Isometric plantar flexion in long sit against ball - knee flexed (Soleus strengthening isometric, against ball, sitting)

Sit with your legs straight out in front of you, for example on a mat.
Your feet should be close to a wall.
Place a ball between the sole of your foot and the wall.
Move forwards or backwards until you have your ankle in a neutral position at 90 degrees against the ball.
Your knee should be slightly bent.
Press the ball of your foot into the ball, keeping your knee in the same bent position.
Hold this position.
Relax, and repeat.