WHY: The goal of the plank is to work on your core stability. Adding in the alternating leg lifts further challenges your core stability.
HOW: Assume the plank position on your elbows and toes. Keeping everything in your body tight and braced, slowly lift one leg off the ground. When you lift, your hips should not move. Pretend that if someone pushed you, you would not fall over! Lower your leg and then switch sides.
FEEL: You should feel all the muscles in your stomach working to hold this position. You may also feel your shoulder and chest muscles working.