The Ankle Dynamometer

Robert Loper

Jessica Sanders

Matthew Moseley

Seth Rhodes

April 29, 2016

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.

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

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

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

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

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

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

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

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

Wei

ght

(gra

ms)

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.

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

Ave

rage

To

rqu

e (N

-m)

TorqueTesting

Plantar Flexion

Dorsal Flexion

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

Functional Decomposition

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