Another cardio training I do is called the elliptical. Since I do have knee trouble, I have to limit my actual running I can do on the pavement outside. The reason why I love doing the elliptical is because it is much smoother and easier on your knees than running outside. Also, you can make it as hard as you want. For instance, the resistance starts off on 15 automatically, but I usually take it up to the mid 30’s in order to maximize working on strengthening my knees while not hurting them by pounding them on the pavement but still getting the advantages of doing a cardio session.