This is another one of those exercises we did in high school gym class.
It’s great for your legs and puts a hurting on your arms too.
Start out on your hands and knees, then come onto your hands and feet. Put one leg forward underneath your chest and have the other straight back behind you.
To do the exercise, keep alternating legs back and forth. It’s like your legs are running, but since your hands are on the ground, it’s like climbing a flat mountain! 🙂
Body Weight Workouts 
Great site I weighed 252 pound May 09 this week I have lost 70 pound in weight, I’m disabled, I can’t move fast enough to raise my heart rate so I used a Mountain bike that cost me £40 and did high, intensity interval training.
I watched my eating which was easy really if you like fresh veg and stir fry. I started doing eccentric body weight exercises to bring my upper body in balance with my now totally toned butt and legs, but I love doing cores,no expensive weights or club membership, just a room and a gym ball the odd band and dumbell and there you have the ultimate fitness studio Nice one
Man, a set of intense mountain climbers thrown into a bodyweight circuit can be pretty brutal. Especially after your arms are already taxed by pushups, burpees, etc.
I find that this exercise works both the stability and spinal flexion (bending forward) functions of the core.
Nice site!
-Ian
Awsome site, thanks!