The balance board is excellent for strengthening the ligaments in ankle joints and knees as well as the foot and calf muscles. I use it twice a day for about 15 minutes each. It strengthens in particular the underlying muscles, with which one tries to keep the board horizontally through the smallest possible compensatory movements. With a little practice you make it to stay for a few minutes in the balance, with no set up with the edge. The amount of 'hemisphere' on the bottom (on the product photo Blue and above) I find fine.
Annoying / pity the smooth hard plastic top is (black in the image and below), on the edge are a few concentric circles, the only protection against slipping. Annoying because one the greatest effect - in particular for small foot muscles - reached if one were to face barefoot or in socks on the board. That's with dry feet or in socks because of the smooth surface but not advisable because you can easily slip off at larger deflections of the disc and just can not keep up. In sneakers slipping is not a problem; but comparatively coarse foot movements are transferred through the shaft and the sole and the little muscles just not trained. A slightly dull gumming the stand I thought better.
