Isometric plantar flexion in long sit against ball - knee extended (Gastrocnemius 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.
Press the ball of your foot into the ball, keeping your knee straight.
Hold this position.
Relax, and repeat.