Woman doing plank exercise on mat

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Must-Know Exercise 3: The Plank
Why: Have you ever felt a painful twinge in your abs while getting out of your car? This may be because you've been neglecting your core. The plank uses all the important core muscles (unlike crunches, which focus on the lower abs).
Do: Start on your knees with your elbows on the floor directly under your shoulders, your forearms on the ground, your palms flat. Slowly rise up onto your toes and straighten your legs. Try not to drop your head or raise your backside, and relax your shoulders away from your ears. Hold for 10 to 30 seconds; then lower yourself back down. To make these easier, stay on your knees.
Don't: As people focus on holding this pose, their form often goes out the window, says Matthews. Prevent the belly from sagging by tightening your core as if you're expecting a punch in the stomach. "That engagement also helps with spinal stability," she says.