Surgery can force athletes into a period of reduced activity, which leads to a decline in overall fitness and strength. This is a major concern for the injured athlete.
Programs to maintain muscular and cardiopulmonary conditioning are important. If you are working with a physical therapist for knee rehab, you can ask for guidance with a fitness program as well.
Two of the most commonly used exercise tools are the stationary bicycle and the arm cycle. The arm cycle works both arms at the same time. The stationary bike is more of a one-legged exercise for those who have had knee or leg surgery. While the healthy leg pedals, the recovering leg can be propped on a chair next to the bike. The uninvolved leg can be strapped to the bike pedal for one-legged cycling.
One study comparing these two forms of exercise found they were equally effective in achieving aerobic conditioning. The arm cycle was perceived as more rigorous, which might suit your need to feel like you've had a good workout.
But if you want an easier form of exercise that still builds cardiovascular fitness, then one-legged cycling may be just what you are looking for. Of course, you can still use free weights or elastic tubing to maintain upper body strength during your course of recovery.