The document describes the development of an ankle dynamometer device to measure ankle strength. It discusses shortcomings of current devices, which are either too expensive, large, or inaccurate. The proposed design aims to be inexpensive, portable, and provide accurate measurements. Several prototype designs are presented. The first prototype included a wood base with knee brace and straps. The second prototype separated the design into a base with adjustable slide for the foot and a cupped back piece with blood pressure cuff. Force sensors were added to measure applied force. The goal is a device that physical therapists can easily use to track patient rehabilitation progress affordably.

1. The Ankle Dynamometer
Robert Loper
Jessica Sanders
Matthew Moseley
Seth Rhodes
April 29, 2016

2. Introduction
Ankle dynamometer is a device that measures the force of the muscle in the ankle to move the
foot in plantar flexion or dorsiflexion. Plantar flexion is the movement of the foot that increases
the angle between the superior surface of the foot and the shin. Dorsal flexion is the movement
of the foot that decreases the angle between the superior surface of the foot and the shin. This
device is typically used in rehabilitation facilities for looking at improvement in muscle strength
of a patient after an injury (Zhang, 2015). By measuring the force of the muscle, the user can tell
how strong the muscle is. By comparing this along the recovery period, one can look at the
physical numbers showing the improvement of the patient. Current models of the ankle
dynamometers can be found as large, stable devices known as isokinetic dynamometers that can
measure the muscle strength of not only the ankle, but other muscles as well. Another model is a
handheld dynamometer which is portable and also capable of being used in places besides the
ankle.
Each of these current models of ankle dynamometers have their own set of problems. The
isokinetic dynamometer model works correctly and gives accurate, desired results. The problem
is that it is a large machine that is also expensive. For starters, the machine cannot be moved
which causes a problem when dealing with injured patients. This requires moving someone with
a debilitation of some kind to the machine to perform the testing. This takes extra time and
discomfort. Another problem with this particular model is that it is also expensive. These models
are not as widespread due to “high cost, limiting its use to advanced medical centers, sports
studies and physical therapy” (Saldias, 2010). The other current model is the handheld
dynamometer. This model solves the problem of portability, but lacks accuracy. It is known that
a major limitation of the handheld dynamometer is “the reliance on the tester to hold the device
to accommodate the force being generated by the person being tested” (Carol, 2013).
The proposed solution is a device that will be inexpensive and portable, but will provide accurate
results that are not reliant on the tester. The idea is to have a device to secure the leg in place,
including the knee, ankle, and foot. The design will include four components. The first
component will include many individual cut outs that will be assembled into one piece. The
cutting of the individual pieces will be done via a computerized numerical control (CNC)
machine. The material of the design will be Starboard, which is a cleanable yet durable hard
plastic used in making boats. Once cut into individual pieces, the base will be secured into one
piece that will include a footpad, a back piece, and two hinges that are each attached to an arm
piece. Next there will be a knee brace attached to the arm pieces to prevent movement in the
patient’s knee. The design will also have a separate component situated between the two hinges
that consists of a blood pressure cuff to minimize unwanted movement in the ankle. Lastly, the
design will have an electrical component that will collect and read the force applied to the
design. A crucial part of an effective reading of the dynamometer is to keep the knee locked in
place and prevent the ankle from any angular motion. This is what the knee brace and the hinge
mechanism, in conjunction with the blood pressure cuff, will solve. The proposed design will be
more rigid, so when the patient is performing the flexion motion, it will provide resistance for
measuring the muscle strength. Load cells will be placed on the bottom and top of the foot pad to
measure the strength of the muscle. The forces measured by the load cells would then be
displayed via Excel to the attached computer. Altogether, this design is a cheaper, portable, and
accurate alternative to the products previously mentioned.

3. Background
An ankle dynamometer is a device used to measure plantar and dorsal torque in the ankle. This
type of device is used on patients who have disabilities or suffered an injury and are trying to
regain physical strength in their ankle. The ankle joint is the most common to experience a
musculoskeletal injury and without rehabilitation can be permanently impaired (Russell, 2010).
Ankle strength in the past has been measured mainly three different ways, manual muscle testing
(MMT), handheld dynamometry, or isokinetic torque measurement systems (Moraux, 2013).
MMT is easy to perform with no extra equipment as the testing is done with the therapist’s hand.
This type of testing ensures eventual movement of the ankle joint as the therapist stretches the
joint but this test does not provide a quantitative value for how much force, torque, and
movement is done by the patient alone. MMT suffers from low reliability and low sensitivity
when it comes to determining the amount of force generated by the patient (Moraux, 2013). The
handheld dynamometry is more useful than MMT as the handheld version is easily portable and
more sensitive. However, this handheld design comes with too much variability as the accuracy
of the measurements depend on the knee joint position, whether flexed or straight, and the
amount of force the therapist generates to maintain a fixed position on the foot of the patient.
Therefore, the ability to reproduce accurate results each test is very unlikely to occur. The
therapist would have to generate enough force to equal or trump the amount of force generated
by the patient’s flexion of the joint which can be problematic due to the amount of flexion torque
(Moraux, 2013). Both previous testing mechanisms have difficulty with differentiating true
plantar and dorsiflexion from the help of eversion and inversion of the foot where depending on
the flexion the eversion or inversion would influence the experimental values based upon the
side angle generated. The last testing method is the isokinetic torque measurement systems
which provide joint stabilization and accurate readings of the force generated by the ankle but
can be rather expensive on the market. Other problems include sizing for children compared to
adults and not being a portable device (Moraux, 2013). The ankle dynamometer design that we
create must be a portable, yet accurate measuring device that is relatively inexpensive in order
for most all therapies are able to purchase a device and able to utilize the device in their everyday
operations.
Justification
An ankle is involved in daily functions that any person may complete throughout the day without
much thought. It is when the ankle strength is affected that people begin to realize something
may be wrong. Ankle strength becomes impaired in common neuromuscular disorders as well as
ankle sprains or direct damage to the ankle itself (Moraux, 2013). Progressive loss of muscle
strength is associated with neuromuscular disorders which in turn can affect the balance and gait
of patient with one of these disorders. The neuromuscular disorders that affect ankle strength
include Charcot Marie Tooth, Duchenne muscular dystrophy, myotonic dystrophy type 1 and
inclusion body myositis (Moraux, 2013). By being able to measure the ankle strength, the ability
to assess the strength of the disorder prevalent in the patient, determine effectiveness of
therapeutic strategies, and predict loss of ambulation or motor skills will help physicians and
physical therapists make more educated decisions on how to treat the neuromuscular disorders in
different patients. Neuromuscular disorder is common in all ages as the disorder can be in the
form of Duchenne muscular dystrophy which is acquired at birth or when an elderly person
develops Lou Gehrig’s disease or suffers a traumatic brain injury. “Each year, 750,000 people in
the US experience a stroke and 11,000 suffer a spinal cord injury. 500,000 Americans currently

4. live with cerebral palsy, 270,000 with multiple sclerosis and 5.3 million with the after-effects of
a traumatic brain injury, and thousands more with movement disorders such as amyotrophic
lateral sclerosis (ALS, or Lou Gehrig’s disease)” (Neuromuscular, 2015). As prevalent and
entrenching as neuromuscular diseases can be, even more common are ankle injuries due to
sports, accidents, or other problems that arise. No matter what the cause of the ankle issue, the
person experiencing it is likely to end up at a rehab center in needs of therapy to improve their
ankle strength. It is very likely that an ankle dynamometer would be used in the patient’s rehab.
However, if the person went to a rehab center that did not have an ankle dynamometer or one
that took much more time due to its inconvenient location and bulkiness, it could make the
patient’s rehab take longer or be less effective. That is why a portable, efficient and inexpensive
ankle dynamometer is needed in the present world.
Potential Opportunities and Market Overview
To be successful in producing a desirable device, the ankle dynamometer must be cost efficient
and portable. Current marketable ankle dynamometer machines such as The Biodex System 4
provide accuracy and safety that is currently unmatched by any other product (System, 2015),
but the problem is that it is both costly and not easily moved (System, 2015). The new ankle
dynamometer design being implemented will be both cost efficient and portable. The advantage
of having a portable device is that it allows the therapist to bring the device to the patient rather
than isolating the device to one specific location in the facility. Having a cost efficient device
allows the facility to have multiple devices on hand which would, in turn, allow for more patients
to be seen at once. Current devices on the market that are competitive to the device lack
accuracy and are less desirable. This product would provide accurate results while eliminating
the current concerns for ankle dynamometers.
Engineering Issues
When designing the ankle dynamometer, a variety of issues were identified. For example, this
device must be adjustable according to each patient’s foot size. The force plates that will be
implemented must be consistently located on the same location on all patients. According to Dr.
Knight, a professor in the department of kinesiology at Mississippi State University, the
effectiveness of measuring plantar and dorsiflexion force in the ankle is dependent on isolating
the movement in the inversion and eversion direction. In order to combat this, the device must be
implemented with locking mechanisms to inhibit unwanted movement. Dr. Knight also
introduced issues with placing the knee at correct angles. Any angle induced while testing the
device can alter the results, and since a constant testing environment is necessary, the device
must ensure a constant angle within the patient’s knee (Knight, 2015). Additionally, the issue of
displaying the results must be taken into consideration. The final issue to take into account
results from the patient’s applied force. When facilitating a test, the patient must apply a force in
order to collect the desired data. When doing so, the patient needs to push against something that
resists the foot force. Current products on the market require the therapist to apply an initial
force. This leads to inaccurate results due to inconsistent forces that may be applied with each
test.
Deliverables
The deliverables of the project are the parts that have tangible aspects to be fulfilled in order to
meet the needs of the consumer. The deliverable of this project must have clear target

5. consumers, and must set and meet certain expectations for those consumers. Its intended use for
physical therapy patients is to provide an easy way for the physical therapist to track the progress
of the patient's rehabilitation as they regain their muscle strength. These are the end products of
the project, but to get there, a proper design will have to be built with the correct frame and
structure to fit any consumer. The design will need to have adjustable parts to fit each
individual’s size, and sensing and output hardware for reading the data.
Designs Conceived and Developed Until Now
Proposed Design #1
The initial design, seen in Figure 1, was expanded upon from the previous senior design ankle
dynamometer. It was a simple foot pad hinged to a single
bar “arm” that ran up the back of the calf. The initial
thought was that the design would be simple and mobile
allowing the therapists to transport this ankle
dynamometer anywhere in the hospital. The hypothesized
design would work by the patient pushing the balls of their
foot into the foot pad causing the bar on the back of the
calf to push against the calf creating a resistive force.
Their exerted force could then be read by the therapist.
However, the problem with this design was the lack of
rigidity making the proposed solution too flimsy and not
sturdy enough. The design did not prevent dynamic
movement of the foot which means the foot would move
in the eversion and inversion directions. The consistency
of the force readings were thought to be jeopardized by
the single bar on the back of the calf. Due to a lack of
stability, the foot was not held firmly in place while
testing meaning this design needed further improvement.
In addition, the design did not adjust to different sizes of
patients. Therefore, this proposed design of the ankle dynamometer never made it to the building
phase.
Proposed Design #2
This proposed design’s frame is similar to the design
of a foot brace or splint that would have a hinge at the
ankle to allow for vertical foot movement, as seen in
Figure 2. It would incorporate adjustable components
to fit any foot size such as adjustable straps. Once the
frame was fitted to the right size and comfortably
placed, it would lock into place preventing inversion
or eversion movement. Force sensitive resistors
would measure the force applied and the maximum
resistance with an ohmmeter before displaying the
results. This would display how much force was
produced by the patient's foot.
Figure 1
Figure 2

6. The hinge would also be modified to include a locking mechanism which the patients would
push against for resistance to measure their force output. There would also be an area on the foot
pad at the ball of the patient’s foot where the force sensors would be placed for the patient to
push on.
Prototype #1
This design, seen in Figure 3, became the initial prototype to the device, which consists of a
wood base in conjunction with a knee brace screwed into the arms of the device. Two adjustable
Velcro straps wrapped around the base
to ensure limited movement of the foot
once attached. The brace included four
adjustable Velcro straps to provide a
tight fit for the patient’s thigh and a
locking mechanism in the hinge to
prevent movement of the knee. In
addition to the Velcro straps, a blood
pressure cuff was implemented at the
ankle to further decrease unwanted
eversion and inversion of the ankle and
to adjust to any size ankle. The last component of this design included force sensitive resistors
that measured the force applied by the patient.
Prototype #2
The Base
The second prototype was a modified version of the previous one, and included two parts. The
first of which was the base containing an attachable slide which can be seen in Figure 4. The
slide had two force sensors attached which
allowed the force to be calculated at the ball of the
patient’s foot, regardless of their foot size. The
slide clamped into each slit located on the side and
locked into place allowing for it to be adjusted to
the patient’s
foot. In
addition, the
base included
two hinges
that allowed
for the
attachment of the second part. The second part of the design,
seen in Figure 5, included a cupped back that would screw
into the two hinges. This back had two rectangular slots to
allow for implementation of the blood pressure cuff
mentioned in the previous design. The two ends of the cuff
fed through the two slots and velcroed together on the other
side of the back piece. This would allow for a snug fit along
the ankle of the patient and limit inversion and eversion movement of the ankle. The base and
Figure 3
Figure 4
Figure 5

7. cupped back was intended to be 3D printed, but the 3D printer was unable to handle the large
print; consequently, wood was used instead, similar to prototype #1.
Force Sensor Pad
The electronics part was used to sense the amount of force that the patients exerted on the foot
plate. It used force sensitive resistors (FSR) to measure the force. The sensors were connected to
an Arduino which was also connected to an LCD
screen, as seen in Figure 6. The FSRs were the input
to the Arduino which then outputted the force to the
LCD screen for the patient and therapist to see and
record. The FSRs worked by changing their
resistance when a force was applied. With a voltage
over the
sensors, the
current
changed as
their resistance
changed which the Arduino measured. The Arduino then
took the measured current and, with some programing
through Arduino’s software, converted that current to a
unit of force in pounds and outputted that value to the LCD
screen. Two sensors were used to help distribute the load
and provide a more accurate reading. All the circuitry was
connected and soldered on a protoboard, whose wires
connected to an Arduino (Figure 7), which then fit into a
black circuitry case. The case had a rectangular hole to
hold the LCD screen. This was all powered by a 9V battery
connected to the Arduino that also fit in the case.
The remaining component of this design was the slide
containing the force sensors that attached to the foot plate.
This slide held the sensors and was the focal point for the patient to push on to measure their
ankle strength. The slide has two cylindrical extensions to ensure all force was being transmitted
through the sensors.
Prototype #3
The third prototype was also a wood model, but provided more
advanced aspects to improve upon the previous designs. The
wood would be stained and sealed to improve its aesthetics and
make it possible to wipe down and clean after each use. A new
aspect to this prototype was the design of the back plate. The
back plate would be made of multiple layers of wood, each one
being half an inch. These layers allow the back plate’s height to
be adjustable and provided a way to hold the blood pressure cuff
on the ankle. The top and bottom layers were wider to prevent the
blood pressure cuff from moving during the use of the device, as
seen in Figure 8. Another aspect of this prototype that improves
Figure 6
Figure 7
Figure 8

8. upon the design is the electronic portion. The Arduino
is still used, but it now outputs the data onto an Excel
interface with a PLX-DAQ program. These programs
allow the data to be automatically uploaded to Excel
and even placed in a graph. This makes recording and
saving the data easier for the user, especially to
compare to later tests. Instead of force resistive
sensors, load cells were implemented for higher
accuracy. Figure 9 shows the Arduino component with
a load cell in the background.
Final Design
The final design is very similar to the previous prototype. It
mostly builds off of that design and improves it. Most of the
improvements were due to making the device more adjustable
to different sized patients. Another major change was in the
material used. Instead of wood, starboard was utilized, as seen
in Figure 10. It has many properties similar to wood which
made it easy to work with and allowed use of the same tools.
Starboard was used rather than wood because it was more
aesthetically pleasing and already capable of being cleaned to
provide a more sterile surface.
Another major change was in the type of slide used, as seen in
the Autodesk professional drawing of the device in Figure 11.
The original
slide containing
the load cells
was one-sided and placed on the top of the foot
pad. It would slide up and down the foot pad to
adjust to the patient’s foot. This was the plantar
flexion direction and then it would be flipped to
the bottom of the foot pad to be used in
conjunction with a Velcro strap for dorsal flexion.
The new slide is an all in one design that surrounds
the foot pad. It contains two load cells: one on
bottom and one on top. The top load cell is used to measure plantar flexion while the bottom
measures dorsal flexion. A Velcro strap connects to the slide and goes over the patient’s foot.
When the patient pulls up for dorsal flexion, it pulls the bottom of the slide allowing the bottom
load cell to measure that force. The double-sided slide allows for dorsal and plantar flexion to be
measured in the same test.
A key component, specifically requested by Methodist Rehab Center (MRC), was the ability to
“lock” the ankle hinge to a specific angle. Different patients have different range of motion in
their ankle; therefore, the hinge needed to be capable of being locked at different angles for each
Figure 9
Figure 10
Figure 11

9. patient. This is seen in Figure 12 with the crescent-shaped slits
on the side of the hinge. Wing nuts and screws are used to lock
the hinge at the desired angle. Another important aspect of the
ankle hinge is its adjustability to the patient’s ankle size. To
account for this, slits were placed along the bottom of the foot
pad that allowed the ankle hinges to be moved inward to fit the
patient’s ankle. This in conjunction with the blood pressure cuff
keeps the ankle secure.
One aspect of the electronics that improved was the
measurement of torque. Torque is the actual value wanting to
be measured, but the load cells measure the force. Therefore,
the Excel program has built-in capabilities to calculate the
torque. The user measures the distance of the patient’s foot
from the ankle hinge to the slide which can be done using the ruler found along the side of the
foot pad. Once the program is running, the user/therapist can input the distance measured which
then gets automatically calculated to torque. A final addition to the final design was a tripod to
support the foot.
Testing
Once the final prototype was completed using the starboard material and other key components,
extensive tests were conducted to determine whether the final design could withstand the force
that would be applied and how precisely the force sensors could read the incoming data.
The first testing procedure was to ensure a tight fit for different sizes of the lower leg. This test
was accomplished by surveying 10 different people. The lower leg of the test subject was placed
in the prototype followed by tightening and fastening all the straps and the blood pressure cuff to
ensure isolation of the ankle. After all fitting and adjusting was completed, the participants were
asked about how the prototype fit, whether it fit very loose, partially loose, partially tight, or very
tight. Out of 10 test subjects, there were five who said the prototype was partially tight and five
who said it was very tight, as seen in Figure 13. These results show that the prototype isolated
the ankle properly and comfortably.
Figure 13: This graph shows the results of the fit testing of the device.
0
1
2
3
4
5
6
Very Loose Partially Loose Partially Tight Very Tight
Fit Testing of Device
Figure 12

10. The second testing procedure was to ensure accurate and repeatable measurements while using
the force sensors. The load cells’ measurements were compared to a scale’s measurements using
five different objects that included a TV remote, a roll of tape, a book, a bottle, and a different
bottle. After multiple tests on each object, the force sensors were found to measure the correct
value within 6% error. The overall results of this testing is seen in Figure 14.
Figure 14: This graph shows the results of the redundancy testing of the load cells using a scale
to compare the weights. The standard deviation is also shown.
The final testing procedure was to confirm that the actual prototype worked for what was
required. The test was done utilizing nine different test subjects where each subject would have
their ankle isolated in the prototype. All sensors were connected to display the data on an Excel
spreadsheet. The test subject would be asked to relax their foot and the sensors zeroed out. The
researcher then inputted the distance, in centimeters, from the ankle to where the sensors were at
the balls of the feet. Once the distance was entered, the test subject was asked to press down
with the balls of their feet to
measure plantar flexion of the
ankle. After a peak torque was
reached, the subject was asked to
relax and return the foot back to
the relaxation orientation. Then
the subject was asked to pull back
with their foot to measure dorsal
flexion. Similar to plantar flexion,
once a peak was reached the test
subject was asked to relax. The
results were then saved to the test
subject’s individual file and can be
seen in Figure 15. For each of the
nine test subjects, three tests were fulfilled.
An average of each peak force was found
for each participant in plantar and dorsal
0
200
400
600
800
1000
1200
Bottle Book Remote Tape Bottle 2
Weight(grams)
Redundancy Testing
Figure 15: This figure shows an example of
what the data looks like in the Excel
interface including the graph of the data.

11. flexion as seen in Figure 16. The structure of the prototype was able to withstand the force
applied by all test subjects that were tested.
Figure 16: This graph shows the results of the actual testing of the device with participants.
Areas to Improve
The final design met many of the criteria set by Methodist rehab center. Although it functioned
well for their needs, there are a few design details that could be improved. First, the camera
tripod used to elevate the dynamometer is flimsy and cheap. With a better budget, this could be
upgraded to a sturdier and longer lasting tripod. Second, the only programs used to process the
data retrieved from the Arduino were Excel and PLX-DAQ. This is one solution that most any
person could use, but some rehab centers may prefer other programs such as LabVIEW or
MatLab. Those programs could be implemented to suit those specific rehab center’s needs.
Thirdly, the load cells have a small nub point that senses the force during testing. This nub places
a lot pressure on the starboard and has started wearing away the outer surface. With enough time
a groove could form in the starboard that could affect testing accuracy. A simple steel plate could
be glued on under the sensing areas of the board to prevent further damage. Fourth, the locking
mechanism used is a simple bolt tightening design that relies on friction to keep the ankle locked
at certain angles. Over time, the hinge mechanism might lose this friction needed to accurately
test. A better design that does not rely on bolt force and friction to keep the ankle in place,
should be used. A solution that might be used is the button clip designed used to lock many
things like crutches in to certain lengths. The downside to this though, is that there is a limited
number of angles you can set the ankle at, whereas the design used now can lock at an entire
range of angles. The last improvement that could be done is the method used to keep the sensor
slide in place. Simple office rubber bands were used as a friction force in between the slide and
the base, and this looks unprofessional. More aesthetically pleasing rubber pads or bands could
be attached to the sensor slide to keep it in place and look professional.
Engineering Analysis
The main aspect of this design to be considered was the forces exerted on it. Every time the
device is used, the patient will exert forces on the hinges and foot pad. Therefore, the ankle
hinges were designed to withstand these forces and a material was selected that could withstand
these forces. Even though forces had a large impact on the design, torque was the actual
0
5
10
15
20
25
30
Data 1 Data 2 Data 3 Data 4 Data 5 Data 6 Data 7 Data 8 Data 9
AverageTorque(N-m) TorqueTesting
Plantar Flexion
Dorsal Flexion

12. component being measured. Therefore, the force measured by the load cell had to be converted
to torque. This was accomplished in the Excel interface. Once the distance of the patient’s foot
was inputted, the Excel program would calculate torque using Equation 1 seen below.
T=f*d
In this equation, T is torque, f is force, and d is distance. These calculations are completed by the
Excel program and then outputted to the screen for the user to see and use.
Conclusion
Methodist Rehabilitation Center was in need of an ankle dynamometer that would be both
affordable and mobile. The device created needed to meet certain standards in order to be as
accurate as the current model used, but be a better, transportable design. These standards
included the ability to measure plantar and dorsal flexion; isolate the ankle; secure the ankle in
place to limit unneeded movement; and adjust to different sizes of ankles/feet. Although there
were areas to improve upon it, the final design used was found to be accurate and portable,
therefore meeting the standards set in place.
Acknowledgments
The authors would like to thank Dr. Filip To for his help with this design, the use of his lab and
equipment, and implementation of the design. Further gratitude towards Methodist Rehab Center
for their help in correcting and improving the design. Lastly, the authors would like to thank Dr.
Thomas Byrd for funding during the fall 2016 semester of the design.
Physical Decomposition

13. Functional Decomposition

14. References
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