If you really want to challenge the core muscles, try this plank on a stability ball. Maintain the position as long as you can -- up to one minute -- for maximum muscle toning.
Get into plank position by supporting your body weight with your chest and forearms on the ball and your toes on the floor.
Lift your chest off of the ball so your upper body weight is supported by your forearms.
Keep your abdominals contracted and your back straight, eyes ahead of you.
Hold this position for as long as you can, building up to 1 minute.