We had to do em non-stop in Marine Corp boot camp, they were a favorite of Drill Instructors trying to break you. They are really hard on your ankles.
mountain climbers. Always a favorite of mine to use with clients and interval training.
Keep up in the 21st century.
To take stress off of the ankles, I use gliders. Which somewhat resemble frisbees a bit.
Same position but instead of the "kicking off" back and forth, you "glide".
You can also go in different planes of motion i nstead of just "knee to chest".
If you are in a commercial gym that holds gliding classes, try them on your own if you don't want to be embarrassed by the members who are used to them.
Sandra