The Walker Team

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					                                     The Walker Team

                                   Product Design Proposal

                                             February 9, 2006

                                           Course: BME 227L
                                         Instructor: Aura Gimm

   Ian Shakil                  Katie Sullivan              Mark Connell                Chris Schumann

We have neither given nor received improper assistance in the completion of this assignment. I understand that a
violation of this assignment can result in failure of this course, and/or suspension from Duke University

Abstract ........................................................................................................................................... 2
Background ..................................................................................................................................... 2
Goal Statement & Functional Specifications.................................................................................. 5
Proposed Work................................................................................................................................ 6
Design Evaluation........................................................................................................................... 8
Conclusions & Next Steps ............................................................................................................ 10
Bibliography ................................................................................................................................. 11

        Assistive walking devices are used by individuals who suffer from decreased mobility.
However, only a few models exist to support a wide range of disabilities, from balance
difficulties to leg amputations. Our clients, Dr. Dan Erb and Dr. Kathy Shipp from the Physical
Therapy department at the Duke Medical Center, plan to redesign existing walkers in order to
better serve their osteoporosis patients. Before doing so, they require data from the patient-
handle interaction from existing walkers. The clients have requested a device that can measure
the force that a user exerts on the left and right handle-grips of a walker or rollator during normal
use. These force measurements will be measured in the Krzyzewski Human Performance
Research Laboratory (K-Lab) over a 1-minute testing period. The resulting data will be used to
prescribe walkers and rollators that minimize excessive loading in the user’s upper extremities.
Ultimately, it is hoped that the data will be used to design superior walkers that maximize
comfort, aesthetics, and load distribution.

This device must meet the following criteria:
   • Measure the force exerted by each hand (right and left) onto the hand grips of a
       traditional and rollator walker
   • Not alter the user’s gait pattern significantly by adding height to the grips or weight to the
   • Output data that is easily synchronized with the K-Lab motion analysis system
   • Be portable and adaptable to multiple walkers
   • Be Easily set up and calibrated
   • Be easily cleaned and very durable

       The team has considered a host of design proposals ranging from floor-based motion
sensor mats to instrumented walkers, and it was decided that a “portable sleeve – cantilever
handle” (PSCH) device is the most feasible and cost effective design that satisfactorily addresses
the needs of our clients.

       Osteoporosis, which translates into “porous bones,” is a disease in which bones lose
calcium structure and become more likely to break. Over 200 million people worldwide are

affected, 80% of whom are women, and millions more are at risk.1 While onset is most common
after age 50, osteoporosis can develop at any age. Throughout a lifetime, old bone is removed
from the skeleton in a process called resorption, and new bone is added in a process called
formation. Until around age 30, bone formation exceeds resorption. After peak bone mass is
reached, resorption occurs at a more rapid rate than formation. In a subset of people, this process
continues until the bone reaches a dangerously low mass (Figure 1). If left untreated, the
structural deterioration of osteoporosis leads to bone brittleness and an accompanying increase in
fractures, particularly in the hip, spine, and wrist.2 These fractures are serious; hip fractures
frequently involve hospitalization and surgery, and spinal fractures can result in height loss, back
pain, and deformity. Both can cause permanent disabilities and impair a person’s ability to walk
unassisted for the rest of his or her life.

                      Figure 1. A normal bone (right) and an osteoporotic bone (left).
                      The osteoporotic bone has lost a significant amount of structure,
                         which leads to frailty and a higher likelihood of fractures3.

        Patients who experience decreased mobility as an effect of osteoporosis are frequently
prescribed an assistive walking device. Assistive walkers and rollators are also prescribed to
patients suffering from post-surgery trauma, balance problems, and general frailty. These devices
effectively allow the user’s upper extremities to provide added support and stability for the
patient. The type of walker prescribed is patient specific and depends on the extent of required
balance and weight-bearing assistance. Canes provide the least support, followed by crutches
and walkers. Assistive walkers are intended to increase mobility, prevent falls and fractures, and
encourage independence. The two most common walkers are the standard aluminum walker and
the rolling walker, or rollator (Figure 2). The standard walkers are constructed from a tubular
aluminum frame, plastic handgrips, and rubber-tipped, height adjustable legs. At about seven
pounds, they are relatively lightweight, however the patient must have the upper body strength to
lift the device and move it ahead after each step. Additional drawbacks include a slow and
awkward gait and the possibility of a curved, kyphotic spine. Use of this walker is usually
limited to indoor, single storied buildings.

         The rollator is better suited to patients who lack the upper body strength and balance to
lift the walker with each step. It is composed of metal tubing and has wheels on the legs to
promote movement. Rollators often include a built-in seat in which the user can rest, a waist
high safety bar to prevent falls, and a front basket for convenience. Patients with unsteady arms
and hands may find the rollator unstable as the wheels react to any applied forces and could
move in an unintended direction. In addition, the device is cumbersome and awkward, does not

allow the user reach forward, and cannot easily be maneuvered upstairs. Again, it is primarily
designed for indoor use.

            Figure 2. Existing assistive walking devices. Conventional aluminum walker (left) and
            rollator (right). While both devices provide stability, they have drawbacks as well and
                    neither was specifically designed with the osteoporotic patient in mind4.

        We are working with two doctors from the Physical Therapy Division of the Duke
Medical Center who plan to design a new assistive walking device for osteoporosis patients to
confront the drawbacks of the standard and rolling walkers. Dr. Dan Erb is an associate clinical
professor in the Department of Community and Family Medicine. His past research has focused
on the functional outcomes of physical therapy interventions in both the elderly and
neurologically impaired patients. Dr. Kathy Shipp is an assistant professor at the Duke Center
for Aging. She has researched osteoporosis and the resulting vertebral fractures, spine
deformity, and rehabilitation. Before designing a novel walker, the clients have requested
quantification of the various forces applied by the user’s upper extremities on the walker, both
compressive (down) and sheer (forward and back). These measurements will determine whether
the existing walker configurations cause harm to the joints, as well as to ensure that their future
design will operate properly under these forces. The force sensors must not alter the patients’
normal pattern of ambulation through additional weight or height. The clients also stressed
portability and adaptability of the force sensor to a number of existing devices.

        Previously, instrumented walkers have been reported in scientific literature. These
devices use strain gauges to measure forces from the users’ upper extremities. In each instance,
the walker has been permanently altered; none of the devices offer the portability requested by
our clients. The group of Bachschmidt, et al, developed a strain gauge-based instrumentation
system for six load measurements—three dimensions of forces and three moments.5 Twelve
strain gauge bridges were placed within the handle. Simpson’s rule was used to compute stresses
and strains through the cross-section, and Gaussian integration allowed for the calculation of
moments and axial forces.

       The group of Pardo, et al, also measured walker reaction forces using strain gauges
placed on the legs of a standard walker.6 The forces from each of the four legs were summed to

yield the total walker ground reaction forces. Walker stability was assessed using the calculated
horizontal position of the center-of-pressure. The results were also used to identify upper
extremity strength requirements. The group of Fast, et al, placed their strain gauges on the
walker legs as well, at 15 inches above ground.7 Axial and bending forces on the aluminum
frame were measured in both frontal and sagittal axes. The interface box was not attached to the
walker in order to avoid extra weight; instead, the examiner carried it during each trial. Software
and calibration data were used to convert the millivolt output into pounds.

         G. Simoneau of Marquette University recognized that walkers require weight bearing of
the user’s upper extremities and adjustments in the kinematic pattern of both arms and legs.8 His
instrumented standard walker, or dynamometer, measured the six loads on the handle using 24
double-pattern resistive foil strain gauges. A customized frame allowed static calibration, and
cross-talk was corrected through data processing. The dynamometer trials were synchronized
with a six camera Vicon motion analysis system. The results indicated significant joint moments
at the shoulders, elbows, and wrists. Another group, Ming, et al, developed a dynamometer
using twelve strain gauge bridges positioned around the walker according to Finite Element
Analysis.9 The strain gauges were powered with a 12 volt DC source, and the system was
statically calibrated using a multiaxis frame.

         Commercially available instrumented walkers are manufactured and sold by Advanced
Mechanical Technology, Inc. (AMTI), a company in Massachusetts.10 Their devices examine
the forces and moments involved with walkers, canes, and crutches, with such applications as
medical research, orthopedics, and rehabilitation evaluation. Specific uses include gait and
stability analysis. The instrumented walkers are available in three styles: pediatric anterior,
pediatric posterior, and adult anterior. Strain gauge load cells output six measurements: three
orthogonal forces and three moment components along the X, Y, and Z axes. The sensors have
high stiffness and sensitivity, low cross-talk, excellent repeatability, and stability over many

Goal Statement & Functional Specifications
        The goal of this design project is to create a portable device that measures the forces
applied to each of the grips of a walker or rollator. There are many functional specifications
indicated by the client. First, the device must measure the compressive force applied to each
walker handle over the course of a one-minute in-lab test. Ideally, all three dimensions of forces
and all three moments would be measured. However, the compressive force is of the highest
interest. Second, the device should be portable. The clients would like to be able to attach the
device to any walker that a patient brings in order to characterize how that patient uses their own
walker. Therefore, the device should not damage the walker, and should be easily attached and
removed. The device should not alter the gait of the patient using the walker. Their use of the
walker should remain as usual. The data from the device should be able to be synchronized with

the three dimensional motion analysis system found at the K-Lab. 1 A simple calibration scheme
should be used, and the device should be durable and easily cleaned.

Proposed Work
        Ultimately, the portable sleeve cantilever handle (PSCH), as shown in Figure 3, was
selected as the proposed design. This design features a hand plate attached to a load cell to
determine the force applied by the user. First, the load cell will be mounted in front of the
current hand grip location. The mounting technique will differ depending on the load cell
selected for the device. Attached through the threaded load cell will be a rigid support that links
the cantilever handle to the mounted cell. This will create a large moment on the load cell but
should not affect the performance of the device. A rigid support between the two key
components is essential to the efficacy of this design, and different materials will be studied to
optimize the performance.

             Figure 3. Cantilever Handle Design Sketch and Potential Mounting Mechanism. The top
            figure describes the cantilevered hand support, while the right figure illustrates the potential
                                        bracket and platform mounting system.

        Four other alternative designs were examined. The first design was implementation of a
force pad. This would allow for the measurement of three dimensions of forces and moments at
a single point on the pad, and would not modify the gait of the user. This design has a few
drawbacks. First, the force pad measurements are only valid if there is one contact point; with a
walker or rollator and a user, there are six contact points. A net compressive force could be
determined, but much information would be lost. The size of a force plate would limit its
usefulness. The largest plate produced by AMTI is 1.2m x 1.2m, which would not be large
enough for a user walking with a walker for one minute. Also, while net vertical forces on the
walker would be measured, the client is most interested in upper extremity forces exerted
specifically onto the hand-grips.

 According to Dr. Queen, Coordinator of the Human Performance Research Laboratory (K-Lab), it is only
necessary for the proposed walker device to output a small analog voltage difference. The K-Lab equipment is
capable of providing further signal conditioning and amplification.

        Walker leg boots were also considered. A boot would be placed on each walker leg and
would be able to capture the compressive force applied to the walker by a user. These boots sre
composed of a cylindrical walker leg casing with a load cell at the bottom of the boot,
surrounded by a rubber sole. It would slip on to a walker leg as shown in Figure 4. The boot
would be secured by a rubber ring near the top. The leg boots would add no more than 0.75’’ to
the length of each leg; the extra height could be compensated for by adjusting the leg length.
This would have some effect on the user’s gait. The wires from each load cell would be attached
to the corresponding leg and then bundled and strung from the back of the walker.

                       Figure 4. Walker Leg Boots Sketch. The boot is tubing that fits
                       around each walker leg, adding a short length and measuring the
                                    ground-walker leg force interaction.

         An instrumented walker design would effectively characterize the user’s employment of
the walker. Such a design would replicate the instrumented walker created by Bachschmidt et al,
as illustrated in Figure 5. The handles would first be removed from the walkers, and twelve strain
gauges would then be carefully applied to a pipe or tubing similar to a walker bar. Next, the two
instrumented grips would be mounted in the laboratory walkers. This method offers limited
portability between reconfigured walkers. It necessitates precise engineering and calibration, but
would allow for the measurement of the six moments and forces of interest.

            Figure 5. The Instrumented Walker Handle. The handle is outfitted with twelve strain
            gauges in various orienations to capture the three forces and three moments applied to
                                                 the walker .

        The portable embedded model, shown in Figure 6, would achieve the high degree of
portability desired by the clients. An array of thin-film load cells would contact both the hand
plate and walker bar, as shown. A potential array configuration (Figure 6) would be two by two;
however, this array size could be optimized during design modification. All forces applied by
the user upon the handle plate would then be applied to the walker through the thin-film load
cells, and the net force in two dimensions could be determined computationally. This design
would not be excessively bulky and would only require two clamps at each end of the device for
stabilization on the walker. By understanding the geometry of the walker grip (ideally a
cylinder) and the compression forces, the four points could be summed to create a net force
vector (vertical and lateral axes).

       Figure 6. Portable Sleeve - Embedded Handle sketch. The sketches illusrtate how the hand plate
      would rest on the walker handle, at 4 contact points with with compressive load cells located at each
                                              of the contact points.

Design Evaluation
        In order to choose between the different designs, a decision matrix was constructed using
five different criteria, each of which was assigned a weighing factor based on their relative
significance. The most important design criterion was feasibility—a measure of how likely
design could be realistically completed given the current restraints (time, available technology
etc.). Feasibility was assigned a particularly high weighing factor (25 out of 100).

        The second category that was used to compare the different designs was whether or not
the device would significantly affect how a person normally moves with the walker—a primary
client concern. Thus, because of the importance of this requirement, it was also assigned a high
weighing factor (25 out of 100).

       The third design criterion was cost, as it is imperative that the expenses necessary to
construct the design within the established budget. This category was assigned a relatively high
weighing factor (20 out of 100).

        The next category used to judge the designs was portability. It is extremely important for
the client to be able to transfer the device from one walker to another. This was not one of the
essential requirements of the client, and as a result, it was a given a slightly lower weighing
factor than the previous categories (15 out of 100).

        The final criterion was performance, or how well the design would be able to measure the
desired forces. This includes the ability of the device to measure force in more than one spatial
direction, as well as the ability of the device to distinguish between the different forces with
mimimal crosstalk. While considered a very important criterion, the performance of the device
was assigned a slightly lower weighing factor than previous categories (15 out of 100).

       Using these five categories, the following decision matrix was constructed on a 1-to-10
scale. As can be seen in Table 1, the force pad design scored a relatively low 655 points (out of
                             Table 1. Decision Matrix for Evaluating Designs
                               Feasibility Does Not           Cost Portability   Performance   Total
                                             Alter Gait                                        Score
Weighing Factor                    25              25          20         15         15
Force Pad                           5              10           2         10         6           655
Walker Leg Sensors                  6               8           5          7          6          645
Instrumented Walker Frame           5               6           5          0         10          525
Portable Sleeve-Embedded            7               8           9         10          7          810
Portable Sleeve-Cantilever          9              8          9        10            9           890
Handle (PSCH)

        The primary strength of this design was that it had little effect on how a person moves
with the walker, since no changes would be made to the walker itself. Also, in terms of
“portability,” it would be ideal, since any walker could be used with the force pad. However, the
design received mediocre scores in performance (since it would be difficult to record forces other
than compression) and feasibility (since force pads tend to be much smaller than what would be
desirable for motion with a walker). But the primary detriment to using a force pad was the cost,
as force pads are simply too expensive to be seriously considered as a possible design.

        The walker leg sensor designed also received a fairly low score (645). This design had
fairly mediocre scores across the board, as it did not stand out in any category. While the design
is feasible, it is somewhat less practical than the other designs just because it would require four
devices (one for each leg) to be built while the handle grip designs require only two. The leg
sensors received a relatively high score in the “does not alter gait” category, since as long as the
sensors do not add too much weight to the walker, it is unlikely that the devices will have much
of an effect on how someone moves with the walker. The mediocre scores that the leg sensor
design received in cost and portability can again be attributed to the fact that four separate
devices for force are required, making this design more expensive and more difficult to transport
from walker to walker. Finally, the design received a relatively low score in performance simply
because it is unlikely that any forces will be able to be measured with this device other than
compression. It should be noted that the walker leg sensors could not function with wheel-based

       The design involving instrumenting the frame of the walker received the lowest score
(525). This design received a perfect score in performance, since it is possible to position strain
gauges throughout the walker’s frame to measure force in all directions. However, in the other
                                           - 10 -

areas, this design scored mediocre to very low. Instrumenting the frame of the walker would be
a difficult process requiring very precise engineering, thus resulting in a fairly low feasibility
score. Also, all of the changes to the walker frame could have an impact on how a person
normally moves with the walker, and as a result, a mediocre score was given in the “Does Not
Alter Gait” category. Furthermore, according to AMTI research, instrumenting a walker would
be expensive, likely exceeding the budget limitations. Finally, this design scored a zero in terms
of portability, as the changes made to the instrumented walker would not be transferable to other

        The design involving a portable sleeve with the embedded sensor received a total score of
810. This device scored well in almost all of the categories. It received its lowest scores in
feasibility (due to the difficulty of embedding very small sensors into the hand grips), and
performance (since the embedded sensors would only be able to measure compression). The
design scored well in “Does Not Alter Gait,” as it is unlikely that a slight change to the current
handle grips will affect a user’s motion with a walker. Furthermore, the device scored an almost
perfect score in cost, as small sensors have been found that are very inexpensive. Finally, the
design received a perfect score in portability, as the device will be able to be easily attached and
detached from any walker.

         The final design, the portable sleeve with the cantilever handle (PSCH), scored the
highest, receiving an 890 out of a maximum of 1000. The device received a high score in every
category. First, the design seems very feasible, as placing the sleeve adjacent to the handle grips
removes the size restrictions on the force sensors that made the embedded sensor design
somewhat impractical. The device also scored high in the “Does Not Alter Gait” category, as
though the design may result in the hand grip being slightly higher than in a conventional walker,
it is unlikely that such a small change will have any affect on how a person normally moves with
the walker. Furthermore, from the research conducted, it appears that the design can be
completed at a relatively low cost. However, the cost is dependent on the quality of the force
sensor, which in this design, could range from a simple strain gauge to a complex three-
dimensional load cell. The device thus scored very high in performance, as it would not be
difficult (though it may be too expensive) to implement a three-dimensional load cell into this
design, and thus be able to measure all of the desired forces. Finally, the design received a
perfect score in portability, as it will be able to be easily transported from one walker to another.

        The portable sleeve with the cantilever handle is the design of choice. This design meets
all of the necessary criteria: feasibility, affordability, portability, minimal gait affect, and
effective performance capability. While the portable sleeve with the embedded sensors also
scored highly and will likely be an excellent second option if the primary design fails, the
portable sleeve with the cantilever handle is the best design for accomplishing the client’s

Conclusions & Next Steps
      The client requires a device that can measure the force exerted on the left and right grips
of a walker/rollator over a 1-minute time period within a clinical setting. This device must
                                                    - 11 -

synchronize with the K-Lab motion analysis system, be portable, cleanable, durable, and easily
calibrated. It is also imperative that the device does not affect the normal gait of the test subject.

        The team has decided that the “portable sleeve – cantilever handle” (PSCH) device
(Figure 3) is the most feasible and cost effective design that satisfactorily addresses the needs of
our client. The PSCH is portable, universally compatible with all walkers/rollators, minimally
invasive, durable, and cleanable.

        The PSCH design makes use of a miniaturized load cell rigidly attached to a cantilever-
styled handle grip, which overlays onto the existing walker grip. The team must now begin an
exhaustive search to identify a cost-effective load cell that provides an appropriate force-
measurement range (0-80 lb)12 while withstanding large moments (~40 ft-lbs) and moderate heat

       The team must also begin designing, machining, and testing the rigid load-cell/grip
connection, clamping mechanism, encasement, calibration system, and ergonomic cantilever
grip. The first step in the process is to embark on a thorough materials selection search.


    Reginster, JY and Burlet, N. “Osteoporosis: A still increasing prevalence.” Bone, 2006 Jan 30;38:4-9.
 National Institutes of Health. Osteoporosis and Related Bone Diseases National Resource Center.
 Osteoporosis and Osteoporosis caused by Parathyroid Disease. Norman Endocrine Surgery Clinic, Tampa Bay,
FL., 2006.
    Your Health Fitness Store. Total Body Works, Weatherford, TX.
 Bachschmidt, RA, et al. “Walker-Assisted Gait in Rehabilitation: A Study of Biomechanics and Instrumentation.”
IEEE Transactions on Neural Systems and Rehabilitation Engineering. 2001 March; 9(1): 96-105.

 Pardo, RD, et al. “Walker User Risk Index.” American Journal of Physical Medicine and Rehabilitation. 1993
Oct; 72(5): 301-5.
 Fast, Avital, et al. “The Instrumented Walker: Usage Patterns and Forces.” Archives of Physical Medicine and
Rehabilitation. 1995 May; 76: 484 – 491.

  Simoneau G, Hambrook G, Bachschmidt R, Harris G. “Quantifying upper extremity efforts when using a walking
frame.” Pediatric Gait:A new millenium in clinical care and motion analysis technology, Harris GF and Smith PA
(editors). IEEE Press, Piscataway, NJ, 2000, p. 210-216.

 Ming, Dong, et al. “A New Dynamometer Walker System for the Measurement of Handle Reaction Vector.”
Measurement Science and Technology. 16(2005) 1272-1280.
  MCW Walker Sensors Brochure. Advanced Mechanical Technology, Inc., Watertown, MA.,
                                                 - 12 -

  Bachschmidt, RA, et al. “Walker-Assisted Gait in Rehabilitation: A Study of Biomechanics and Instrumentation.”
IEEE Transactions on Neural Systems and Rehabilitation Engineering. 2001 March; 9(1): 96-105.
  The client expects that, during peak loads, the walker-user will place a maximum of 80% bodyweight onto the
handle grips. 80% of a larger 200lb patient would yield 160lb. Thus, each hand grip must be able to endure 80 lb
(1/2 of 80% * 200).