RESEARCH
DOCUMENTATION
* Note: Tables and graphs will be added at a future
date.
PERFORMANCE VALIDATION
OF A
POWERED WHEELCHAIR MOBILITY SIMULATOR
by
Mark R. Schmeler, M.S., OTR/L
August 19, 1993
A thesis submitted to the
Faculty of the Graduate School of the
State University of New York at Buffalo
in partial fulfillment of the requirements for the degree of
Master of Science
Department of Occupational Therapy
ABSTRACT
A Powered Wheelchair Mobility Simulator was developed as an assessment
and training tool for powered mobility readiness in individuals
with severe physical impairments as well as cognitive and/or perceptual
deficits by an assistive technology team at the Center for Assistive
Technology at the University at Buffalo. This
device is a powered platform to which an individual's manual wheelchair
and seating system can be mounted enabling the experience of actual
powered mobility. A performance validation study was conducted
on the prototype of this device to determine its viability as
a simulator of actual powered wheelchair movements and control.
Performance of the device was compared to that of an actual powered
wheelchair by having experienced powered wheelchair users drive
both devices through a course that consisted of several tasks
with performance measures. Results indicated substantial statistical
and clinical similarity between the two methods of powered mobility
substantiating that the simulator performs as an actual powered
wheelchair. The advantages and limitations of using powered wheelchair
simulation are discussed and areas of future research identified
INTRODUCTION
Powered wheelchairs provide means for independent mobility to
individuals with disabilities who do not possess the physical
capacity to propel a manual wheelchair. The psycho-social aspects
of independent mobility have an impact on the development and
maintenance of intellect, socialization, and self-initiated behaviors
(Butler, 1984 & Paulsson & Christoffersen, 1984). A person's
ability to explore and move around under their own volition is
the foundation for the development of a lifestyle of independent
and self-initiated behaviors. The degree of mobility a person
has is directly related to their level of independence (Warren,
1990).
The prescription and provision of a powered mobility device involves
a comprehensive evaluation of the individual's needs as well as
a significant working knowledge of available equipment. Recent
advances in technology have not only improved upon the design
and performance of powered mobility devices but have also provided
the consumer with a variety of products to suit many disability
and functional needs. The selection and provision of the most
appropriate powered mobility system has become a difficult task
due to this large and constantly changing market. At the same
time, there is no formal published system for the evaluation of
these devices to assist clinicians with the prescription of the
most appropriate device for their client (McFarland, 1990). One
must rely on either past experience, the advice of others, persuasiveness
of advertisements, or manufacturers' reports to select an ideal
powered wheelchair.
The provision of powered mobility to individuals with severe physical
disabilities as well as cognitive and/or perceptual deficits is
a common practice. Yet it is often controversial, as some professionals
do not feel these individuals possess the skills or insight to
operate or maintain a powered wheelchair in a responsible manner.
This results in their being placed in a manual chair where they
are dependent on others for mobility needs. A standardized objective
protocol for the assessment and/or training of a person's ability
to safely and effectively use a powered mobility device is lacking.
The provision or denial of a powered mobility device to an individual
is dependent on many factors, none of which are based on an objective
assessment of prerequisite abilities and skills needed to successfully
use the device, nor do they focus on the person's ability to learn
them (Verburg, 1987). The methods traditionally used to gather
information regarding an individual's readiness to use powered
mobility vary significantly depending on the resources available
to a therapist. These methods range from evaluating the candidate's
ability to perform related activities, such as controlling a joystick
to play a video game, to a brief road test in a borrowed powered
wheelchair.
These methods are not without limitations. Related activities,
such as a video game, are not ideal as there is no indication
that an individual's skill will generalize from one activity to
another. The use of a borrowed powered wheelchair for a road test
is not ideal either as it requires that an individual be removed
from his/her specialized seating environment. Removing the individual
may result in a lack of postural support necessary to effectively
operate the controls. At the same time, a borrowed device is often
only available for a brief period of time which may be insufficient
to assess a candidate's ability to learn the necessary skills
to operate the device.
The cost of a powered mobility device, with specialized seating
system and switch controls included, can run as much as ten thousand
dollars. For this reason, third party reimbursement is often an
issue. In order to receive approval for the provision of such
an expensive device, one must demonstrate not only an absolute
need for the device, but also provide objective information indicating
that it is the most appropriate device available, and prove that
the candidate is able to use it in a safe and effective manner.
This is an arduous task on the part of therapists and other team
members as there is no standardized means of providing this objective
information.
There is little empirical activity and few published studies in
the research literature related to powered mobility. The majority
of the published research is descriptive in nature and found mostly
in conference proceedings. This does not mean there are no issues
that need to be investigated or resolved in the process of assessment,
training, and provision of powered mobility. There is a lack of
empirical data that indicates what effects powered mobility has
on users and their caregivers, how the assessment and prescription
process can be improved, and how the training process of new users
can be enhanced (Verburg, 1987).
Case studies have described the process of training and providing
powered mobility to individuals with mental retardation and developmental
disabilities. One study (Chase & Bailey, 1990) described the
five year process as a progression of activities related to access
and mastery of a controller, remedial cause and effect training,
simulated activities, and trials in a borrowed wheelchair. The
length of the process was attributed to difficulty in finding
the most appropriate switch controller, lack of an appropriate
device to simulate powered mobility effectively, and difficulty
in carry-over of powered mobility skills from simulated ones.
A second study (Treffler & Taylor, 1987) described similar
processes and added that the provision of powered mobility to
two young boys has facilitated their ability to explore their
environments, interact with their peers, and develop a higher
sense of control of their lives.
Methods of providing candidates with simulated activities have
been described mostly through the use of computer based activities
(Lefkowicz & Wierville, 1992, Pronk et al., 1980, Snell et
al., 1989, & Verburg et al., 1990). These pertain mainly to
moving through a progression of simulated tasks on a computer
screen through the use of a joystick or other type of control
system. This method has substantial benefit for teaching remedial
cause and effect activities as well as fine motor mastery in the
use of a control system. However, for some individuals with cognitive
impairments, these simulated tasks are too far removed from the
experience of actual wheelchair motions. This in turn prompted
the concept of simulation that provides actual wheelchair movements
(Bressler, 1990).
The Powered Wheelchair Mobility Simulator (PWMS) is a prototype
of a device that was developed at the University at Buffalo as
an assessment and training tool to determine powered wheelchair
mobility readiness in individuals with physical, cognitive, and/or
perceptual disabilities who have a manual wheelchair with specialized
seating which they are unable to propel (Schmeler et al., 1993).
Based on the original work of Bressler (1990), the device is a
powered platform to which an individuals manual wheelchair can
be mounted, allowing the unique experience of driving a powered
wheelchair while remaining in a familiar seating environment.
The device also has the advantage of being available to clinicians
and candidates on a continuous basis for long-term training.
The PWMS was fabricated by cutting the chassis of an existing
Invacare Arrow XT power wheelchair base and attaching it with
a platform and caster system. The electronic component of the
Invacare Arrow XT is controlled with a computer based system which
allows for electronic compatibility with virtually any type of
switch control system. This system also provides for various modifications
to driving performance including variations in acceleration, braking,
speed, and turning. An attendant control allows the therapist
to interrupt the user's controls in hazardous situations.
The PWMS was developed specifically as a comprehensive tool to
assess a candidate's ability to drive a powered wheelchair while
they remain in their own seating system and at the same time,
provide the opportunity to try a variety of switch controls. In
order for the PWMS to be considered a viable means of providing
an accurate prediction of powered mobility readiness, it was necessary
to demonstrate that the PWMS was simulating the performance of
an actual powered wheelchair.
The PWMS, compared to an actual powered wheelchair, was designed
such that the drive system was located at the rear of the device
and behind the user. This change in position was necessary to
provide an open space on the platform that would accommodate the
mounting of a manual wheelchair. Placement of the drive system
behind the user resulted in a significantly longer wheel-base
which could alter its driving performance. Standard powered wheelchairs
are also designed with two front casters, whereas the PWMS was
designed using a single caster which was necessary to provide
for a platform that would sit low to the ground and also leave
an open space for the wheels of the user's chair.
Collectively, the increase in wheel base length and the change
from two front casters to a single one, changes the performance
of the PWMS compared to an actual powered wheelchair. For this
reason, some rehabilitation professionals have felt the user is
not experiencing the full effects of powered mobility.
The purpose of this study was to show that the PWMS does
simulate the performance of a standard powered wheelchair. The
results were intended to indicate there is little or no statistical
or clinical difference between the driving performance of the
PWMS and an actual Invacare Arrow XT powered wheelchair. This
in turn would indicate that the PWMS is a viable tool for therapists
to use in the provision of powered mobility to their clients who
have a pre-existing seating system on a non-powered wheelchair
which they are unable to propel independently.
The hypothesis was tested by having subjects, who were experienced
powered wheelchair users, complete a course on two occasions,
once using the Invacare Arrow XT powered wheelchair and once with
a manual wheelchair mounted on the PWMS. The course, that consisted
of a variety of tasks and performance measures, was taped out
on the floor of a large room. Each subject also provided subjective
feedback regarding their assessment of the PWMS's driving performance
through the completion of a bipolar questionnaire.
METHODOLOGY
Subjects
Individuals considered to be experienced powered wheelchair
users were recruited for this study as it was felt they would
provide expert opinion on the performance of the PWMS. These individuals
would also decrease the likelihood of a learning effect across
trials. In a previous study experienced powered wheelchair users
indicated flat learning curves when evaluated using a powered
wheelchair with a new type of joystick as compared to non-powered
mobility users (Widman et al., 1993). Criteria for participation
required subjects to be between the age of 18 and 65, have at
least one year of experience using a powered wheelchair, use a
standard proportional joystick to control their wheelchair, and
have no medical or educational diagnosis that would impair their
ability to follow multiple step directions or navigate a powered
wheelchair safely.
Eight subjects who met the above criteria were selected and recruited
for the study through the solicitation of local service providers
and disability organizations. Subjects were contacted by mail
to determine interest in participating. All responded with an
intent to participate. Subjects were all male from the Western
New York area ranging in age from 22 to 50 (mean 35+
9.65) and with a range of 3 to 20 years of experience using powered
mobility (mean 11+ 7.07). Disabilities included: spinal
cord injury (3), cerebral palsy (1), traumatic brain injury (1),
multiple sclerosis (1), polio (1), and muscular dystrophy (1).
All subjects considered themselves to be active individuals with
gainful employment, families, and leisure interests. Subjects
were paid thirty five dollars for their participation as well
as any costs endured for transportation to the test site.
The Course
The dependent variable in this study was performance in navigating
the PWMS in comparison to the Arrow XT powered wheelchair (WC)
on a course that was outlined with black electrical tape on the
floor in a large room. The course consisted of three ninety degree
turns, two forty five degree turns, and five straight paths (see
Figure 3). This methodology was a replication of the methodology
used by Lefkowicz (1993) to study the validity of a computer based
powered wheelchair simulation program he developed.
Performance on the straight sections of the course was measured
as the average deviation from the center of the path taken by
the mobility device. Perpendicular measurements in inches were
taken every six inches along the length of the median of each
straight path to calculate the average deviation. A second performance
measure for straight paths included the number of times the median
line was crossed by the mobility device. A considerable amount
of median crossings would indicate a continuous need to overcorrect
the direction of the mobility device due to the inability to maintain
a straight path. A final measure of performance on the straight
sections of the course included the maximum deviation from the
median. This measure was important in assessing the safety of
the PWMS as a large deviation would constitute a sudden loss of
control which would be undesirable.
Performance measures on the turns were different from those on
the straight paths as turns are navigated in several ways depending
on user preference. Turns are negotiated by either spinning the
wheels in opposite directions or by locking one wheel and turning
the other forward. Therefore, performance measures on turns were
based on the amount of deviation from the median 18 inches from
the apex of the inside angle before and after each turn (see figure
3). This process was followed to chart the start and finish location
of the mobility device for each turn.
A final performance measure included the amount of time it took
to complete the course. This was measured in minutes.
In addition to the performance measures mentioned above, a questionnaire
was filled out by each of the participants upon completion of
all trials. This evaluation was intended to gather subjective
feedback regarding the realism and usefulness of the PWMS. This
further provided the subjects with an opportunity to express opinions
and make suggestions regarding further developments of the PWMS.
Responses to the questions were in the form of bipolar rating
scales with a rating of 7 being most favorable, 4 being neutral,
and 1 being not favorable.
Apparatus
The Invacare Arrow XT was chosen as the control chair for this
study as the PWMS was developed using the components of this model
and driving performances would vary across individual brands of
wheelchairs used by the subjects. For the purposes of this study,
the standard proportional joystick was used to control both devices.
The performance parameters were set identically on both computers
including; speed, acceleration, braking, and turning speed. The
seating on the Arrow XT was a standard sling back and seat with
adjustable arm and footrests. A standard Invacare Rolls manual
wheelchair of identical size and features was used on the PWMS.
Several different seat and back cushions were used by some of
the subjects to address comfort and positioning needs. Any positioning
equipment used or adjustments made to the chair in one trial were
arranged in the same manner for the other chair for the second
trial.
Procedures
The experiment was carried out at the university in a
large room where the course was mapped out on the floor. Prior
to participation in the study, an explanation of the study's procedures
was given and informed consent was obtained. Demographic and background
information was then obtained. Subjects then transferred into
either the Arrow XT wheelchair or manual wheelchair mounted on
the PWMS. Transfer method and level of assistance depended on
the subjects' needs and preferences. Provisions were arranged
in advance to have available equipment and qualified personnel
to provide transfer needs in a safe and effective manner.
To familiarize themselves with the course and to practice using
the mobility devices, subjects were instructed prior to the start
of each trial to make five to ten completions of the course. This
practice time was based on the previously mentioned research by
Widman et al. (1993) where they found that powered wheelchair
users required trials in this range to reach steady-state performance
using a new powered mobility device. At the start of each trial
subjects were verbally instructed to follow the path as quickly
yet as comfortably as possible while staying close to the center.
As the path was only twenty four inches wide, participants were
told to expect some deviation from the median however, they should
attempt to keep deviations to a minimum. They were also told that
turns could be negotiated in any manner but to remain consistent
across trials.
The course was completed on four occasions by all subjects; once
in both directions with the WC and PWMS. The order of trials alternated
between subjects. A trial consisted of two complete laps around
the course. As subjects navigated the course, a dry erase marker,
attached between the rear wheels of the device with burette clamps,
traced the path taken by the user. A different colored marker
was used for each trial and the floor was wiped clean after each
subject had completed the experiment and all measurements were
recorded.
Two research assistants, both undergraduate Occupational Therapy
students, were employed to assist in carrying out the research.
As subjects were completing trials, one research assistant was
timing the trial with a stopwatch, while the other was videotaping
the process. The amount of time subjects spent during the experiment
did not exceed two hours.
Measurements for each trial were then completed by the research
assistants where one took the measurements with an angled ruler
every six inches along the course while the other transcribed
the measurements on a form. Each measurement site was labeled
with a site number. The person transcribing also repeated each
measurement verbally into a microcassette recorder for back-up
purposes. Measurements were taken to the nearest quarter inch.
Raters alternated duties during the measurement process as a total
of 440 measurements were taken for each subject. A random selection
of 20 measurements was used to determine inter-rater reliability.
RESULTS
Separate performance scores were calculated for each
of the eight outcome measures. Outcome measures for the two wheelchair
conditions were analyzed using repeated measures analysis of variance
(ANOVA). A measure of shared variance (effect size) was also computed
to obtain an indication of the magnitude of difference between
the two wheelchair conditions. The effect size (d
-index) provided a standardized method of analyzing the impact
of the independent variable and was intended to supplement the
tests of statistical significance. Cohen (1988) considers an effect
size to be small if it falls in the range d - index
0.20 - 0.49; medium, d - index 0.50 - 0.80; and large,
d - index > 0.80. The performance outcomes of
the two wheelchair conditions would be considered equivalent with
a d - index < 0.50. Graphical presentation of
the results was also conducted for visual analysis and clinical
interpretation purposes. The subjective questionnaire was analyzed
using descriptive statistics.
The ANOVA for each performance measure indicated no statistically
significant difference between the driving performance of the
PWMS and WC (p > .05) with relatively small effect
size measures (d - index < 0.50) (see table 1).
This was especially true for average deviation from the median
on straight paths, maximum deviation from the median, number of
median crossings, and course completion time. Larger differences
(effect sizes) were noted for performance on the turns (d
- index > 0.50), especially before the forty five degree
turns and before and after the ninety degree turns.
Results of the subjective questionnaire were consistent with the
other analyses. All measures fell between the neutral and favorable
range (4.0 - 7.0). Subjects felt the PWMS was most similar to
the WC on straight paths (4.6 + 1.7) followed by performance
on the forty five degree turns (4.4 + 1.7) and ninety
degree turns (4.2 + 2.0). When asked to evaluate the
overall performance of the PWMS as compared to the WC the responses
were more favorable toward the PWMS (5.0 + 1.8). Subjects
also rated the PWMS as a predictor of powered mobility readiness
in the favorable range (5.6 + 1.4). Inter-rater reliability
was measured at 90% with eighteen out of the twenty measurements
being identical. The two differences were only by 0.25 in.
Participant/
Question 101 102* 104 105** 106 107 108 Average #1 Operation
on Straight 2.00 5.75 6.50 3.00 3.00 4.00 6.00 4.32
#4 Operation on Straight 5.00 7.00 3.25 2.00 6.00 4.00 6.00 4.75
#2 Operation on 45 Turns 2.00 3.00 5.75 2.00 4.00 6.00
5.00 3.96 #5 Operation on 45 Turns 1.00 6.00
3.75 2.00 5.00 5.00 5.00 3.96 #3 Operation on
90 Turns 2.00 2.00 5.75 6.00 2.00 6.00 4.00 3.96
#6 Operation on 90 Turns 1.00 6.00 3.75 6.00 1.00 5.00 4.00 3.82
#7 Ability to Predict 4.00 6.25 6.75 3.00 6.00 6.00 6.00
5.43 #8 Overall Simulation 1.00 5.00 5.75 4.00
5.50 6.00 5.00 4.61 Average 2.25 5.13 5.16 3.50 4.06 5.25
5.13 4.35 Scale: 1= not at all similar4= neutral7= very
similarCOMMENTS * The turning radius seemed
wider on the PWMS.* The acceleration on thePWMS seemed more jumpy
than the Arrow XT's.** The controller on the PWMS was very unresponsive
which made it very difficcult to learn how it turns
DISCUSSION
The results of this study confirm that the PWMS performs within
the realm of an actual powered wheelchair and could be considered
a viable tool for the assessment and training of powered mobility
readiness. By looking at the results descriptively one could see
a consistent difference in the performance results. This difference
was clinically insignificant when considering the unit of measurement
was in inches and individual variations on each performance measure
averaged to 0.61 in. with a difference of 0.26 in. of deviation
from the median on the straight paths to 1.29 in. difference in
position after the ninety degree turn. This seemed logical as
a device with a longer wheelbase would perform the same as a device
with a shorter one when driving on a straight path but would begin
to show increased differences when negotiating turns. The difference
would become even more noticeable as the turns became sharper.
Most importantly, the PWMS performed in a safe manner as there
was no major differences in maximum deviation from the median
or the amount of median crossings between the two conditions.
This indicates that all subjects were able to maintain constant
control of the device throughout the course.
Course completion time, although not significantly different,
was overall faster using the WC versus the PWMS with a mean difference
of 0.20 minutes (12 secs.). This was not attributable to differences
in power of the drive system as both devices were calibrated and
tested to have identical maximum speeds, turning speeds, acceleration,
and braking. The controllers on the devices were the same model
proportional joysticks providing subjects with complete control
of the devices. The consistent difference could be attributed
to the fact that driving the PWMS is a new experience, given the
longer wheelbase and sitting higher off the ground, and therefore
subjects tended to be more cautious when driving it.
Although subjects noted differences in the driving performance
of the PWMS, they felt the device was useful as a tool for the
assessment and training of powered mobility readiness. They felt
performance was most favorable on straight paths followed by forty
five and ninety degree turns. One subject felt the response time
of the PWMS was somewhat slower than the WC which could be attributed
the added weight load on the drive system of the PWMS as compared
to the WC. The next generation prototype of the device will consider
the use of aluminum in the fabrication process to decrease the
amount of load on the motors. One subject also felt that it was
slightly more difficult to master control of the PWMS as compared
to the WC. One might more easily master the use of an actual powered
wheelchair after driving the PWMS for a prolonged period of time.
Enough evidence was provided in this baseline study to justify
continued development and eventual marketability of the device.
Further study should focus on investigating the reliability of
the device as a predictor of powered mobility readiness in individuals
with cognitive and/or perceptual deficits. Additional work is
needed in quantifying and standardizing the assessment, training,
and reporting of a person's ability to use a powered wheelchair
in a safe and effective manner especially for justification and
reimbursement purposes.
CONCLUSION
A performance validation study was conducted on the prototype
of a device developed to simulate powered wheelchair performance
for the purposes of assessing and training for powered mobility
readiness. Performance of the device was compared against that
of an actual powered wheelchair by having experienced powered
wheelchair users drive both devices through a course that consisted
of several tasks with performance measures. Results indicated
there was no statistical or clinical difference between the driving
performance of the two devices, thus, substantiating performance
of the PWMS as a powered wheelchair mobility simulator. Further
development and marketing of the device will now be pursued. Additional
study in the development of the PWMS should focus on the role
of the device as a tool to predict powered mobility readiness.
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