This is a quads exercise, for early rehabilitation, but only after you are confident with static quads (quads sets/quads isometric exercises).

PVC tubing

By supporting the thigh with a piece of ordinary PVC pipe you can buy from a plumber, the quads only have to lift the shin from the bed to full extension. Here is a close-up of the type of tube. Any plumber should have it. It is better than a rolled-up towel as you can be sure of always having a definite height.




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

Short arc extensions Lie on a firm surface on your back with your hands by your sides and your head supported by a pillow. You need something underneath your knees, and in this example we show you how to use a length of PVC tubing that can be bought from a hardware store. You can also use a rolled-up towel or small blanket.
short arc extensions With the PVC tubing or rolled-up towel under both knees slowly straighten one knee and focus on getting a good contraction on the quad muscles. Keep the leg there for a count of five.
short arc extensions Repeat this with the other knee and alternate them until you have done this routine ten times. If the exercise is difficult, use a smaller gauge PVC tube and later you can progress onto the wider gauge. The tubing you buy will depend on what is available, but 90mm and 140mm are about the right sizes, with each one being 50cm long.