The captain’s chair helps you targets the abs by performing a vertical crunch against the force of gravity. You may have observed people cranking out repetitions of this exercise with the knees bent and with the legs straight. The basic form has your knees bent, but straightening the legs increases the level of difficulty by engaging your core more than your legs.
The captain’s chair, aptly named for its appearance, forces your abdominals to work directly against the pull of gravity. The most visible muscle at the surface of your abdomen, the rectus abdominis, is home to the coveted “six pack.” The captain’s chair directly works this large, flat muscle with the assistance of the oblique muscles.
Hinging at the knees and hips and lifting the legs towards the chest activates the rectus abdominis into a concentric contraction by forcing the spine to curl into a vertical crunch. However, the abdominals are only activated if the spine curls. Keeping your knees bent makes it easier to curl the spine as you raise your legs. The knees must come above the level of the hips in order to make sure all your efforts are going toward building your abdominals. Without curling the spine, the abdominals won’t engage fully and lifting your knees will become more of a leg exercise as you feel the burn in your quads and hip flexors rather than your abs. Keeping the pelvis tucked under and drawing the knees as high up toward the chest as you can will make sure all your efforts go toward building your abs.
Performing the same exercise with the legs straight increases the difficulty of the exercise. Physics steps in here to up the ante with the principle of leverage. Our joints work on a lever system, manipulating force and resistance by moving levers -- bones -- around the pivot point of your joint, or fulcrum. The further away the load is from the fulcrum, the greater the torque and resistance. Technical jargon aside, keeping the legs straight adds more resistance to the movement. Be sure you hinge from the waist and continue to curl the spine the same as when your knees are bent to ensure the rectus abdominis does the work.
The captain’s chair pits you full on against the force of gravity and requires a certain level of existing core and leg strength. If you’re just starting out with abdominal and core exercises, you may want to start with more basic exercises like planks and balance ball crunches to develop basic core strength. Taking on the captain’s chair without using proper form can potentially crank your lower back and hip flexors and end up doing nothing for your abs. However, if you’re already cruising in the chair, challenge yourself further by holding a medicine ball between the knees or ankles, or twisting at the waist as you hinge from the hips to target the obliques on either side of the abs.
Jullie Chung writes regularly for various websites. She is a nationally certified fitness trainer and performance enhancement specialist through the National Academy of Sports Medicine and trains regularly in yoga, flatwater kayaking, boxing and mixed martial arts. An avid outdoor fan, she regularly hikes, climbs and trail runs.