...it doesn't have to!
If you've tried using an elliptical machine but felt like it hurt your knees, there's an easy fix for that.
Instead of coming up on your toes or putting pressure through the balls of your feet, the widest part of your feet, I want you to shift back and put the pressure through your heels. That will keep your shin vertical and that will take the strain off of the back of the kneecap.
Keeping your shins more vertical and using your hips to drive the motion, you'll create less strain on your knees.