Isometric inversion in long sit against ball (Ankle inversion strengthening isometric, against ball, sitting leg straight)

Sit with your legs straight out in front of you, for example on a mat.
Your legs should be close to a wall with the foot to be exercised furthest from the wall.
Place a ball between the inside of your foot and the wall.
Bend your other leg so that is out the way.
Move sideways until you have your leg parallel to the wall.
Press the inside of your foot into the ball, keeping your knee straight.
Hold this position.
Relax, and repeat.