One of the best ways to get in shape (and I know this well from my younger Captain America type days) is to jump rope. Heavy exercise requires good wind power for the oxygen needed. That is why boxers and other atheletes swear by jumping rope. But my knees began to hurt from too much hard court tennis and I have not been able to jump rope for some time.
I recently purchased a Bellicon and was able to jump over thirty minutes with no hurting in the knees. I loved it. No sound either while listening to music and jumping to the rhythym. Incredible rebounder and well worth the investment in your health.