since they do not react in time (12). The effectof differences in neck musculature is further reduced bythe fourth assumption, which was that the head isalways in initial contact with the headrest pad . BothMertz and Patrick and Foret-Bruno et al . found that asthe distance between the head and head restraintincreased, the rotation of the head and the load on therestraint increased (9,11) . This assumption was madebecause wheelchair users who utilize headrests do so toprovide maximum support of the head, and the headrestis adjusted to contact the head during their typicalpostures.

The procedure developed to evaluate headrestsresulted in repeatable and predictable results (Figures 5to 12) . The percent variation of the values of energy andmaximum load for a repeated test was below 10 percentfor the majority of the tests, with the largest variabilityreaching about 16 percent . This variation is quiteacceptable, considering the number of adjustments on aheadrest system and that inconsistent adjustment be-tween repeated tests could cause significantly differentresults . The average value of the two tests performed ateach height adjustment was used to determine if therestraint met the criteria . In all cases, if the average metthe test criteria, the original values obtained for eachtest also met the criteria, showing repeatability.

A noteworthy result is the fact that in all cases inTable 3, the minimum height adjustment withstoodhigher loads than the maximum setting . This outcomewas predictable since the longer the length of the verticaladjuster, the longer the moment arm would be for theforce applied at the headrest pad, resulting in a greatermoment applied to sections of the vertical adjuster.However, the headrest system is more complex than asimple beam in bending, and a relationship between themaximum load sustained and the height adjustment ofthe headrest could not be established . The number of

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adjustable components of the headrests, resulted inseveral modes of deformation or failure of the headrest,precluding a simple relationship between the heightsetting of the headrest and the load required to displacethe headform 9 cm . More tests performed at heightadjustments between the lower and upper limit should becarried out to determine whether a relationship can befound to predict the failure loads at a certain setting.

The headrests tested, except for AEL, should notbe adjusted to their maximum height during transport,since they are unable to sustain any significant loadingat this setting. Maximum height settings can be avoidedwith the proper location of the attachment hardware andan adequate seat back height.

The determination of the mode of deformationallows for improvement in design for increased strengthand safety . In the cases where the plywood seat backfailed, the load required was above 890 N . Failure ofthe plywood occurred when the headrest did not absorbthe energy through its own deformation . These head-rests were able to limit the displacement of theheadform and should prevent critical rotation of thehead in the event of a crash.

The majority of the tests resulted in the plasticdeformation of the vertical adjuster, which often causedthe hardware to fail to withstand the necessary appliedloads and fail to limit displacement of the headform to anoninjurious range . The knowledge that the verticaladjuster is a weak component of the device and bendsupon loading can be used to redesign headrest hardware.To provide maximum protection, the vertical adjustershould be designed to have a yield load greater than the587 N it is required to withstand . This will `ensure thathead excursion is limited to within a safe range . Sincethe maximum elastic moment is determined from thesection modulus and the yield strength of the material,the required section modulus can be calculated from theload required, the material yield strength, and themaximum height of the vertical adjuster. As demon-strated in the test results, the maximum height of theadjuster will be the worst case. From the sectionmodulus, the required dimensions of the adjustment barcan be determined . In addition, consideration should begiven to using more compliant mounts for attaching thevertical adjuster to the seat back . However, the primaryfunction of the headrest to provide support for the headas part of the wheelchair seating system must not becompromised.

Other guidelines that might be useful when design-ing a headrest include choosing a material such as a

mild steel (1018 low carbon steel) or a high gradealuminum (6061-T6 aluminum) to construct the ad-juster. These materials have a high yield strength andwill improve the performance of the headrest underbending loads. Also, a square cross-sectional area forthe adjuster bar will provide the most resistance torotation within the attachment hardware . Use of tubing,versus solid bar or rod, allows for greater strength peramount of material and allows for a lighter headrest . Asquare shape will also provide more bending resistancethan a thinner flat stock. The Techni Seat headrest didnot meet the criteria at any height adjustment and isconstructed from a stainless steel metal sheet.

Some vertical adjusters may contain offsets result-ing from bending or angling of the adjustment bar . TheOtto Bock headrests both contained offsets that were thepoint of weakness on the headrest . This part of theadjuster yielded first, even though larger moments wereapplied to other sections of the bar. Vertical adjustmentbars containing these offsets may yield at lower loadsthan calculated for the bar's section modulus . This mustbe considered when designing the system, and a largersection modulus will be required to sustain the neces-sary applied loads.

The size of the headrest pad should also beconsidered. In FMVSS 202, the federal governmentspecified size requirements for motor vehicle headrestraints . These requirements are most likely defined toallow for various sized occupants from the 5th percen-tile female to the 95th percentile male . The widthrequirements may also have been determined to preventhead excursion under dynamic conditions when thehead has shifted . The larger the headrest pad, the betterthe chance of it contacting and supporting the head in acollision. FMVSS 202 specifies a minimum width of 17cm for bucket seats . This requirement should also applyto the design of wheelchair headrests used duringvehicle transport . Table 1 shows that all the restraintstested in this study met the size requirement.

Although the static tests performed intentionallyisolated the headrest from the rest of the seatingsystem, and thus did not take into account the seatback, seat back stiffness should be considered . Thedynamic studies performed by Mertz and Patrick andForet-Bruno et al . concluded that seat back stiffnessaffected the amount of load seen by the head restraint(9,11). Under dynamic loading, as seat back stiffnessincreases, the load on the head restraint increases . Asthe seat back deforms, less load is transferred to thehead restraint .

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The move toward improving wheelchairs for ve-hicle transport may result in increased seat back rigidityto provide the strength necessary to maintain integratedoccupant restraints . The increased stiffness will necessi-tate the use of headrests and affect the load require-ments. Currently, most wheelchairs do not provide rigidback support . In the event of a vehicle crash, thisflexibility reduces the loads on headrests as compared tothose seen in a vehicle seat, since the wheelchair seatand seat back will dissipate some of the energy as itdeforms. In addition, the seat back deformation reducesthe rearward acceleration of the head, requiring theheadrest to resist less force . Dynamic testing ofwheelchairs with headrests is necessary to determine theinfluence of wheelchair seat backs and the loads that theheadrest will experience in a rear end collision . With aless rigid seat back than a motor vehicle seat, the loadsthat must be withstood may be lower than thoserequired by FMVSS 202 . The energy criteria woulddecrease and more headrest models may be effective ina crash . However, the less rigid seat backs may result inother spinal injuries . Ideally, seat backs and headrestsshould be optimized as a system.

CONCLUSIONS AND RECOMMENDATIONS

Based on the results of the methodologies per-formed on the sample of products, the followingconclusions may be drawn:

1. The data obtained from the testing showed themethodologies developed yielded repeatable re-sults.

2. Minimizing the height of the vertical adjustmentbar of a headrest will provide the greatest resis-tance to loading from the head and limit headexcursion . Bar height can be minimized by properwheelchair back height and location of the mount-ing hardware.

3. The headrests tested exhibited three modes ofdeformation : 1) plastic deformation of the verticaladjuster, 2) plastic deformation and/or failure ofthe attachment receptacle, and 3) yielding of theplywood board . In some cases, combinations ofthese types of deformation were observed. Basedon the observed mode, design improvements canbe determined.

4. The majority of the tests resulted in bending of thevertical adjusters . This knowledge can be used to

determine the required material and dimensions ofthe adjustment bar necessary to prevent bending atthe predicted loads.

5. Headrests with offsets obtained by a bending orangling of the vertical adjuster failed at the offsetpoint, showing this point to be the weakest.

6. Commercially available headrests, with a fewdesign improvements, can be effective in prevent-ing neck injury in rear-end collisions.

The findings of the study led to the followingrecommendations for future research:

1. Dynamic tests on the products should be per-formed to further validate use of static procedurefor determination of crash performance.

2. Dynamic testing is also required to determine themagnitude of the loads on the headrest that canthen be used to validate or change the pass/failcriteria of the static methodologies.

3. Dynamic testing should also be performed onwheelchair seating systems, including combina-tions of the various adaptive seating componentsto assess their interaction.

4. Design improvements, especially ones that reducestress concentrations and design for gradualchanges in stiffness, can be implemented and theproducts retested for their effectiveness.

REFERENCES

1.La Plante MP. People with disabilities in basic life activitiesin the U .S . Disability Statistics Abstract, No.

3 . Disability Statistics Program, Institute for Health and Aging,School of Nursing, University of California at San Francisco,Apr 1992.

2. Dalrymple GD, Hsai HS, Ragland CL, Dickman FB . Wheel-chair and occupant restraint on school buses . Final reportsubmitted to National Highway Traffic Safety Administration,U.S . Department of Transportation. No. DOT-HS-807-570.Washington DC, May 1990.

3. Department of Transportation . 49 CFR, Part 38 : Americanwith Disabilities Act (ADA) : Accessibility specifications fortransportation vehicles, 6 Sept 1991.

4. ISO Working Document 10542 . Wheelchair and occupantrestraint systems for use in motor vehicles, March 1993.

5. National Highway Traffic Safety Administration (NHTSA).Final rulemaking : FMVSS 222-School bus passenger seatingand crash protection . Washington, DC : Department of Trans-portation . Fed Regist 1993 :58(10) :4586-98.

6. Schneider LW . Protection for the severely disabled : a newchallenge in occupant restraint . In: The human collision .

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Proceedings of the International Symposium on OccupantRestraints, AAAM, 1981 :217-31.

7. Seeger BR, Caudrey DJ . Crashworthiness of restraints forphysically disabled children in buses . Rehabil Lit 1983 :44(11-12):332-5.

8. National Highway Traffic Safety Administration . FederalMotor Vehicle Safety Standards 202, 49 CFR Part 571.

9. Mertz HJ, Patrick LM . Investigation of the kinematics andkinetics of whiplash . In : Proceedings of the Eleventh StappCar Crash Conference . Society of Automotive Engineers, Inc .,

I967 :Technical paper 670919:2952-80 .

10. Mertz HJ, Patrick LM . Strength and response of the humanneck . In : SAE Transactions, Vol . 80 . Society of AutomotiveEngineers, Inc ., 197I :Technical paper 710855 :2903-28.

I I . Foret-Bruno JY, Dauvilliers F, Tarriere C . Influence of theseat and head rest stiffness on the risk of cervical injuries inrear impact . In : Proceedings of 13th International Conferenceon Experimental Safety Vehicles (ESV) . Paris, France,1991 :Paper #91-S8-W-19:968-74.

12 .

Seletz E . Whiplash injuries : neurophysiological basis for painand methods for rehabilitation . JAMA 11/29/1958:1750-5 .

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