You will need an assistant for this exercise.
Lie on your back with a buoyancy collar around your neck and a noodle under your hips.
Place an inflatable ring under your unaffected leg and bend your affected knee to 90 degrees.
Ask your assistant to stand at your feet facing towards you with one of their hands over the front of the ankle of your affected leg.
On your assistant's command, straighten your knee by pushing your ankle towards the water surface into your helper’s resistance.
Your assistant will need to adjust their position to accommodate this movement.
Return to the start position and repeat.