One of the best exercises to rehabilitate your knee or just to get it moving again is the stationary bicycle. If you have back problems, then the recumbent bike will be the choice you want, but if you do not, then use the upright bike. The reason this exercise is great is because it allows you to get full range of motion without putting a lot of stress or pounding on the joint that you would get from running. In addition, it will work all of your major muscle groups in your lower leg, which is what you want.
If you are experiencing knee pain or need to adjust your workouts because of an old injury, make sure you let your personal trainer Portland Or know about this so they can alter an modify the exercises and include the bike too.
Portland Oregon Fitness