Get down on all fours with your wrists under your shoulders. Place a folded blanket under your wrists if you need support, or lower onto your forearms. To get into Plank position, straighten one leg at a time behind you, balancing on the balls of your feet, abs pulled up and in, and head in line with your spine (i.e., neither straining up nor hanging down).
Elongate your spine and press into the balls of your feet. Inhale, then exhale as you draw your right knee in toward your chest, contracting the muscles in your lower belly (shown). Return to Plank position, then switch legs. Alternate knee tucks for 8 to 20 reps.
Benefits: Increases stamina and strength, especially in your deep abdominal and back muscles.