MANUAL WHEELCHAIR RIDE COMFORT

Brad Lawrence, Rory Cooper, Rick Robertson, Mike Boninger, Jess Gonzalez, and David VanSickle Dept. of Rehab. Science and Technology, University of Pittsburgh, Pittsburgh, PA 15261 Human Engineering Research Laboratories, Highland Drive VA Medical Center, Pittsburgh, PA 15206

Abstract

Wheelchair ride comfort is a subjective assessment by the wheelchair user of his or her comfort within the wheelchair and environmental system. Static ride comfort involves discomfort as a result of prolonged sitting with minimal movement or prolonged postural deviation. Dynamic ride comfort includes the effects of acceleration and vibration. Due to the lack of wheelchair ride comfort studies, occupational posture, automobile ride comfort, and human vibration studies were investigated to provide various experimental techniques for a wheelchair ride comfort study. The results of this study will optimize rider comfort as well as lead to wheelchairs that are safer and more durable.

Background

The wheelchair user assesses comfort based on the presence of discomfort. Ride vibration, postural support, pressure distribution, ergonomics, and material breathibility are important parameters which affect the user's assessment of comfort. These parameters produce physiological changes in the user's body including circulation and nerve occlusion, ischemia, heat buildup, and visual and auditory interference. These changes result in short term human experiences of discomfort such as pain, annoyance, and displeasure. Additionally, these changes can cause long term damage and deformity such as tissue necrosis, nerve damage, and spinal deformity (1).

Static ride comfort deals with the user's response to prolonged sitting with minimal movement. For example, in an office setting, the user is primarily stationary for most of the work day. Static loads affect the user continually in each position she or he acquires. Posture and postural support are therefore extremely important factors when considering rider comfort. The spinal column supports the upper extremities and compresses under these loads. The loads are transmitted through the spine to the gluteus maximus where continuous pressure can lead to decubitus ulcers. In addition, these compressive loads induce moments, and these moments increase as postural deviation increases. These moments can cause spinal deformities. Seat supports can reduce and redistribute pressure as well as minimize postural deviation.

Dynamic wheelchair applications incorporate accelerations and cyclic loading compounding the existing static loads. For example, wheelchair motion as the result of user propulsion, or propulsion from external forces such as automobile transportation, are illustrations of dynamic situations. During an acceleration, the spinal column acts as a shock absorber, an energy absorber, and a transmission couple for vertical forces. Vibration, especially vibration that is near the first human resonance frequency of 5 Hz, can lead to spinal deformities, herniated discs, and chronic back pain over time. It is important to design wheelchair stiffness and damping characteristics to minimize vibration transmission.

Several studies have been performed examining rider comfort in static and/or dynamic situations. Unfortunately, there has been very little research done in the field of wheelchair rider comfort. Therefore, studies on occupational posture, automobile ride comfort, and human vibration will provide the basis for modeling wheelchair ride comfort experiments.

Genaidy and Karwowski (2) studied the effects of posture deviation on perceived joint discomfort ratings. Nineteen college students performed 24 body movements while seated and were asked to rank the associated discomfort with each movement. The results could be used to modify seats by adding supports that accommodate uncomfortable movements. [.1]Magnusson, et al. (3) studied the effect of seat back inclination on spinal height. Previous study showed that the disc pressure and the EMG activity of the erector spinae muscles decreased markedly when the backrest inclination was increased from 90 to 120 degrees. Further decrease of disc pressure and muscular activity occurred through the use of a lumbar support (4). In the Magnusson, et al. study twelve women were subjected to 5 minutes of sinusoidal vibration at a frequency of 5 Hz. The women had a 20 minute rest before each exposure. Each woman was seated in a 90, 110, and 120 degree posture and instrumented with a linear displacement voltage transducer. It was found the use of a 110 degree angle backrest caused less height loss than use of a 120 degree angle backrest in a static situation, but the opposite was found for a dynamic situation.

Zimmermann, et al. (5) studied the effects of posture on erector spinae EMG activity. 11 male college students were vibrated vertically at 4.5 Hz in three different unsupported postures: neutral upright, forward lean, and posterior lean. Subjects were vibrated for 2 minutes and EMG activity was collected. They concluded posture has a significant effect on the response of the erector muscles. Results showed that posterior lean resulted in decreased EMG and decreased vibration response, and reduction in vibration response yielded decreased disc compression.

Some research has examined the use and comparison of seat suspensions and rider comfort. Wilder, et al. (6) studied the effect of posture and seat suspension on discomfort. Six males were subjected to vibration in 2 different seats and three different postures simulating truck driving for 10 minutes. One seat was a standard spring seat; the other seat had a gas spring suspension. The three different postures were leaning forward, sitting upright, and seated back against the backrest. Three uniaxial accelerometers were used to measure the transmission of acceleration. One was placed on the baseplate, one was placed on the seatpan/driver interface, and one was placed on a bite bar. A transfer function gain value less than one signifies attenuation. Gain values greater than one indicate amplification which could lead to mechanical failure. Results revealed amplification in the 4-6 Hz range for all postures except the full-back posture. In addition to acceleration transfer, EMG, spinal height loss, subjective comfort (using a visual analogue scale) have been used to evaluate and quantify comfort. In the simulated truck driving study performed by Wilder et al., a visual analogue scale was used. It consisted of a continuous 10 cm long line that ranged from 'very comfortable' to 'very uncomfortable'. Subjects marked a point on the line they felt best described their level of comfort and this point corresponded to a numerical value. In the study, the full-back posture yielded the lowest (most comfortable) values, illustrating that subjective comfort may be correlated with an objective measure such as acceleration transmissibility.

Statement of Problem

Many different tools have been used to measure rider comfort, both objective and subjective, but none of these have been used to evaluate wheelchair ride comfort.

Ride comfort will be assessed using a test course. The proposed test course is designed to present a range of normal obstacles that impart road loads. These include curbs, door thresholds, ramps, and pavement joints. A random sample of subjects (minimum n=50) will be used in the test. Each subject will complete the course in three instrumented wheelchairs at three different speeds (0.5 m/s, 1.0 m/s, and 1.5 m/s), yielding at least 150 data sets per obstacle.

Subjective assessment by the user will be addressed by using a visual analogue scale during testing and an evaluation survey before and after testing. The visual analogue scale consists of a ratio scale 10 centimeters in length. Along the scale are numbers with corresponding levels of comfort (most comfortable-most uncomfortable). Ratings will be requested from the user when encountering an obstacle. The user will look at the scale and give a number that best represents his perception of comfort. These values will be correlated with the accelerations imparted to the wheelchair and rider.

Before and after testing, the users will participate in a short interview. The interview prior to testing will assess the user's perceived static comfort of the current wheelchair being used. The interview after testing will assess the wheelchair's performance in the test course.

Implications

By quantifying wheelchair ride comfort, users can benefit in short term and long term situations. Optimization based on subjective and objective results will produce more comfortable and more efficient wheelchairs.

Designs will be incorporated that optimize comfort by considering backrest inclination, frame stiffness and damping, cushion stiffness and damping, lumbar support, and ergonomics. Designs will decrease discomfort incidence by reducing vibration, pressure, and postural deviation, leading to reductions of pressure sores, nerve damage, and spinal deformity.

Discussion

Along with the visual analogue scale, the evaluation surveys before and after testing comprise the subjective tools that will be used in this study. Survey questions will address the wheelchairs' performance characteristics. The following list is a sample of questions:

How well is your back supported? How often do you have to shift your weight to maintain comfort in this chair? How bumpy is this wheelchair's ride? How easy is this wheelchair to propel?

Each question will contain a set of answers with five degrees and be numerically coded from 1 to 5 (i.e. not at all bumpy-1 to extremely bumpy-5). The results from these questions and from the visual analogue scale will comprise the perceived ride comfort correlation data.

Postural deviation, EMG, spinal height loss, and accelerometry are several objective tools which have been used in previous studies. EMG and spinal height loss would be difficult to implement with a user propelling a wheelchair, and therefore will not be considered for this study. Postural deviation will be measured in a static situation and correlated with users' perceived static comfort. Accelerometers will be placed on the wheelchair and an instrumented mouthpiece will be placed in the users' mouths. The recorded accelerations will be correlated with the users' perceived comfort during propulsion through the test course.

Based on previous studies the following parameters will be varied and compared during the 150 test course trials: frame stiffness and damping; and back inclination. Results will be correlated with comfort and acceleration data.

References

(1) Viano D. and Andrzejak D., "Research Issues on the Biomechanics of Seating Discomfort: An Overview with Focus on Issues of the Elderly and Low-Back Pain," SAE Transactions, # 920130, pp. 122-131, 1992.

(2) Genaidy A. and Waldemar K., "The Effects of Neutral Posture Deviations on Perceived Joint Discomfort Ratings in Sitting and Standing Postures," Ergonomics, Vol. 36, No. 7, pp. 785-792, 1993.

(3) Magnusson M., Hansson T., and Pope M., "The Effect of Seat Back Inclination on Spine Height Changes," Applied Ergonomics, Vol. 25, No. 5, pp. 294-298, 1994.

(4) Andersson G., Murphy R., Ortengren R., and Nachemson A., "The Influence of Backrest Inclination and Lumbar Support on Lumbar Lordosis," Spine, Vol. 4, pp. 52-58, 1979.

(5) Zimmermann C., Cook T., and Goel V., "Effects of Seated Posture on Erector Spinae EMG Activity During Whole Body Vibration," Ergonomics, Vol. 36, No. 6, pp. 667-675, 1993.

(6) Wilder D., Magnusson M., Fenwick J., and Pope M., "The Effect of Posture and Seat Suspension Design on Discomfort and Back Muscle Fatigue during Simulated Truck Driving," Applied Ergonomics, Vol. 25, No. 2, pp. 66-76, 1994.

Acknowledgments

This project was supported through grants from the US Department of Veterans Affairs, Rehabilitation Research and Development Service (B805-RA), the US Department of Education, the National Institute on Disability and Rehabilitation Research, Rehabilitation Engineering Research Center on Technology to Improve Wheelchair Mobility (HE133005), and from the Paralyzed Veterans of America.

Brad Lawrence Human Engineering Research Laboratories Veterans Affairs Medical Center 7180 Highland Drive, 151R-1 Pittsburgh, Pennsylvania 15206 [.1] WHEELCHAIR RIDE COMFORT