'Heel slides' are simple early exercises to maintain some active range of movement.

They can be done in bed even while the knee is still painful, as long as the surgeon allows it and the knee is not in an immobiliser. Pop something slippy under the heel (like the tea tray pictured here) to make the slides easier.



In case you cannot see the video, here is a still sequence -

heel slides Heel slides are simple exercises to start building knee flexion and to get the quads moving once you are out of an immobiliser splint. Resting on a firm bed and the back supported in a comfortable position, ask someone to help you put a smooth tea-tray under your leg. Have your hands by your sides, both legs extended. The bad leg should be extended as far as you can comfortably manage.
heel slides Concentrating on the heel of the painful knee, slowly slide the heel up towards your buttock and then slowly slide it down again. Repeat 5 to 10 times. You can repeat the exercise with your good knee just to keep that leg moving also.