Prosthetic Foot for Yoga

In a course led by Vishal Sachdev at the University of Illinois, our team explored how to increase balance for prosthetic leg users who love yoga! Our inspiration came from our mentor, Jenna Fesemyer, an above-the-knee amputee who uses a C-Leg prosthetic. Through the course, we were able to make a prosthetic foot attachment that she could interchange with her normal prosthetic foot when practicing yoga and we want to share it with all of you!

Created by Ruei Kao, Samantha McClary, and Yingyi Wu

Step 1: Find Your Resources

Our design is printable from a Fusion360 file on a 3D Printer. To be able to create your own yoga prosthetic, you need to understand the basic functionality of Fusion360 in order to customize the file to your dimensions then have access to a 3D Printer to print your design. For materials, you will need 3D filament (PLA is fine, nylon is stronger) and a sheet of thin cork, similar to the bottom of coasters, to use on the bottom of the final product to prevent slipping.

1. Fusion360

Fusion360 is a great 3D Modeling software that can be downloaded on any Mac or PC. It is easy to use and offers free trials of 1-year for non-commercial users and 3-years for student users. You can find download instructions here.

2. 3D Printer

3D Printers are oftentimes found in makerspaces across the United States. These are very common in universities, libraries, high schools, and stand-alone makerspaces. While this directory is not all encompassing, it is a good start to finding a 3D Printer near you. If you cannot find a makerspace near you in that directory, we encourage you to ask your local library or start googling as there are many that are not registered in the directory! We used an Ultimaker 2+ Extended for our prints, as Jenna is a tall user and in order to print the necessary size we needed an extended printer. You can find Ultimaker build volumes and dimensions here.

3. Filament

The most important physical material for this design is filament. Filament is what is used in a 3D printer to make the designs a reality. PLA is the most common type of filament, and at the right fill density can be sufficient for this design. If you are looking to use this product a lot, using a stronger filament like nylon would work better. We recommend that you first try the product in PLA and if you love what you've made and you think you will use it often, print it again in nylon so you have a sturdier final project! Most makerspaces will have filament for you to purchase and use, but often times at different prices per gram so we recommend shopping around for the best price. For example pricing, you can view our university's Makerlab here.

4. Cork

We used a thin sheet of cork cut to the size of the base of our product on the bottom to prevent slippage. Plastic filament is very slippery on carpet and hard flooring, and we found that the material similar to the bottom of a coaster has the best grip on yoga mats. You can find sheets of this material at most craft stores, like Hobby Lobby and Michaels, or on Amazon. You can also test different materials to see what you prefer on your yoga mat!

Step 2: Take Your Measurements

Our Fusion360 design was created to fit Jenna's C-leg, but each type of prosthetic has different dimensions. The most important dimensions needed are as follows:

1. Overall Height

The height from the base of the prosthetic to the top of the prosthetic. In our design you can see this is two sections of 5.025in, for a total height of 10.05in. Jenna uses an Allen Wrench to put on her normal prosthetic foot, so we measure from the spot that the Allen Wrench bolt is to the ground as the height the prosthetic needed to be. For this measurement, it is best to make your design slightly taller than your measurement, as long as your prosthetic can take in rod above the Allen Wrench spot, to give you a little wiggle room so that you can have the perfect height!

2. Overall Base

The base of the prosthetic is a perfect circle, and this measurement is to your discretion. Increased surface area increases your ability to balance, but you don't want it so large that the base gets in the way. For Jenna, we created a 4in radius base, for a total of an 8in diameter. This may also be limited by the 3D Printers you have access to so make sure you know your printers' maximum dimensions!

3. Amount of Tapering

For Jenna's prosthetic, we had the base taper to the rod for exactly half of the overall height, or 5.025in. This is again to your discretion, but you need enough straight rod to sufficiently fit into your prosthetic and be adjustable in height! This should be based on the distance from the Allen Wrench bolt to the base of your prosthetic. For Jenna, this length was about 3in, so we made half of the design completely straight to ensure she had wiggle room to get the perfect height.

4. Rod Diameter

The rod will be inserted into your prosthetic, so this measurement should be based on the diameter of the prosthetic foot you already use! The rod that you insert on a daily basis for your normal prosthetic foot should be the same diameter as the rod you print for your yoga prosthetic! For Jenna, this amount was about 1.34in, or 0.945in + 0.197in + 0.197in.

5. Rod Thickness

Depending on your prosthetic, the insert-able rod cannot be completely filled. You will need a particular amount hollowed out to be able to slide into your prosthetic. The rod thickness measurement should again be based on the prosthetic foot you already use. This thickness in Jenna's case was about 0.2in, which we made a touch smaller at 0.197 to ensure the rod would be able to slide into her prosthetic without being too tight or too loose.

6. Insert Length

If you need an inset because your insert-able rod cannot be completely filled, the inset should be as short as possible. Having full fill in as much space as possible increases the amount of weight and pressure the design can handle. For Jenna, we estimated about 3in (2.953 in the design) for the inset length, to allow for height adjustability and maximum fill.

Step 3: Edit the Design

Using your measurements and the design file "Yoga Prosthetic" attached here, you can create your custom yoga prosthetic base! The key measurements you have should be all the editing you need, but you can also adjust the amount of tapering and other parts of the design as you see fit. There is a video attached showing an example of how to edit a dimension via a screen recording. Google and Youtube are your best friend in learning the in's and out's of Fusion360, but with just a few dimension edits your design should be perfectly customized to you!

Step 4: Print Your Design

Now that you have your customized file, you can print and try your design! Cura is a free software that you can export your design to in order to 3D print on Ultimaker printers, which will orient it in the 3D printer for you. From Fusion360, select "Make" > "3D Print" and select the body. In Cura, you can make certain selections regarding the printer such as printer type, material, and nozzle size. For the most part, you can proceed with standard settings shown in the picture. However the important settings for this print are infill density and speed of print.

We set our prototype print to a 25% infill density in grid design. The higher the infill density, the stronger and more weight-bearing your print will be - but also the more expensive! We also set the speed of print to 45 mm/s. This is slower than the standard print speed, but ensured good quality with such a large print. However, the slower the speed the longer the print will take! If your makerspace charges based on time, this will also cost more.

This design does not need supports generated and is easily printed with a brim build plate adhesion.

Overall, the cost of this print for us was around $115. However, based on other prosthetic feet like running blades that can cost upwards of $20K each, we see this as a steal to do something you love!

Step 5: Finalize and Try Your Design!

Now that you have your design printed - try it on! If it fits and feels sturdy, you can finalize the design by cutting out a cork circle to glue to the base of the design with a strong glue like super glue. This will give you more traction!

If anything about the fit is off, you can adjust the measurements and print again - the beauty of 3D printing.

Be sure to comment below if you print your own and show what you can do with your new yoga prosthetic!