The plank is an isometric exercise that targets your abdominal and back muscles, and it's used for developing core strength and balance. It's also a versatile exercise with a few key variations, such as the side plank with either a bent leg or a straight knee. Isometric exercises develop muscles by engaging them in a prolonged held position, meaning there is less wear and tear on joints such as the elbow and wrist.
Stiff as a Board
To perform a plank, first lie on the floor or an exercise mat, stomach down, elbows at your sides and under your shoulders. You should be resting on your forearms, fingers pointing away from your head, palms down. Holding in your abdominal muscles, flex your thighs and push your heels out, with your feet resting on tiptoes. Slowly lift your body from the floor, keeping your back and head straight. Breath normally and hold the position for as long as is comfortable. When you're ready, slowly lower yourself back to the floor.
Variation on a Theme
A side plank with a straight leg is performed by lying on your side, resting on your elbow and forearm for support, with your hip and thigh resting on the floor. From this starting position, straighten your spine, lifting your hip off the floor, and hold for as long as is comfortable. When ready, return to the starting position. If you bend your knees when performing this exercise, you'll be performing a side plank with a bent knee.
Abs of Steel
The main muscles contracted by the plank are the rectus abdominis and the hard-to-reach transversus abdominis. These are the muscles in your belly, and the muscles that sit in between your belly and your side muscles. Stabilizing muscles contracted include the obliques in your waist, your hip flexors, your quads located in your thighs, the pectoralis major in your chest, and a group of muscles covering your ribcage called serratus anterior, or "boxer's muscle."
Always consult your doctor before embarking on any physical exercise. Seek a qualified personal trainer or gym instructor to take you through any exercises like the plank before trying it.