Start in a forearm plank with your elbows under your shoulders and your body in a straight line. Shift your weight onto one forearm and extend the other arm straight out to the side like a wing, holding it level with your shoulder. Bring it back under you and extend the other arm. With one support gone, your entire midsection has to fight to keep your hips square to the floor - the wing is easy, staying level while it's out is the exercise.

Works: abs, obliques, shoulders, upper back

Breathing: Exhale as you extend the arm out, inhale as you bring it back under you.

Common Mistakes: Rolling the hips open toward the extended arm, letting the hips sag or pike to compensate, and rushing the extensions instead of holding each one under control.

Make Easier: Drop to your knees and practice the arm extension from there, or build up with Plank Arm Raises to get used to losing one support at a time.

Make Harder: Hold each wing out for a 2-count, or bring your feet together to shrink your base even further.