Fitness Magazine caught my eye with "Burn 160 Calories in 15 Minutes."
The exercises are pretty much all jumping rope, and my 40+ knees sometimes like jump rope more than others. (Many women have told me a bigger problem is post-mom bladder.)
I have to say that jumping rope is a surprise, and though it looks like a highest-impact activity, it actually isn't. The lower leg muscles have to control the rebound, and you don't leave the ground hardly at all.
(Bonus: lower leg muscles toned for spring dresses and heels.)
I find what folks really hate though, is tripping on the rope. How can it be so hard to do something I did for hours when I was 10 years old?
Two options that are a bit more knee-friendly (and less frustrating!):
1) No rope. That's right - just pretend you are jumping. As silly as it sounds, you won't get frustrated tripping on the rope and you won't have to quiet the "Cinderella, dressed in yella..." chants from your childhood (which make you feel even more silly).
2. Jump Snap. I've gotten emails from these folks for quite awhile, and apparently Dr Oz gave them quite an endorsement. Try it!