A dense foam pad that makes flat ground slightly unstable on purpose. Stand on it and the small muscles around the ankles and knees start working to hold you steady, which is the basis of most balance and rehabilitation drills.
The filling is 50D high-resilience sponge foam, which springs back rather than compressing flat over months of use. The underside is non-slip, so it stays where you set it on a hard floor, and the cover has a sturdy zip so it can be taken off and machine washed.
It works the other way round too: kneel on it for floor work, tuck it under the ankles during core exercises, or sit on it through a long stretch session. Allow about an inch of variation in the size.
