A common mistake when performing this exercise, referred to as a plank, is to drop the hips too low and sag in the lower back area. This compresses the spine and makes the exercise less effective. When you practice this move, think of using your abdominal muscles to hold the pelvis up.

1 Begin on all fours with your wrists directly below your shoulders, fingers facing forward. Curl your toes under and, one leg at a time, straighten your legs behind you until you are holding the top of a push-up position. Press your heels back and slide your shoulder blades down and away from your ears. Keep your hips and shoulders squared to the floor as you raise your left leg to hip height.

2 Press down firmly into the heels of your hands and pull in your abdominals as you bend your left knee toward your left elbow. Pause for three to five seconds, return your foot to the floor behind you and repeat on the other side, lifting your right leg. Repeat twice on each side.

This article originally appeared in the Los Angeles Times in the "Good Form" feature of the Health Section. "Good Form" is updated with a new Karen Voight article each Monday.