Absolutely Everything You Need to Know About Planks

Whether you call them "commandos," "plank up-downs," or "up-down planks," this moving plank variation taps into your high- and low-plank skills. You'll feel this one in your shoulders as well as your core. (If this hurts your forearms or elbows, try adding a towel underneath them for extra padding.)

• Start in a high plank position with your feet shoulder-width apart.
• Lower your right elbow to the floor directly below your shoulders, then do the same with your left, coming into an elbow plank.
• Place your right hand on the mat, and straighten your right elbow. Do the same on the left to return to a high plank. Throughout the movement, try not to leg your hips pike up or rock from side to side.
• That's one rep. Repeat, starting the next rep with your left arm. Do three sets of 10 reps alternating arms, resting for 30 seconds to one minute between sets.