Intervals on the Elliptical
 

For those of you the do intervals on the elliptical, I'm curious to know how you do them. I run intervals on the treadmill. That's easy to figure out; I just set the treadmill at the highest speed I can keep up with. But I'd like to figure out how to do intervals on the elliptical because I have some ongoing knee and ankle problems that limit how much time I can spend on the treadmill.

On the elliptical, how fast I go is up to me and it's hard for me to maintain a fast pace, especially if I have the resistance up. So do you:
1) Set the resistance low to moderate and go as fast as you can for, say, 1 min, then slow for 1 min, etc.
2) Same as #1 but with the resistance set high.
3) Set the resistance high for 1 min, then low for 1 min, etc.

Do you try to achieve a certain speed on your intervals or do you just get your heart rate up by making the resistance harder?

My gym also has an EFX machine, which has a crossramp (incline) setting in addition to the resistance setting. I'm really stumped on how to do intervals on that machine. What do I set the crossramp at? Right now, I use a variety of crossramp settings but I'm just doing steady state cardio on both the elliptical and EFX.