XV. Wheelchairs and Powered Vehicles


A. General


Rory A. Cooper, PhD; David P. VanSickle, MS; Mike L. Boninger, MD; Jess Gonzalez, BS; Brad Lawrence, BS; Andy Rentschler, BS; Dehou Lui, MS; Dan Woodbury, BS; Mike Tassick, BS
Human Engineering Research Laboratories, VA Pittsburgh Healthcare System, Pittsburgh, PA 15206; Departments of Rehabilitation Science and Technology, and Bioengineering, Schools of Health and Rehabilitation Science, Engineering, and Medicine, University of Pittsburgh, Pittsburgh, PA 15261; Division of Physical Medicine and Rehabilitation, Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, Pittsburgh, PA, 15213-3221; email: rcooper+@pitt.edu

Sponsor: Department of Veterans Affairs, VA Rehabilitation Research and Development Service, Washington, DC 20420
(Project #B805-2RA)

PURPOSE--The goals of this research are to determine factors related to ride comfort; to record actual wheelchair road loads; and to evaluate the performance of selected wheelchairs on ANSI/RESNA standards. Ultimately, this research should improve wheelchair design and selection procedures leading to more durable and comfortable wheelchairs which are less likely to lead to disabling secondary conditions (e.g., pain and spinal-pelvic deformities).

METHODOLOGY--This is a complex study addressing wheelchair design in a comprehensive way. Surveys and accelerometry are being used to assess wheelchair rider comfort. Seven different wheelchair models from various manufacturers are being evaluated. Subjects ride the wheelchairs over a predefined obstacle course that has events similar to those encountered during normal activities of daily living. Upon completion of the course, each subject is asked to complete a survey about the obstacles and the course as a whole.

  Accelerometer data is collected from the seat and bite-bar (head) in three dimensions. The data are used to develop periodograms and transfer functions. This information is being used to determine whether some wheelchairs are better at attenuating impacts and vibrations from the ground. It is also being compared to rider reported comfort.

  Road load data are being collected to determine the actual loads experienced by users and their chairs, data critical for innovative design using computer-aided engineering and essential for using new materials that may lead to improved quality and lower life-cycle cost among wheelchair products. Actual road load data are being compared to that collected with the same instrumentation on ANSI/RESNA test machines. These data will be useful in improving the test methods to obtain accelerated results which are comparable to actual usage.

PROGRESS--As part of the ANSI/RESNA standards evaluation process, we have tested nearly 30 wheelchairs (depot, lightweight, and ultra-lightweight) to class III failure which would resulted in a significant degradation of performance or safety risk. Our results indicate quality striation increasing from depot to lightweight to ultralight weight wheelchairs. This is reflected in quality (i.e., number of cycles to failure) and value (cycles per dollar). Our results further indicate that similar products from different manufacturers tend to yield similar results. Strain gage data on some of the models of chairs is being used to develop S-N curves for fatigue equations to predict cycles to failure at critical points.

  Ride comfort data have been collected using methods adopted from ISO 2631 and a survey on seven different wheelchairs evaluated by about 20 users. The survey addresses comfort while traversing various obstacles over a predefined course and ergonomic questions. The results indicate that there are differences between rider perceived comfort related to wheelchair model. Accelerometer data were also collected between the seat and rider's head. These data are being analyzed to determine whether variables relating seat-to-head acceleration are correlated with rider comfort.

  The ANSI/RESNA fatigue tests were largely developed through consensus with little supporting data. The tests have served to improve the quality of wheelchairs. However, it is difficult to determine whether the tests are realistic and provide an accurate assessment of useful life. Moreover, data are required to apply computer-aided engineering techniques to wheelchair design. To this end, we developed instrumentation to record rear hub and front caster forces and moments. The instrumentation is portable and is being used to collect data in the laboratory, during normal use in the community, and on ISO test machines. Data have been collected with 20 subjects in the laboratory, 11 subjects during community use, and with an ISO dummy during ANSI/RESNA fatigue testing.

RESULTS--Our results indicate that ride comfort does vary among different models of wheelchairs. While there is considerable spread among some of the data, they do tend to show significant trends that appear to be stable as the number of subjects increases. Accelerometry data does appear to be correlated with self-reported rider comfort.

  The road load data indicates that wheelchairs experience more frequent and higher magnitude loads on the test machines than during actual driving. This may explain why some wheelchairs tend to experience low-cycle fatigue on testing machines. Some of the loads experienced infrequently by wheelchairs during community driving are quite high. High loads are likely to be associated with curb descents. Community and test machine load data can be accurately described by statistical distributions. This should promote incorporation of the data into design processes.

FUTURE PLANS--We plan to continue increasing our sample size, to look at the influence cushion type may have on rider comfort, and the relationship between road and test loads. Currently, the focus has been on manual wheelchairs. It is important to extend this work to electric-powered wheelchairs and scooters. Very little comfort, load, or testing data are available for these devices. Our results indicate that modifications to the ANSI/RESNA fatigue tests may be appropriate to obtain results more similar to those obtained during actual use. Of course, the test procedures must take into account the accelerated time frame required of standards testing in order to be useful for design and evaluation. This line of research shows promise to benefit people who use wheelchairs, and the results have opened several areas for more intense investigation.




Lucas H.V. van der Woude, PhD; A. Dallmeijer; R.H. Rozendal; A.P. Hollander; H. van As; E. Angenot; M.T. Hopman
Institute for Fundamental and Clinical Human Movement Sciences, Vrije Universiteit, Faculty of Human Movement Sciences 1081BT Amsterdam, the Netherlands; Rehabilitation Center Amsterdam, Amsterdam, The Netherlands; Department of Physiology, Catholic University Nijmegen, The Netherlands; email: L_H_V_van_der_Woude@FBW.VU.NL

Sponsor: Dutch Prevention Fund

PURPOSE--Systematic analysis of manual wheelchair propulsion from a combined biomechanical and physiogical perspective may eventually improve the mobility of the wheelchair-user combination. We study the impact of wheelchair design characteristics upon the physiology and biomechanics of the wheelchair user, with special reference to functional load and mechanical efficiency and kinetics, loading of structures during mere manual wheelchair propulsion. From this, a set of theoretically based guidelines for wheelchair design and wheelchair fitting emerges. We also study the factors that determine work capacity and power output (among others: functionality and propulsion technique) of the wheelchair user. This should lead to guidelines of wheelchair training in sports and rehabilitation, and additionaly serve in the development of design and fitting guidelines of wheelchairs.

METHODOLOGY--Wheelchair propulsion is studied during standardized submaximal aerobic wheelchair exercise and sprint tests on a motor-driven treadmill and during simulated conditions on different computer-controlled wheelchair ergometers. During the treadmill tests (used in studies on prototype evaluation, performance capacity, and propulsion technique), physiological measures are combined with 3-D kinematics and electromyography. Force measurements and kinematics during propulsion on the wheelchair ergometer enable an additional 3-D reconstruction of the movement pattern of arms and trunk and the study of force and power production. Together with electromyography of shoulder, arm, and trunk muscles and overall physiology, phenomena of the low and varying mechanical efficiency in manual wheelchair propulsion may be studied from a biomechanical and anatomical perspective.

  A detailed model of the shoulder-arm complex allows calculation of the contribution of different muscles on power production during static and dynamic activity of shoulder and elbow in wheelchair arm work and other tasks. Thus the high prevalence of repetitive strain injuries (RSI) in the shoulder and hand-wrist area among the wheelchair-user population may be understood more clearly. Currently, a system is implemented that allows the ambulant measurement of 3-D wrist excursions during wheelchair propulsion. Arm work during different forms of manual wheelchair propulsion is thus studied: lever, crank, hubcrank, and hand rim propulsion.

PROGRESS--Detailed studies were conducted on lever and (synchronic and asynchronic) crank propulsion in relation to different gear ratios. Results indicated a significantly better performance using the synchronic mode. The levers showed a better performance when using a "high resistance/low speed" condition. Overall, levers and cranks are much less straining and more efficient than handrims. The latter also holds for hubcrank propulsion. A first study of hand-wrist motions indicated large excursions around the flexion/extension axis and ulnar/radial deviation axis.

FUTURE PLANS--We shall work on further refinement of fitting quidelines for groups of impaired subjects, also during the process of rehabilitation. Detailed analysis of wheelchair arm work during hubcrank and handrim propulsion must contribute to a further understanding of the mechanisms and risks of RSI and possible preventive measures in terms of wheelchair design or propulsion technique.




Sheldon R. Simon, MD; Wayne E. Carlson, PhD; Donald L. Stredney; Rebecca Jackson, MD; Rosalind Batley, MD; Torsten Moeller; Po-Wen Shih
The Division of Orthopaedics, ACCAD, and the Department of Physical Medicine/Rehabilitation, The Ohio State University, Columbus, OH 43210; email: Simon.1@osu.edu; carlson.8@osu.edu; don@osc.edu; jackson.20@osu.edu; batley.1@osu.edu; moeller.19@osu.edu; shih.4@osu.edu

Sponsor: National Institute for Disability and Rehabilitation Research, Rehabilitation Engineering Research, Washington, DC 22202

PURPOSE--In an effort to utilize technology that can have a direct and immediate relevance to the problems confronting persons with disabilities, we propose to examine human performance in negotiating barrier-free environments through the use of computer-generated virtual simulations. The project will be instrumental in defining standards for use in evaluating user proficiency, which will provide information for more suitable selection of assistive technology (AT). In addition, this research will demonstrate direct implications to the development of AT through virtual testing and analysis, and provide improved methods for designing barrier-free environments.

METHODOLOGY--We will interface Invacare's Action Power Evaluation and Training simulator with a computer simulations. We will incorporate "real world" architectural databases, so as to maximize transfer from the simulator to actual user environments. We propose that this research will demonstrate feasibility toward a practical interface for for use in analyzing and evaluating human performance, and provide us with new insight into the nature of disabilities and new understanding of certain limitations.

PROGRESS--We have developed a system that integrates computer visualization with innovative interface technology to create a virtual environment that simulates the movement of a power wheelchair through synthesized architectural environments. For architects and designers, this system provides structure, previsualization, and analysis that can both improve the accessibility of building designs and test a structure for compliance with the ADA; for wheelchair users, it provides more appropriate device fitting and training with control systems; and for health care professionals, it provides a way to assess user performance and for determining the best power chair control mechanism for a particular user.

  The system consists of an instrumented, joystick-driven power wheelchair connected to a high-performance graphics workstation. The user, wearing 3-D glasses, sits in a wheelchair utilizing a standard joystick interface to navigate through a computer-synthesized environment. The system depicts the dynamics of the wheelchair in the particular environment. The virtual chair moves with the same speed and turning radius as a chair in the physical world.

  Two primary modes of operation were supported for the joystick: passive and active. In passive mode, the joystick will discourage the driver from going toward obstacles by making it harder to push the joystick in those directions. Active mode allows the joystick to exert a force that will guide the user toward a less congested area.

  We are integrating a head-mounted display with the system to provide the precise location of objects in the user's field of view at the time of reaction. All of this information is automatically filed for the evaluator, providing a comprehensive analysis of the user's performance.

  Pilot trials have been run at the Ohio Supercomputer Center with individuals with disabilities of varying severity and controls. Simple steering tasks were performed and evaluated as the subjects navigated a simple environment using a joystick controller.

RESULTS--To date, we have developed a robust system that provides previsualization of architectural data sets and assists in assessment for ADA compliance. In addition, the system provides an immersive environment for users to train themselves in the use of a powerchair, thereby limiting the application of unsuitable technology that may never be fully, or even partially, utilized. Also, we are completing an evaluator's environment for trials to evaluate individuals in power chair proficiency.




Michael J. Dvorznak, BS; Rory A. Cooper, PhD; Thomas J. O'Connor, MS; Michael L. Boninger, MD
Human Engineering Research Laboratories, VA Pittsburgh Health Care System, Pittsburgh, PA 15206; Departments of Rehabilitation Science and Technology, University of Pittsburgh, Pittsburgh, PA 15261; Division of Physical Medicine and Rehabilitation, Department of Orthopedic Surgery, University of Pittsburgh Medical Center, Pittsburgh, PA, 15213-3221; email: rcooper+@pitt.edu

Sponsor: Paralyzed Veterans of America, Washington, DC 20006; Eastern Paralyzed Veterans of America

PURPOSE--Many power wheelchair users have paraplegia, quadriplegia, or cerebral palsy. Because of their conditions, they lack sufficient muscle strength or control to hold the body in an upright seated position, especially when subjected to forces associated with braking. There are about 36,000 serious wheelchair-related accidents annually, the majority of which are attributed to falls and tips. Our goal is to reduce this number by investigating the reason why these falls occur. Our hypothesis is that safety and support devices such as seatbelts and footrests are not always used by the wheelchair occupants, compounding problems. We will correlate the risks of injury due to falls with the use of said devices.

METHODOLOGY--A fiftieth percentile anthropometric crash test dummy (ATD) was used to simulate a human occupant of a power wheelchair. The dummy was seated in a power wheelchair and the chair accelerated to its maximum velocity. When the chair reached a line on the floor, a braking condition was enacted. The response of the dummy and the chair was recorded via a digital video camera. Reflective markers were placed on 9 anatomical positions of the dummy and 6 locations on the chair so that the camera could capture the position versus time of the wheelchair and body segments of the ATD. The recorded images on video are digitized and analyzed using a Peak5 Motion Analysis System.

  The test protocol includes three braking conditions: kill switch activation, joystick release, and full reverse of the joystick. Four adjustments were made to each wheelchair: seatbelt and footrests on, seatbelt on and footrests removed, without the seatbelt and footrests attached, and with neither seatbelt nor footrests in use. This gives a total of twelve combinations tested on each chair. For statistical reasons, each condition was repeated three times.

PROGRESS--Eight power wheelchairs have been tested, filmed, and digitized. The data output from the motion analysis system was processed through a program written in Matlab to calculate the different kinematic parameters of the chair and dummy body segments. We are beginning statistical analysis to correlate the risk of falls and injury to the kinematic data.

PRELIMINARY RESULTS--The set-up of the wheelchair and the braking condition appear to influence driver safety. A pilot test study with two wheelchairs involving only a few wheelchair and ATD kinematic parameters showed that wheelchair condition had no significant effect on the dummy kinematics for the joystick release braking condition. However, kill switch braking conditions, in which deceleration rates are higher, showed significant differences in the kinematic data between trials in which the ATD fell from the chair and trials where it did not.

FUTURE PLANS--In the future, we would like to conduct a more comprehensive study with an increased sample size. We intend to use a HybridIII test dummy, equipped with accelerometers and force sensing gages, to acquire force data during the trials. From this kinetic data, we can acquire information as to whether bones will be broken during falls or whether restraining devices could put users at risk to injuries.

  Until now, all tests have been done on a clean, dry, level, linoleum floor. This does not encompass all driving conditions the user may encounter. We would like to test on different surfaces (carpet, concrete) and on different obstacles (ramps, curbs).

  Part of the problem is lack of awareness. Power wheelchair users need to be notified so that they understand that they are accepting a potential risk by removing footrests and not wearing seatbelts.

  We would like our research to result in the establishment of safety standards associated with characteristics of the power wheelchair, such as maximum deceleration and braking distance.



B. Powered Controllers


Pascal Malassigné, MID, IDSA; Audrey L. Nelson, RN, PhD; Mark W. Cors, BFA; Carl H. Sutton, MD; Thomas L. Amerson, PhD; Rosemary Bonifay, MS
Research Service, Spinal Cord Injury Center, and the Social Work Service, Clement J. Zablocki VA Medical Center, Milwaukee, WI 53295-1000; James. A. Haley VA Medical Center, Tampa FL 33612-4798; Department of Physical Medicine and Rehabilitation, Medical College of Wisconsin, Milwaukee, WI 53226;

Sponsor: Department of Veterans Affairs, VA Rehabilitation Research and Development Service, Washington, DC 20420
(Project #B1522-3DA)

PURPOSE --The goal of this project is to develop a new motorized prone cart, the last phase in the design of new prone carts by this team of investigators.

  Prone carts are used by individuals with spinal cord injuries (SCI) who have pressure sores on the sacral region and the legs. Once pressure ulcers develop, the patient must stay off affected areas until they are healed. In most instances, this precludes the use of a wheelchair. Prone carts provide an alternative to prolonged bed immobilization for persons with ischial or sacral pressure ulcers who must lay on the side or in prone position.

  Motorized prone carts are used by individuals with SCI who have limited, or no, arm and hand functions (quadriplegics), while manual prone carts are used mostly by individuals with arms and hand functions (paraplegics).

METHODOLOGY--This project will be acocmplished in three phases. First, we shall design and fabricate two motorized prone carts with Ortho-Kinetics, the collaborating manufacturer. Each will be equipped with a different controller system. Then we shall evaluate the carts with patients at the Milwaukee and Tampa SCI Centers as well as in wheelchair accessible homes. This evaluation will include a combination of questionnaires, interviews, and photography to validate the prototypes. Finally, we shall Test the new carts with applicable ANSI/RESNA Electric Wheelchair Standards: determination of dynamic stability (WC/02); determination of efficiency of brakes (WC/03); determination energy consumption (WC/04); determination of overall dimensions, mass, and turning space (WC/05); determination of maximum speed, acceleration, and retardation (WC/06); static, impact, and fatigue strength tests (WC/07); climatic test (WC/09); determination of the obstacle climbing ability (WC/10); testing of power and control systems (WC/14); and determination of flammability.

PROGRESS--The design and fabrication of two prone cart frames has been accomplished. The investigators await power base and controller units, in order to mount them on the frames prior to beginning clinical evaluation.

FUTURE PLANS--Based upon the evaluation, the prototypes will be modified if necessary, before completion of the project and testing with applicable ANSI/RESNA Electric Wheelchair Standards.


C. Seating Systems


Denise Reid, PhD, OT(C); Steve Ryan, BESc, PEng; Patty Rigby, MHSc, OT(C); Michael Doell, AOCA
The University of Toronto, Department of Occupational Therapy, Toronto, Ontario, Canada; Bloorview MacMillan Centre, Toronto, ON Canada M4G 1R8

Sponsor: Ontario Rehabilitation Technology Consortium funded by the Ontario Ministry of Health

PURPOSE--The purpose of this project is to develop school furniture that has universally accessible design features. We plan to identify consumer needs and the issues related to posture and school work to design and develop new furniture for use by most students with or without physical disabilities.

METHODOLOGY--Qualitative methodologies were applied to acquire new information about the product needs of elementary school students. Initially, unstructured interviews were conducted with 15 key informants: teachers, teacher assistants, principals, occupational and physical therapists working in schools, and educational consultants. Next, we developed and administered a self-report questionnaire using the relevant variables identified by the key informants and issues discussed in the literature.

PROGRESS--Sixty-four responses were received from a sample of 123 questionnaires (52 percent return). Respondents expressed a pressing need for chairs and desks that fit children better. However, capital funding constraints in schools will clearly curb the commercial success of our products if we compete against simple, low cost school furniture. The results are now being used to establish criteria for various designs. We plan to work with consumers to evaluate these novel design solutions.



Wes From, MASc, PEng; Denise Reid, PhD, OT(C); Patty Rigby, MHSc, OT(C); Steve Ryan, BESc, PEng; Joy Sommerfreund, MEd, OT(C); Kubet Weston
The University of Toronto, Department of Occupational Therapy, Toronto, Ontario, Canada; Bloorview MacMillan Centre, Toronto, ON Canada M4G 1R8; Thames Valley Children's Centre, London, Ontario; The University of Toronto, Department of Physical Therapy, Toronto, Ontario, Canada

Sponsor: The Ontario Rehabilitation Technology Consortium funded by the Ontario Ministry of Health

PURPOSE--In our effort to understand the concerns of consumers who use seating products, we have discovered that postural belts appear to be the source of greatest dissatisfaction. To address this, a project was begun to focus our developments on these components.

METHODOLOGY--We will work at improving the functional design of the anterior postural control systems with the help of consumers and service clinicians. Survey, focus group techniques, and family trials will be used to evaluate current belting approaches to support the development and commercialization of new, innovative alternate designs.

PROGRESS--Since 1994, development has progressed to a state where pre-production prototypes of two products have been or currently are being clinically tested. Based on needs expressed by consumers and clinicians, two directions were pursued: the Hip Huggers;tm were developed to increase the efficacy of regular lap belts, while the Rigid Pelvic Stabilizer System was designed to replace postural belts entirely, particularly for children with complex seating problems.

FUTURE PLANS--The design phase for both devices is virtually complete. The clinical trials will be completed, and the devices will be turned over entirely to their respective industrial partners.



Steve Ryan, BESc, PEng; Michael Doell, AOCA; Patty Rigby, MHSc, OT(C); Wes From, MASc, PEng; John Hancock; Ihsan Al-Temen, PEng; Joy Sommerfreund, MEd, OT(C); Margaret Young, OT(C)
Bloorview MacMillan Centre, Toronto, ON Canada M4G 1R8; Thames Valley Children's Centre, London, Ontario, Canada

Sponsor: Rotary Club of Leaside (Toronto), Ontario Rehabilitation Technology Consortium funded by Ontario Ministry of Health, and General Motors Canada (Oshawa, Ontario)

PURPOSE--Many children with physical disabilities need custom-made seats to be comfortable and well-supported while in their wheelchairs. This creates safety concerns for parents wanting to transport their child in the family car. Commercially available car seats are often not suitable because many of these children do not fit into them. As a result, parents use the custom-made seat from their child's wheelchair for this purpose. This arrangement may not provide adequate protection for the child in case of collision.

METHODOLOGY--We are working to develop a custom car seat for school-aged children weighing from 40 to 75 pounds. In this way, they will be comfortable, yet be offered the same level of occupant protection as other passengers in motor vehicles. Parents are also concerned about how to transfer their child safely to and from the car without injuring their backs. To help them deal with this problem, we also developed a portable lift and transfer device for this purpose.

PROGRESS--To reduce manufacturing costs and improve the designs, efforts focused on repackaging both the child restraint system and lifting device. The power source of the lifting device was changed from a rechargeable unit to the car battery to increase its reliability and battery life.

FUTURE WORK--We will develop marketing strategies to interest companies. As part of this initiative, we will seek patent protection to guard the commercial value of our products.



Thomas Adams, ME; Derek Kamper, PhD; Steven Reger, PhD, CP; Maureen Linden, MS; Vinod Sahgal, MD
Department of Rehabilitation Medicine, Cleveland Clinic Foundation, Cleveland, OH 44195

Sponsor: Transportation Research Board, The Ohio State University Center for Automotive Research; the Cleveland Clinic Foundation, Cleveland, OH 44195

PURPOSE--Information is needed to assess the application of simple, clinical evaluations in a static environment of the stability of individuals seated in wheelchairs during automobile driving. Reliable clinical evaluations are needed for the design of postural support belts and contoured seating systems necessary to compensate for the loss of muscular control and strength used to maintain upper body stability.

METHODOLOGY--In normal body mechanics, the stability of the torso is achieved when the trunk musculature exerts a resisting torque to balance the moment created by external forces (gravitational and inertial). The maximum torque that an individual needs to apply to maintain balance can be estimated by the test subject leaning as far as possible without falling. The horizontal displacement of the torso center of gravity (cg) at this position is defined as the stability limit. During dynamic vehicle maneuvers, the inertial acceleration generates a force to act perpendicular to the gravitational vector at the cg. The maximum tolerable inertial acceleration without loss of balance can be estimated from the stability limit and the height of the torso cg.

PROGRESS--Testing was performed with four subjects with paraplegia (T2-3 to T9) and four with quadriplegia (C5-6 to C7). The height of the torso cg of each subject was found by measuring the displacement of the cg at two tilt angles while the subject was well supported upright in the wheelchair. The forward and lateral stability limit was measured and the magnitude of the inertial acceleration to cause instability was predicted.

  To compare the predicted stability to the actual, the dynamic stability of each subject was also measured using a computer-controlled tilt-table, The pitch and roll of the table simulated conservative (0.2 g) and aggressive (0.4 g) driving maneuvers, reproducing previous on-road tests. The subjects were seated in a conventional manual wheelchair with a foam cushion on a platform seat with a vinyl sling back. A floor-mounted lap belt was used for safety. The wheelchair was rigidly mounted to four load cells to record the cg data throughout the test. Each subject was asked to maintain upright posture, look straight ahead, and keep his/her arms crossed to assure no assistance for stability. Release of arms from this posture was an indication of lost stability.

PRELIMINARY RESULTS--The stability limits for the subjects with paraplegia ranged from 2.39 to 7.32 cm. laterally, and 2.59 to 14.10 cm forward. From this data the predicted dynamic stability was calculated to be greater than 0.2 g for three subjects during turning maneuvers and for two subjects during braking. In each of these cases, the subjects maintained stability during conservative driving maneuvers. In each of the other driving conditions, stability was not maintained. The stability was predicted correctly in each of the eight test conditions.

  For subjects with quadriplegia, the lateral stability limits varied from 3.66 to 4.78 cm, and the forward stability limits ranged from 2.36 to 5.44 cm. These measurements predicted the loss of stability below 0.2 g. Two of these individuals, however, were able to resist inertial forces during the 0.2 g turns.

  The statically measured stability limit was found to be a more accurate predictor of dynamic stability than was the level of disability. This was particularly evident with one of the subjects who is a low level paraplegic, but had the smallest stability limit and was unable to maintain stability during any of the simulated maneuvers. Another subject with a static forward stability limit more than twice of any other subject was the only one who could maintain stability during aggressive braking.

FUTURE PLANS--These results indicated accurate static evaluations of the level of stability experienced while traveling in a wheelchair. Further simplification of the evaluation process will be sought to eliminate the need for complex equipment such as the tilt table and load cells. The promising concepts of the maintained maximum angular torso displacement, and the trunk muscle generated horizontal force at chest level will be investigated.



Jonathan M. Owens; Beth A. Todd, PhD
The University of Alabama, Tuscaloosa, AL 35487-0276; email: jowens@eng.ua.edu; btodd@coe.eng.ua.edu

Sponsor: The University of Alabama, College of Engineering, Tuscaloosa, AL 35487

PURPOSE--Computer modeling can be used as a design tool for various types of assistive technology, including wheelchair seating systems. The long-term goal of this project is to improve the model of the material properties used in seating design. A finite element (FE) model will be generated using data collected for both soft biological tissues and cushioning materials. The ultimate goal is to develop a computer-aided design tool for creating seating systems that will reduce the risk of decubitus ulcers.

METHODOLOGY--In the initial phase of the work, a hemispherical axi-symmetric model of a portion of a single buttock was developed. For initial model development, linear elastic material properties were used with an elastic modulus of 15 kPa and Poisson's ratio of 0.49.

  The model includes the soft tissues under and around the ischial tuberosity, with a rigid core representing the bony prominence, of a typical adult weighing 779 N with a hip breadth of 400 mm. It consists of a vertical cylinder, radius 25 mm, length 50 mm, with a hemispherical end (radius 100 mm). The outer hemisphere represents soft tissue and consists of 75 elements. The cylindrical core does not consist of any elements at all: rather, it is created by the use of displacement constraints placed along its interface with the soft tissue.

PROGRESS--For the purpose of initial development, several load cases were applied to the model, including hydrostatic loads over the outer surface of soft tissue, hydrostatic loads over the lower portion of the outer surface of soft tissue, and concentrated forces. The first load case approximated the external pressure created if the buttocks were submerged in 61 m of water. The second load case approximated sitting on a flat, rigid surface. These load cases were selected to facilitate comparison with published results in the literature.

PRELIMINARY RESULTS--The first load case resulted in the lowest stresses and deformation. The second load case created high stresses at the base of the rigid core, representing the ischial tuberosity. The concentrated force load case led to the highest local stresses and deformation, but the rest of the model remained in a low stress state.

  Results of these load cases compared favorably with the work of previous investigators. Thus, the initial model has been verified.

FUTURE PLANS--The next step in this project is to modify the FE formulation to include nonlinear characteristics in the soft tissue. This will be followed by either modifications to the loading schemes to simulate a more realistic seat interface or the actual inclusion of FEs to represent the seating surface.


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Last revised Fri 04/30/1999