Stand against a wall and place the ball either at the back of the knee or just above at the lower part of the back of the thigh. Slowly extended your knee by pushing back into the ball until your knee is straight - squeezing your quads (thigh) to get maximum quads muscle tone and activation. Hold this form 10sec up to 30sec as directed. Slowly release and repeat.

During the contraction keep trying to maximise the squeeze - but stay below any knee pain. Quads fatigue is ok -but not knee pain