For decades, people have sworn that the best way to get strong abs is to do sit ups. The logic behind it is, if you’re able to lift yourself up from the floor using your abdomen muscles, you must be making them stronger. Well, physicians at Harvard Medical School think different.

They believe that doing sit-ups do not target all the muscles you need to target to achieve a six-pack. Which is probably why a lot people feel frustrated with their lack of results with it. Not only is it not the most effective approach, but it is also thought to be a recipe for injury.

The better way to achieve a strong core is to do planks. This is no mystery for some fitness buffs and yogis. Planks engage all of your core muscles, when done correctly. But, you have to make sure you’re not straining your back or wasting your time with ineffective planks. In order to make sure you’re doing it right, you should hold yourself on your hands and toes in a pre-push up position.

Planks work so well because they challenge muscles all across your abs from the front, sides and back. You’ll have a better chance of achieving definition with that.

To hold the plank properly, start out by bending your elbows slightly, directly under your shoulders with hands clasped together. Keep feet hip-width apart. Tighten your abs while you hold your pose. Hold the position for at least a minute. As you get stronger, you’ll be able to hold yourself up longer.

READ MORE: Harvard doctors say this neglected move is a better way to get strong abs than sit-ups