London Bridge Plank: Get down on your forearms and toes, and keep your body in a straight line. Your hands should be shoulder-width apart. Do not arch your back; maintain a neutral spine at all times. Slowly twist your body from one side to the other. That's one rep. Repeat 15 times and do three sets.
Assume a pushup position with your arms completely straight. Your body should form a straight line from your shoulders to your ankles (A). Lift your right knee toward your left elbow (B), lower, then raise your left knee toward your right elbow. That's one rep.