If you have a knee injury, staying in shape without making the knee worse is a major concern. Cardiovascular exercise is important for general health and also aids in healing injuries by providing freshly oxygenated blood cells to the area. Many exercises that get your heart rate up -- such as cycling, jogging and using an elliptical trainer -- use both legs and may be impossible with a knee injury. Activities involving primarily your arms will enable you to keep up your cardio workout regimen.
Some knee injuries, as well as osteoarthritis, may improve with bicycling. In addition to providing cardiovascular and healing benefits, cycling improves your knee flexibility and strength. The American Arthritis Society recommends cycling as an acceptable form of exercise for osteoarthritis, but points out that avoiding hills is crucial, so you don't need to brake suddenly and increase the pressure on your knee. Instead, biking on flat terrain or a low resistance level at the gym are better choices. Raising the seat level slightly decreases the pressure on your kneecap.
If your leg is immobilized or unable to bear weight, an arm ergometer is an excellent substitute for bicycling. The arm ergometer is a machine that allows you to use your arms in place of your legs to push the pedals of a bicycle. Another option is trying an upper body circuit training program, using high repetitions and low loads, and short rest breaks between exercises. Using upper body machines or free weight exercises that work your latissimus dorsi, deltoids, pectorals, biceps, triceps and rhomboids will get your heart pumping.
If you're not the gym type, there are plenty of sports that do not actively involve both legs. Rock climbing is an excellent option -- plenty of injured climbers use their arms and uninjured leg to scale the walls, which builds muscle and provides an intense cardio workout. Kayaking and rowing are also fantastic cardio workouts, if your knee can tolerate being in a bent position for that long. Swimming is another option, and if it bothers your knee, walking briskly in the water can get your heart pumping.
Before starting any new exercise program, talk to a medical professional to ensure you will not injure your knee further. Also, if you have a preexisting cardiovascular condition, check with your doctor to get clearance before altering your exercise routine.