Want a killer core? Ditch the sit-ups and plank it up instead.

A strong core forms a strong foundation for health and fitness. Most people associate core strength with endless sets of crunches, but those only work the muscles on the front and sides of the abdomen. If you want to target all of the muscles of the core and get more defined abs, opt for the plank.

Planks help train muscles to stabilize the spine and pelvis. They can improve posture and reduce back pain—and because planks work more muscles, they also torch calories.

Ready to plank it up? There are endless variations of this effective exercise:

Forearm Plank: Assume a plank position with forearms on the ground and elbows directly beneath shoulders. Hold for 30 seconds to one minute.

Side Forearm Plank: Lie on your side with your forearm on the ground beneath your shoulder. Lift the hips off the ground so that your torso is straight from the ankle to shoulder. Hold for 30 seconds to one minute.

One-Arm Forearm Plank Reach: Assume a forearm plank position. Lift one arm and reach and hold it in front of your body for 15 seconds, then switch. Want to make it harder? Lift your opposite leg simultaneously as you reach forward with an arm.

Stability Planks: Use a stability ball or a BOSU ball flipped upside down. Place hands on the ball and feet on the floor and hold the plank.

Decline Planks: Place your feet on a stability ball and your hands on the floor and hold the plank. Make it harder by lifting one arm.

Climbing Planks: Assume a full plank position in front of a low bench. Move one hand up to the bench, followed by the other, then walk each hand back to the floor.

Walkouts: Assume a full plank position with hands wider than shoulders. Walk your hands as far forward as possible and then back to the start position.

Full to Forearm: Start in a full plank. Drop one forearm to the floor, followed by the other. Then place one hand on the floor, followed by the other. Continue.

High to Low: Assume a full plank position. Hold it for 30 seconds. Lower to a low push-up position and hold for 10 seconds before pushing back up to a full plank.