Summary

This is a parametric printable ankle board / balance board / wobble board. It is used for rehabiliating ankle injuries, improving balance and agility, core strengthening, improving athletic coordination, increasing ankle motion, and general jedi training.
This component is part of an open source lab library, by the Michigan Tech's Open Sustainability Technology Research Group.

For similar see the Open-Source Lab How to Build Your Own Hardware and Reduce Research Costs

Instructions

Ankle boards range from $20 to over $100 for the really good ones that allow you to customize the size. This design not only does everything that the commercial versions do but is better than all of those as it allows you to alter the size of the hemisphere in OpenSCAD so as you get better you can increase the difficulty by printing out a new piece.

The OpenSCAD files is included so you can change the size and thus the difficulty of the board. The images show the default setting and STL.

Print the STL with a reasonable infill (e.g. this one was with 2 exterior layers and only 15% and it works fine - but the more support you give it the longer it should last) on your favorite open-source 3D printer.
Cut out a circle in plywood -- the easiest way to do this is to put a nail in the middle and then tie a string to your jigsaw the length equal to the radius of your wanted ankle board circle. Commercial versions of ankle or wobble boards are usually around 16 inches in diameter. This one was larger - if I were to make another one I would use thicker plywood.
Drill a hole in the middle of the circle
Bolt the printed hemisphere to the board with a 3/8 hex nut/bolt. The hemisphere has a hexnut hole in it meant to lock the nut in so you can tighten from the top and easily switch out hemispheres. This actually worked really well -- you can tighten the bolt down by turning the printed hemisphere.

One of the main features of the ankle board is that it improves proprioception, which is your sense and awareness of the position of your body parts. Having good proprioception helps to reduce the risk of injury.

Here are the basics of using it:

While sitting down on a chair place the board under your feet and slowly rotate it a number of times in each direction to improve ankle range of motion and control.
Stand on the board with both feet while holding on to a chair for balance. Rock the board forwards and backwards, then side to side.
When you are comfortable with (2) stand on the board and rotate it around so that the edge of the board is in contact with the floor at all times.
As you get better try balancing on the board for as long as you can without the edges touching the floor.
As you master your balance try to make it harder by rotating the board in a circle without allow the edge of the board to touch the floor.
Balancing on the board with both feet, perform small knee bends to challenge your balance. Gradually bend your knees further into a squat and then a deep squat.
Again while balancing with both feet, reach both arms out in front of you. Rotate your upper body around slowly from side to side.
Balancing with both feet, throw a ball against a wall and catch it on its return. You can also do this with a partner.
Close your eyes. Do all of the above exercises. This will make them considerably more challenging.
Now try one leg and repeat 2 through 9.
At this point you are probably pretty jedi like - open your eyes and have a friend throw stuff at you to catch. Add friends, throwing speed and danger of objects....
Repeat 11 with eyes closed and then get to work on a printable light saber ;)

Have fun!