The most common mistake people make with mountain climbers is to roll up their whole body," Montenegro says, meaning they lift their hips and throw their weight forward on to their arms. "They'll tell me they really feel it in their shoulders, when the abs should be doing all the work." She says that putting towels under your feet, as in this move, will isolate the ab muscles and keep your back flat to help you maintain proper form.
1. Start in a high plank position with your palms on the ground underneath your shoulders, arms straight, core engaged, with a small towel under the toes of each foot.
2. Use your abdominals to bring your left knee in toward your right elbow. This should be a fluid, controlled movement. You should feel the twist in your lower abs, not as much in your back or hips. Keep your shoulders squared.
3. Return to starting position, with both feet behind you.
4. Repeat by bringing the right knee to the left elbow.
5. Alternate sides for 12 total reps.
Next: 5 ways to get a flat stomach standing up
