Douglas Hobson

Comparative effects of posture on pressure and shear at the body-seat interface

This study considers the effects of seated posture and body orientation on the pressure-distribution and surface shear (tangential) forces acting at the body-seat interface. Nine postures typically assumed by wheel-chair users were studied. Comparisons were made within and between two study groups, made up of 12 subjects with spinal cord injuries (SCI) and 10 nondisabled subjects. Both interface pressure and the surface shear were measured simultaneously in each of nine reproducible, seated postures. The same seat cushion was used for all trials. The Oxford Pressure Monitor, a pneumatic cell device, was used to measure and record the interface pressures. Instrumentation for measuring and recording the surface shear force was constructed specifically for the study. Analysis consisted of statistically comparing changes in pressure values and shear forces derived from eight sitting postures with reference to values recorded in a defined neutral sitting posture. The pressure-distribution findings suggest that in the postures studied SCI subjects have maximum pressures that are higher than nondisabled subjects in all postures, ranging from 6% to 46% depending on the posture. Maximum pressures can be reduced by postural changes: forward flexion to 50 degrees, -9%; backrest recline to 120 degrees, -12%; and, full body tilt, -11%. On average, the SCI group members have peak pressure gradients (PPG) that are 1.5 to 2.5 greater than the nondisabled group. The maximum reduction in PPG occurred at backrest recline of 120 degrees, -18%. Tangential shear force acts at the body-seat interface in all nine postures studied. Extrapolation of results suggests that full-body tilt to approximately 25 degrees reduces the surface shear force to near zero. In contrast, a backrest-only recline of 20 degrees causes a 25% increase in the surface shear force. These results suggest that caution must be taken when using nondisabled subjects as surrogates for people with SCI because of the inherent differences between the groups. Also, researchers and clinicians should recognize that posture and body orientation in space are additional variables that can have a profound effect on the interaction between a seated person and his or her supporting surface.

Development of a consumer-driven Wheelchair Seating Discomfort Assessment Tool (WcS-DAT)

This study examined the concept of seating discomfort in a population of full-time wheelchair users with intact sensation. The goal was to construct a tool that would quantify seating discomfort experienced by wheelchair users. Ten participants were interviewed using ethnographic interview techniques. Data were analyzed using a cross-classifying matrix to examine commonalities among the 10 participants’ responses. There were 16 discomfort and 13 comfort descriptors used by the participants. Of these, eight discomfort and five comfort descriptors were selected to include in the Wheelchair Seating Discomfort Assessment Tool (WcS-DAT). The discomfort descriptors selected were: aches and pains, need to move, pressure points, feeling poorly positioned, unable to concentrate, instability, not comfortable, and feeling too hot, cold or damp. The comfort descriptors selected were: absence of discomfort, feeling good, having no pain, able to concentrate, and feeling stable. The WcS-DAT also includes general information, such as the amount of time spent sitting and whether the individual was transferred into the chair properly—factors thought to affect discomfort—and ratings of discomfort intensity—in general and differentiated by body area. The WcS-DAT is a comprehensive tool for quantification of wheelchair seat discomfort for this population.

Development of transportable wheelchair design criteria using computer crash simulation

The Americans with Disabilities Act (ADA) has led to an increase in disabled travelers, many of whom are unable to transfer to a vehicle seat and are required to use their wheelchair to fulfill this function. ANSI/RESNA is currently developing a transportable wheelchair standard which will identify design requirements and testing methods for wheelchairs suitable for transport. Wheelchair manufacturers should begin to modify their existing design criteria established for a normal mobility function to design criteria appropriate for a transportation function which may subject the wheelchair to large dynamic crash forces. A thorough understanding of the crash environment and its effect on the wheelchair is necessary to insure the safety of the wheelchair user. To assist manufacturers in the design effort, this study uses mathematical crash simulations to evaluate loads imposed upon a wheelchair when subjected to a 48 kph/20 g frontal crash. Using a four-point belt tiedown system to secure the wheelchair, securement point, seat, lap belt anchor, and wheel loads are evaluated under three different securement configurations. Results show that positioning of rear securement points near the wheelchair center of gravity can serve as an effective strategy for managing crash response and loadings on the wheelchair. Force ranges for each of the evaluated parameters, derived for a 50th percentile male using a simulated power wheelchair, are provided for use as a preliminary guide when designing transportable wheelchairs.

Computer simulation and sled test validation of a powerbase wheelchair and occupant subjected to frontal crash conditions

The Americans with Disabilities Act (ADA) has led to an increased number of wheelchair users seeking transportation services. Many of these individuals are unable to transfer to a vehicle and are instead required to travel seated in their wheelchairs. Unfortunately, wheelchairs are not typically designed with the same occupant protection features as motor vehicle seats, and wheelchair seated occupants may be at higher risk for injury in a crash. To study the effects of crash level forces on wheelchairs and their occupants, it is useful to simulate crash conditions using computer modeling. This study has used a dynamic lumped mass crash simulator, in combination with sled impact testing, to develop a model of a secured commercial powerbase and restrained occupant subjected to a 20 g/30 mph frontal motor vehicle crash. Time histories profiles of simulation-generated wheelchair kinematics, occupant accelerations, tiedown forces and occupant restraint forces were compared to sled impact testing for model validation. Validation efforts for this model were compared to validation results found acceptable for the ISO/SAE surrogate wheelchair model. This wheelchair-occupant simulation model can be used to investigate wheelchair crash response or to evaluate the influence of various factors on occupant crash safety.

Test-retest reliability, internal item consistency, and concurrent validity of the wheelchair seating discomfort assessment tool.

Discomfort is a common problem for wheelchair users. Few researchers have investigated discomfort among wheelchair users or potential solutions for this problem. One of the impediments to quantitative research on wheelchair seating discomfort has been the lack of a reliable method for quantifying seat discomfort. The purpose of this study was to establish the test-retest reliability, internal item consistency, and concurrent validity of a newly developed Wheelchair Seating Discomfort Assessment Tool (WcS-DAT). Thirty full-time, active wheelchair users with intact sensation were asked to use this and other tools in order to rate their levels of discomfort in a test-retest reliability study format. Data from these measures were analyzed in SPSS using an intraclass correlation coefficient (ICC) model (2,k) to measure the test-retest reliability. Cronbachs α was used to examine the internal consistency of the items within the WcS-DAT. Concurrent validity with similar measures was analyzed using Pearson product-moment correlations. ICC scores for all analyses were above the established lower bound of .80, indicating a highly stable and reliable tool. In addition, alpha scores indicated good consistency of all items without redundancy. Finally, correlations with similar tools, such as the Chair Evaluation Checklist and the Short Form of the McGill Pain Questionnaire, were significant at the .05 level, and many were significant at the .001 level. These results support the use of the WcS-DAT as a reliable and stable tool for quantifying wheelchair seating discomfort. Its application will enhance the ability to assess and to research this important problem and will provide a means to validate the outcomes of specialized seating interventions for the study population of wheelchairs users.

A dynamic seating intervention for wheelchair seating discomfort

Crane BA, Holm MB, Hobson D, Cooper RA, Reed MP: A dynamic seating intervention for wheelchair seating discomfort. Am J Phys Med Rehabil 2007;86:988–993. Objective:The objective of this study was to examine the effectiveness of a new user-adjustable wheelchair seating system designed to relieve discomfort for long-duration wheelchair users. Design:This objective was carried out using the newly developed Tool for Assessing Wheelchair disComfort (TAWC) as the primary outcome measure. Two wheelchair users each tested two different designs and feedback from the wheelchair users regarding the first design was used to guide development of the second design. A single-subject research methodology was used, allowing long-duration (up to 2 wks per test) evaluation of the wheelchair seating systems and comparison of subject discomfort levels with those experienced during a baseline period using their own wheelchairs. The experimental wheelchair seating systems employed existing automotive seating with embedded pneumatic bladders that allowed adjustment of the seat and back-support characteristics. The test wheelchair also had tilt, recline, and elevating leg rests. Results:The two subjects completed limited periods of testing with the first design, both finding poor results with either stable or increased levels of discomfort. Subject feedback was used to redesign the wheelchair seat. After redesign, both subjects tested the second design and found it substantially more comfortable. Conclusions:The selected research methodology was a very positive method for a progressive wheelchair seating design and the second design provided improved comfort for both users when compared with that experienced using their own wheelchairs and the first test wheelchair. Future research of this type of user-controlled technology is recommended.

Development of a wheelchair occupant injury risk assessment method and its application in the investigation of wheelchair securement point influence on frontal crash safety

To promote proper wheelchair securement in transportation, the proposed ANSI/RESNA Standard on Wheelchairs Used as Seats in Motor Vehicles will require that all transit wheelchairs be equipped with four securement points compatible with strap-type tiedowns. Through computer simulations, the location of these securement points has been found to influence wheelchair user response to a frontal crash. This study develops and employs an injury risk assessment method to compare the crashworthiness of various securement point configurations. The comparative injury risk assessment method is designed to predict the risk associated with internalized crash forces, as well as risk associated with secondary occupant impact with the vehicle interior. Injury criteria established by Federal Motor Vehicle Safety Standards and General Motors, along with excursion limitations set by the Society of Automotive Engineers (SAE) J2249 Wheelchair Tiedowns and Occupant Restraint Systems (WTORS) Standard were used as benchmarks for the risk assessment method. The simulation model subjected a secured commercial powerbase wheelchair with a seated 50th percentile male Hybrid III test dummy to a 20 g/30 mph crash. The occupant was restrained using pelvic and shoulder belts, and the wheelchair was secured with four strap-type tiedowns. Results indicated that securement points located 1.5 in to 2.5 in above the evaluated wheelchairs center of gravity provide the most effective occupant protection.

Postural changes with aging in tetraplegia: effects on life satisfaction and pain.

OBJECTIVES (1) To measure kyphosis and scoliosis in individuals with tetraplegia; (2) to examine the relation between kyphosis and scoliosis and years since injury; and (3) to determine the association between kyphosis and scoliosis and measures of pain, depression, and life satisfaction. DESIGN Cross-sectional, case-control study. SETTING University medical center and a free-standing university-affiliated rehabilitation hospital. PARTICIPANTS (1) Ten individuals with tetraplegia I to 3 years postinjury (NT); (2) 10 individuals with tetraplegia 10 to 20 years postinjury (OT); and (3) 10 control individuals (C) matched to the other subjects on the basis of age, height, and weight. MAIN OUTCOME MEASURES Radiographic measurements of kyphosis and scoliosis taken in a seated position, pain as measured by the short form of the McGill Pain Questionnaire (SF-MPQ), depression as measured by the Center for Epidemiological Studies-Depression Scale (CES-D), and life satisfaction as measured by the Life Satisfaction Index Assessment (LSIA) and the Craig Handicap Assessment and Reporting Technique (CHART). RESULTS No significant differences were seen between the OT and NT groups with respect to age, height, or weight. In addition, no significant differences were found between the NT and OT groups with respect to measures of kyphosis and scoliosis. Individuals with tetraplegia had significantly higher (p < .05) measures of kyphosis (42 + 16.0 ) and scoliosis (14 degrees +/-9.2 degrees) than the C subjects (kyphosis, 32 degrees +/- 7.9 degrees ; scoliosis, 5 degrees+/-3.8 degrees). No correlation was found between scores on the SF-MPQ and degree of kyphosis or scoliosis. Significant differences were seen between the NT and OT groups on both CES-D (NT, 15.2+/-8.1; OT, 5.8+/-5.5) and LSIA (NT, 9.9+/-2.8; OT, 14.4+/-2.9). CONCLUSION This study indicates that seated kyphosis and scoliosis develop early in individuals with tetraplegia and may not be progressive. No association was seen between pain and kyphosis or scoliosis in this relatively young sample (mean age of OT and NT combined, 34.8 years). Future research is needed to determine whether pain becomes a problem in individuals with significant kyphosis or scoliosis as they age.

Wheelchair Seating: A State of the Science Report

Regardless of the field, agenda-setting processes are integral to establishing research and development priorities. Beginning in 1998, the National Institute on Disability and Rehabilitation Research mandated that each newly funded Rehabilitation Engineering and Research Center (RERC) hold a state-of-the-science consensus forum during the third year of its 5-year funding cycle. NIDRRs aim in formalizing this agenda-setting process was to facilitate the formulation of future research and development priorities for each respective RERC. In February 2001, the RERC on Wheeled Mobility, University of Pittsburgh, conducted one of the first such forums. The scope encompassed both current scientific knowledge and clinical issues. In preparation, expert interviews were carried out to establish the focus for the forum. Because a stakeholder forum on wheelchair technology had recently been held, opinion favored wheelchair seating as the focus and included the following core areas: seating for use in wheelchair transportation, seated postural control, seating discomfort, and tissue integrity management. The aim of this report is to present a summary of the workshop outcomes, describe the process, and increase awareness of this agenda-setting process in order to enhance future participation in a process that critically influences the field of wheeled mobility.

Transportation Safety Standards for Wheelchair Users: A Review of Voluntary Standards for Improved Safety, Usability, and Independence of Wheelchair-Seated Travelers

Safe transportation for wheelchair users who do not transfer to the vehicle seat when traveling in motor vehicles requires after-market wheelchair tiedown and occupant restraint systems (WTORS) to secure the wheelchair and provide crashworthy restraint for the wheelchair-seated occupant. In the absence of adequate government safety standards, voluntary standards for the design and performance of WTORS, and for wheelchairs used as seats in motor vehicles, have been developed. The initial versions of these standards qualify equipment for use in all types and sizes of motor vehicles using a 30-mph (48-kph), 20-g frontal sled-impact test. The wheelchair standard requires four accessible, crash-tested securement points on wheelchairs so they can be more easily and effectively secured using a four-point strap-type tie-down system. Future voluntary standards are aimed at reducing injury risk for wheelchair-seated occupants in rear impacts and at providing a method for evaluating the crashworthiness of wheelchair seating systems independent of wheelchair base-frames. They also address improved usability and independence for wheelchair-seated travelers using public transportation by specifying universal docking interface geometry for wheelchairs and design and performance requirements for rear-facing wheelchair passenger stations for use in the very low-g environments of large fixed-route transit buses.