SP-4b: Development of Usage Guidelines and Design/ Performance Criteria for Postural Support Devices

Task leader: Patricia Karg, MSBME

Co-investigators: Greg Shaw, PhD, Miriam Manary, MSE, and graduate student

Other participants: Bette Cotzin, MS, PT (clinician), Focus Group (parent/caregiver, wheelchair user, manufacturer, school bus transporter)

Duration/Staging of task: This 36 month development task will be conducted in months 25-60 of the 60 month RERC cycle

Design of research activities

This task responds to the priorities by investigating the crashworthiness of secondary PSDs and integrating the results into existing voluntary performance standards. The task performs the necessary first step of identifying injury risk associated with use of PSDs in vehicle transport, investigating and characterizing their performance in a dynamic crash environment, and developing design and performance criteria that can be translated into test methods to evaluate crashworthiness, as appropriate.

Literature review

Motor vehicle travel while seated in a wheelchair, either as a driver or passenger, presents additional challenges to mobility. Those individuals who also use their wheelchair as a vehicle seat need a seating system that addresses their needs in and out of the vehicle. Use of a seating system in a motor vehicle may require additional need for postural supports to provide postural stability during vehicle movement, additional arm or hand function, prevention of additional risk for tissue trauma, and the need to address potential increases in spasticity, abnormal reflexes, and spontaneous abnormal movements (Hunt, 1995). These needs must be addressed to ensure postural stability and safety during normal driving, evasive maneuvers, and crashes.

To meet the additional demands on the seating system due to use as a motor vehicle seat, certain postural support devices are often indicated, as well as used throughout the day to address postural needs. This includes a pelvic stabilization strap or bar, lateral thoracic supports, added height and rigidity to a back support, shoulder straps or bars, chest harnesses, horizontal or diagonal chest straps, and the addition of armrests and/or headrests with or without chin or forehead straps (Hunt, 1995; Cotzin and Marks, 1996). Additional postural supports that are used in daily activities that my pose a risk or benefit during vehicle transportation include neck rings, thigh abductor pads, and footrests.

Safety standards and guidelines have made great progress in addressing the safety of the wheelchair-seated vehicle occupant. A large amount of research has gone into developing voluntary industry standards addressing the wheelchair used as a seat in a motor vehicle (ANSI/RESNA, 2000; ISO, 2000). Additional research has been done on the primary postural support devices, which includes the seat and back support (see Task SP-4a). However, very little research has been done on the effect of secondary postural support devices and wheelchair accessories on transportation safety and the existing standards barely touch upon issues related to their use. We use the term “secondary postural support device” to refer to all postural support devices other than the seat and back support. Wheelchair accessories, such as lap trays, oxygen tanks, IV poles, backpacks, etc. also may pose a risk and require investigation. However, this task will specifically address secondary PSDs and not wheelchair accessories.

Existing voluntary standards currently address the use of secondary postural supports mainly with informative language (i.e., not part of requirements) and primarily address use of anterior trunk and pelvic supports. SAE J2249 and ISO 10542 standards for wheelchair securement and occupant restraint systems state that belts should not be relied on for occupant restraint and protection unless they comply with the requirements of the standards, and that belts or components placed over the abdomen or chest should break away at a force less than 1000 N (225 lb) to reduce injury (SAE, 1995; ISO, 2000). Voluntary standards for wheelchairs used as seats include requirements that all sharp edges be covered with energy-absorbing material to prevent injuries, that postural support belts provided with the wheelchair be clearly labeled they are not intended for occupant restraint in a motor vehicle, and provides recommended pelvic belt angles for belts intended to be used for occupant protection and postural support or for postural support alone (ANSI/RESNA, 2000; ISO, 2000). The draft voluntary standard ISO 16840-4 for complete seating system crashworthiness states that seating system components greater than 100 grams cannot become completely separated during the required sled test of the system, or components cannot fragment or separate such that they produce sharp edges, as well as wording similar to the wheelchair standards on postural support belts (ISO, 2001). The American Academy of Pediatrics policy statement on transportation of children with special needs addresses the use of postural support devices only in that it states “head bands should not be used to restrain the child’s head separately from the torso” (AAP, 1994).

There is very little in the literature on injury risk or benefit associated with the use of secondary PSDs in a motor vehicle. Bunai et al. (2001) recently reported on blunt pancreatic trauma caused by a passenger restraint system during a motor vehicle accident. The passenger restraint was stated as intended to “prevent the user from being ejected from the chair” and not for postural support. However, it does indicate the potential for injury from anterior supports placed over soft tissue. It consisted of webbing that was stitched to the sling seat of the wheelchair between the occupant’s thighs, the ends of the webbing fastened with a plastic buckle behind the wheelchair back. To fit over the armrests, the webbing passed over both sides of the abdomen. During the accident, a crash with an oncoming vehicle, the belt applied a large force to the abdominal wall, causing pancreatic injuries that resulted in death. The author concluded that “inadequate safety devices like the present safety belt can be harmful, if not lethal.”

Literature addressing wheelchair transportation safety contains recommendations for use of a head restraint on a wheelchair used as a vehicle seat to prevent neck injury (Schneider, 1981; Seeger, 1983). Other literature discussed the risks to wheelchair users in rear impacts, especially to whiplash injury (Paskoff, 1995). The crashworthiness of headrest systems, anterior trunk supports, and pelvic belts were evaluated both statically and with low-speed dynamic testing during the previous Transportation RERC (Karg, 1996; Forziati, 1994). The studies determined modes of failure and made recommendations for design improvements. The authors found that with some design improvements, commercial headrest systems could be effective in preventing neck injury. They also found that the anterior postural supports tested, when used with a vehicle occupant restraint, helped maintain proper positioning in low-speed frontal impacts and did not fail in a hazardous manor. However, when used improperly or without vehicle restraints, could be dangerous in a crash. Recommendations for future research included the need for dynamic sled tests to validate the procedures used, to determine the load magnitudes on the PSDs, and include seating systems as a whole to assess a variety of PSDs and their interactions.

In summary, there is minimal information documenting the injury risk associated with secondary PSDs during vehicle transport. The work that has been done needs to be validated with dynamic sled testing. Literature review, case studies, anecdotal evidence, and the experience of this task’s investigators leads us to the first step in addressing the risk associated with secondary PSDs, to further identify primary risks associated with PSD use and characterize their effect on safety and their performance in a crash environment.

Research objectives

The immediate goal of this task is to develop general usage guidelines, design criteria, and/or performance criteria for secondary postural support devices that pose the greatest risk or benefit during transportation of wheelchair-seated vehicle occupants (primarily children). The long-term goal is to develop test methods that evaluate the crashworthiness of secondary postural support devices based on the outcomes of this task.

This task will reach its goal by accomplishing three objectives:

  1. First, we will identify/confirm key questions or concerns regarding use of secondary postural support devices during vehicle transport by gathering anecdotal evidence and experiences from constituents, and through a review of previous dynamic tests.
  2. Second, we will perform additional dynamic sled tests to further characterize injury risk or benefit associated with use of the PSDs.
  3. Third, we will develop general guidelines, design criteria, and/or performance criteria for the PSDs. This work will provide the framework for inclusion of wording in the current voluntary standards and the development of low cost testing to evaluate whether devices meet design or performance criteria, as needed.

Anticipated Outcomes

Three main outcomes are anticipated as a result of this project: key questions/concerns regarding use of secondary postural support devices during vehicle transport will be defined and documented; the characterization of injury risk or benefit associated with use of the targeted PSDs; and the development of general guidelines, design criteria, and/or performance criteria for identifying PSDs appropriate for use during vehicle transport. This work will provide the framework for extending the current voluntary wheelchair transportation standards to address secondary PSDs, and the development of low cost testing to evaluate whether devices meet design or performance criteria, as needed. Additionally, one graduate student will receive training in rehabilitation technology research as a part of this task.


American Academy of Pediatrics (AAP). Transporting Children with Special Health Care Needs. Pediatrics, 104 (4): 988-992, 1999.

ANSI/RESNA WC19: Wheelchairs Used as Seats in Motor Vehicles. American National Standards Institute (ANSI)/Rehabilitation Engineering Society of North America (RESNA). 2000.

Bunai, Y., Nagai, A., Nakamura, I., Ohya, I. Blunt pancreatic trauma by a wheelchair user restraint system during a traffic accident. Journal of Forensic Science 2001, 46(4): 965-68.

Cotzin, B. and Marks, J. Preconference Instructional Course, 5th National Conference on Transporting Students with Disabilities, Birmingham, AL, March 1996.

Forziati, K. Development of a Methodology to Dynamically Evaluate the Efficacy and Safety of Wheelchair Occupant Support Devices, Masters Thesis, University of Virginia, Charlottesville, VA May 1994.

Hunt, J. Therapeutic Principles of Wheelchair Seating in Transport. Preconference Instructional Course, 11th International Seating Symposium, Pittsburgh, PA Feb 1995.

Karg, P., Sprigle, S. Development of test methodologies for determining the safety of wheelchair headrest systems during vehicle transport. Journal Rehabilitation Research Development 1996, 33(3): 290-304.

Paskoff, G. Whiplash injury risk to people with disabilities traveling in wheelchairs. Proceedings of the RESNA ’95 Annual Conference, RESNA Press, Washington DC, 1995: 72-4.

Schneider, L. Protection for the severely disabled: a new challenge in occupant restraint. In: The human collision. Proceedings of the International Symposium on Occupant Restraints, AAAM, 1981:217-31.

Seeger, B. and Caudry, D. Crashworthiness of restraints for physically disabled children in buses. Rehabil Lit 1983, 44(11-12): 332-5.

Society of Automotive Engineers (SAE). SAE J2249-Wheelchair tiedowns and occupant restraints for use in motor vehicles, Society of Automotive Engineers, Warrendale, PA, Jan 1995.

International Organization for Standardization (ISO). ISO 10542 Wheelchair tiedown and occupant restraint systems. ISO, 2000.

International Organization for Standardization (ISO). ISO 7171-19 Wheeled mobility devices for use in motor vehicles. ISO, 2000.

International Organization for Standardization (ISO). ISO 16840-4 Seating devices for use in motor vehicles. ISO 16840-4 Committee Draft, June 2001.

Progress Report May 1, 2004

This task became active November 2003 and is currently on schedule. The first months of the task have been devoted to characterizing secondary postural support devices on the market and documenting what is known with respect to their use as part of a motor vehicle seat. This has involved an updated review of published literature and technical standards, gathering of anecdotal evidence and experiences from constituents, and review of previous research and testing at two test facilities, University of Virginia Automotive Safety Laboratory and the University of Michigan Transportation Research Institute. The results are currently being compiled into a matrix that documents what is known about the performance of secondary PSDs with respect to their potential to result in injury or benefit to the wheelchair-seated occupant. A graduate student has joined the project as of June 2004 and will be devoting 50% effort. The remainder of the year will be devoted to completion of the matrix and the use of this information to prioritize and identify the devices and usage scenarios the project will address with additional research (sled testing and computer simulation).

Progress Report May 1, 2005

A questionnaire was developed to better understand prescription patterns for secondary postural support devices (SPSDs), concerns related to their use during vehicle transportation, and requirements for use of SPSDs for transportation by school districts and programs. Seventeen clinicians, engineers and seating specialist responded and indicated that pelvic positioning belts, headrests, lateral support and chest harnesses were the most frequently prescribed SPSD. Head restraints, chest harnesses, subASIS bars and headrests were the SPSDs of greatest concern. Pelvic positioning belts, headrests and chest supports were cited by 35% of respondents as requirements for transport in their school districts. A follow up questionnaire (n = 9) on combinations of SPSDs indicated that adults are prescribed headrests with chest harness with the same frequency and they are prescribed headrests alone. Children are prescribed headrests with chest harnesses with twice the frequency that they are prescribed headrests alone.

Based upon the review of the literature, the review of prior test data and the input from clinicians the project will address two objectives in parallel:

1. To create a guidance document on SPSD usage in transportation based upon current knowledge, that not only discusses concerns for injury, but recommends how to use specific SPSDs, and

2. Performing an in-depth investigation on pediatric use of head rests using modeling and limited sled testing.

The current research and guidelines on use of SPSDs for wheelchair transportation were identified, documented and summarized in the RERC's State-of-the-Science Theme 2 white paper. This information is the basis for the draft guidance document that is now in the early stages of preparation.

Therapists have been consulted to aid in identifying the positioning issues associated with pediatric use of headrests. Several texts on postural support have been reviewed including the Whitmeyer Head School Handout. Currently available SPSDs were characterized based on devices currently available on the market.

The UMTRI wheelchair crash test data were extensively reviewed. The 409 WC19 tests to date were reviewed and catalogued for presence of SPSDs and test failure. One hundred fourteen (114) contained no SPSDs; the remaining tests included a variety of SPSDs and combinations of SPSDs. The tests were conducted according to the manufacturers' requests for the test set-up and thus do not represent a designed experiment. After careful review of 150 WC19 test reports and recording of relative head and head support heights, no statistical differences were found based on headrest height. Since this clearly differs from known information, this reflects the variability of the test set up rather than a true lack of statistical differences in seatback/headrest heights. It also reflects the frontal impact nature of the WC19 testing.

The graduate student on the project has installed, upgraded and maintained our licenses for the modeling software (MADYMO, HyperWorks and Easi-crash) on our computers as well as attended the MADYMO training class. She will begin developing a pediatric, rear-impact computer model for evaluation of injury risk associated with current headrest technology and clinical practices.

Progress Report May 1, 2006

Guidance documents: A draft version of the guidance document has been completed and circulated within the members of the RERC for final comment. The document includes an overview of the general principles of wheelchair transportation safety as well as specific recommendations on best practices for using secondary postural supports while seated in a wheelchair during vehicular transportation. Key information within the document focuses on the need to first ensure good support and posture in the wheelchair, then to properly use a crash-tested wheelchair tie-down and three-point occupant restraint system. The document includes strategies and suggestions for best transporting wheelchair users who cannot sit in an upright position. In all cases, it is strongly recommended that all PSDs needed to ensure clear airways and safe positioning be used. When possible, belts and foam surfaces are preferable to rigid bars and hard edges during transportation. The document clearly outlines the safety issues associated with the use or non-use of each PSD and highlights the recommendations based on experience and engineering principles. Illustrations are being added to the document for clarification.

In-depth investigation of wheelchair headrests: Crash tests: Baseline study: We conducted an investigational baseline study to characterize the kinematics of the wheelchair and describe wheelchair tiedown and occupant restraint system (WTORS) loading during rear impact for a pediatric wheelchair and occupant.
In this initial study, two identical, representative, manual pediatric wheelchairs that comply with ANSI/RESNA WC19, Wheelchairs used as seats in motor vehicles, were sled tested using a proposed 26 km/h, 10 g rear impact test pulse, Hybrid III 6-year old anthropomorphic test device (ATD), and surrogate WTORS. The rear impact test pulse used for this sled test was chosen based on its correspondence in severity to the ANSI/RESNA WC19 frontal impact standard.

Pendulum headrest testing: Dynamic pendulum testing of the proposed pediatric headrest was conducted before conducting sled tests of the Zippie Pediatric Wheelchair with the headrest attached. A calculation was made to determine the appropriate test configuration. All pendulum testing was conducted using the pendulum impact tester (PIT). All calculations are based on data from the two previously conducted sled tests with a 16 mph crash test sled pulse. Targets were placed on the Hybrid III 6-year old ATD: two on the head and one on the knee.

High-speed video (1000 frames/sec) recorded the crash test. Target coordinate data were acquired for the test crash. Head velocities were calculated from this data. Maximum head velocity was determined to be 7 meters/second. The mass of the ATD head is 3.47+0.05kg (7.66+0.10lbs). Maximum momentum of the ATD head was calculated using: momentum = mass x velocity.

A calculation was made to determine the pendulum displacement needed to create a momentum equivalent to the momentum of the ATD head during rear impact testing. This information was used to pendulum test the headrest in anticipation of the rear impact sled tests and to also determine the dynamic force-displacement response of the headrest and seatback configuration to input into the MADYMO modeling.

The pendulum test jig includes a pendulum bob weighing 16 lbs, and a 48 inch steel pendulum arm weighing 30 lbs. Ten identical pendulum drops we conducted against a wheelchair seat back. The load cell data was extremely noisy with a large noise component at 60 Hz. Initially, the data was averaged over 16-data points - creating a simple low pass filter. Although the data was noisy, it was repeatable with peak loads of 80+ 3.8 kg - within the accuracy of the load cell.

For the actual headrest testing, a variety of filters was tested including notch filtering and low pass Butterworth filter. A fast Fourier transform (FFT) was done on the load cell data, and it indicated that the impact event occurred at frequency of less than 15 Hz. Based on this, the minimum sampling rate should be 0.02 seconds. A trial was done at the rate, but it was not adequate to capture the peak loads. A sampling rate of 0.001 seconds was used. A Butterworth filter was used to remove all frequencies over 30 Hz and to obtain the force forcedisplacement curve. The vertical headrest stem sustained a 5-degree permanent deformation. Load displacement data will be used in the computer model.

Crash tests: Four additional crash tests were conducted: an additional test without a headrest and three with the headrest. All sled tests (6 total) used the identical wheelchair model, identical surrogate WTORS, identical ATD, and the identical test pulse. Three of the tests used the identical headrest model mounted in the identical manner. The headrests were positioned so that the leading edge of the back of the ATD's head was directly in front of the center of the headrest. The anterior/posterior stem of the headrest was pinned in the a/p direction, limiting slippage at the a/p - vertical headrest stem joint, but permitting rotation in the x-z plane.
Computer modeling: The graduate student on the project has begun developing a pediatric, rear-impact computer model for evaluation of injury risk associated with current headrest technology and clinical practices. Results from the previously conducted sled tests will be used both as input into the model and to establish response corridors used to validate the computer model Two models are in the process of being developed: one without a headrest, and one with a headrest.

The baseline crash tests performed without headrests on the wheelchair provided some interesting findings. While the rear impact test pulse was less severe than the WC19 frontal impact pulse, slightly higher maximum tiedown loads were measured than for a comparable frontal impact test - albeit on opposite tiedowns. The sled tests produced some unexpected wheelchair kinematics. In both tests the wheelchair rotated rearward and the casters rose off the sled platform at the time that the sled reached its maximum horizontal excursion and reversed direction. The wheelchair continued to rotate rearward until the caster vertical excursion reached its peak and remained in the tipped back position for the remainder of the test. A key, positive finding was that although this commercially available wheelchair model had not been either designed or tested under rear impact conditions, the wheelchair remained structurally intact and the ATD in an upright posture. The front tiedowns limited the rotation of the wheelchair, potentially preventing ATD ejection during the test. This exploratory study of tiedown loads during rear impact demonstrates that the tiedown loads levels are comparable, but reversed, in frontal and rear impacts, with front tiedowns carrying the larger load during rear impact. During rear impact sled testing, peak frontal tiedown loads (3473-4000 N) greatly exceeding peak rear tiedown loads (346-1117 N). This is critical information since front tiedowns frequently meet lower strength requirements, and highlights the need for all four wheelchair tiedowns to have equally robust design. This would be of particular concern for larger wheelchairs and adult occupants. In addition, the rearward wheelchair rotation and the large front tiedown loads found in rear impact reinforce the critical need to use all four tiedown straps every time the wheelchair is used as a vehicle seat during transportation. Furthermore, the wheelchair structural integrity displayed during the rear impact sled tests indicates that commercial pediatric manual wheelchairs that comply with WC19 have the potential to meet rear impact test requirements; key information in the future development of rear impact test standards for wheelchairs.

The additional crash tests performed with headrests on the wheelchairs also had some preliminary findings of interest. Initial evaluation of the results indicates that the headrest performed as anticipated with permanent headrest stem deformation of 4 degrees. The wheelchairs equipped with headrests were extremely effective in reducing head accelerations and neck bending in the Hybrid III 6-year old ATD despite neither being designed nor tested as an automotive restraint system. Videos of the test crash show both the seatback and headrest moving together and deflecting rearward to cushion the rearward motion of the ATD; this type of motion is similar to Volvo's WHIPS.

Last updated: August 18, 2006