Running is a great form of cardio, but not if you have bad knees. Give one of these other great exercises a try instead!
Great for low- to no-impact lower body exercise.
A recumbent bike is less stressful on the knees.
Swimming or pool exercises are very gentle on all joints.
Circuit training with weights can increase the heart rate as much as a cardio workout!
Jab, cross, and kick up a sweat! Air punching is great cardio if you have lower body injuries.
Get health benefits with low-impact walking. Aim for 30 minutes, 5 days/week.
Ropes can give you a full body workout without ever moving your feet.