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Sit-ups are so 2017.

If you're looking to tone your entire core and work your way to 6-pack abs, the physicians at Harvard Medical School say planks are the one move you should nail. So the next time you get ready to roll out a yoga mat and lie on your back, take the reverse approach and hold yourself on your hands and toes in a pre-push-up position.

Unlike crunches, which target only your abdominal muscles, planks recruit several groups of muscles along your sides, front, and back. And if you want a strong core — especially the kind that would give you 6-pack-like definition across your abs — you need to challenge all of these muscles, researchers write in a Harvard Medical School health report called "Core Exercises."

"Sit-ups or crunches strengthen just a few muscle groups," write the authors of the Harvard Healthbeat newsletter , which summarizes the report's key takeaways. "Through dynamic patterns of movement, a good core workout helps strengthen the entire set of core muscles you use every day."

Why planks are superior for core strength

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Although sit-ups are a good basic move to help tone your abdominal muscles, planks are less likely to cause injury and better for building a stronger core.

When you lay down to do a sit-up, your back gets pushed against the floor. When you pull your body up into a crunch, you're also putting strain on a group of muscles called the hip flexors. These muscles run from your thighs to the lumbar vertebrae in your lower back. When they get tight, they can yank on your lower spine, causing pain or discomfort in your lower back.

And just like crunches, planks don't require a single piece of equipment, meaning you can do them virtually anywhere.

How to plank properly

Start out by lying face-down with your legs extended and your elbows bent, directly under your shoulders, with your hands clasped, the folks at Health.com advise . Make sure your feet are hip-width apart while your elbows are shoulder-width apart. Tighten your abs and tuck your toes to elevate your body, keeping your forearms on the ground. Your body should be in a straight line from your head to your heels. To start, hold it for one minute.

As you get stronger, you can gradually build up to maintain the position for longer periods of time. For even more of a challenge, hold yourself on your palms rather than your forearms.