The plank works the entire core, as well as your upper and lower body muscles. Lie face down on your stomach. Lift your body off the floor with your forearms (keeping your elbows at 90° degrees) and your toes. Keep your body position straight as you can (without arching your back). Hold position for 30-60 seconds. If this is too easy, lift one foot in the air for added difficulty.
You can also do a variation of the plank on a Swiss ball in two ways:
(1) Place your forearms on the Swiss ball with your feet on the ground or,
(2) Place your feet on the ball with your forearms on the ground. Keep your abs and glutes tight without arching your back. Hold the position as long as you can. For a more advanced workout, move the ball slightly from side to side.