Because stiffness and pain are often linked it can be hard to get a stiff painful knee moving using your own muscle power. The solution here is to use gravity to help you.
There are two simple exercises to concentrate on in the early days after your surgery, one for getting the knee straight, and other for getting the knee bent.
For the straightening exercise you need a comfortable chair or sofa, and a foot rest such a as a coffee table with a cushion on it. Relax in the chair with the heel resting on the foot rest at about the same level as your buttock. The leg should be a straight as you can manage, and the knee must be unsupported. The weight on the leg will try to pull it out straight, but initially your muscles will try to resist this. It is only by holding this position for several minutes that your muscles will gradually relax and you will start to benefit from the stretch, so you should time yourself for ten minutes with the knee in this position, then gently bend the knee up and move it again. It will be quite stiff and sore to move after being out straight for ten minutes, this is normal. Bend and straighten for a minute or so, then go back in to the straight position and sit for another ten minutes. Watching the television, and keeping the knee out during the program, but moving it in the breaks is ideal!
Once you are finding this OK, the next step is to add a little weight to increase the effects of gravity. First try hanging an open telephone book or yellow pages over the knee for the exercise. When this is OK, then try crossing your good leg over the stiff leg and letting the full weight of your good leg pull the knee out straight.
This is a very easy exercise which does not take much work, but holding the position with the leg out straight for long periods is critical as the first few minutes of each exercise is spent allowing the muscles to relax.
Chair based knee flexion
To improve knee flexion using gravity you need a sturdy chair without wheels such as a dining chair. One with arm rests is useful but not essential. Sit normally in the chair and bend up your stiff knee as far as you can keeping your foot flat on the floor, bring your heel closer to the chair. Your good leg should also be well planted on the floor under the chair. Grasp the sides of the chair firmly with your hands, then slide your bottom forward off the seat of the chair. With your bottom unsupported, gravity will force the stiff knee to bend. Use your arms and your good leg to support your weight and control the amount of bend in the stiff knee. You should really feel this bending your knee up, and some discomfort is expected and required to make progress.
Hold the bent position for ten seconds then slide your bottom back on to the chair and catch your breath or mop your brow! Then repeat the process five to ten times depending on how the knee feels.
By doing these exercises daily you will make steady improvement. They can be hard work and you may find it helpful to take pain killers before doing the exercises and anti inflammatories or ice packs afterwards to allow you to push yourself a bit harder.