You heard time and again the classic yoga pose, Paripurna Navasana (Boat Pose), described as the perfect ab toning move. But is it really? Well, that all depends on how it is taught. For example, some teachers will describe the asana as a pose that one performs while balancing on the “sitting bones” (ischial tuberosities). So it is natural to give a cue like, “roll forward towards your sitting bones to find the point of balance.” But if you follow this cue you will only use your abdominal muscles to stabilize your torso (or your back extensors, depending on the shape of your lumbar spine and the distance between your back and the floor and the relative length at which your muscles most like to work). Rolling forward also shifts most of the work into your Psoas (hip flexor). Once you’re there if you are attempting to “open your chest” and you don’t have the awareness or articulation to extend your thoracic spine versus your lumbar, you may be placing your rectus abdominus into an elongated shape. Since muscles don’t like working on very long or very short lengths (for most of us, at least, unless we have been practicing otherwise) this actually hinders that muscle’s ability to do work.
For an example of what this looks like, check out the Yoga Journal model’s demo and their instructions. There is no denying that her strength is obvious and the abs will work to keep her stabilized. But the majority of the work is in her hip flexors and not her abdominals proper.
Now check out this teacher’s demonstration. He is in a more obviously rounded shape than the previous one. Here he is rolled back, so that his weight is moving off his “sitting bones” and moving onto his sacrum. His lower back is flexed, which makes him look “rounded.” This shape actually puts the work in the rectus abdominus and internal and external obliques. He looks like he is just performing an extra hard “crunch;” the front of the ribs and the pubic bones are moving towards one another. His abs are contracting and he’ll “feel the burn” in them much more as he has to work harder to hold this position with the added weight of his legs attempting to pull him out of the rounded crunch shape. In this “boat” you are still using your hip flexors to lift the legs, but now the abdominals have work to harder. For some people it may be harder to extend through the thoracic spine, but this is truly the ab defining move.
One is not necessarily better than the other, but people who have tight hips will only get tighter hips by practicing the first boat as opposed to strengthening the abs if they practice the second. So beware of the cues given by less discriminating teachers