Long before Pilates teachers began reminding Americans to tone our abdominal, lower back, and pelvic muscles, Indian yogis created poses to strengthen them. Even though yoga seems to be everywhere these days, people who haven't tried it often don't realize how well it builds strength, not just flexibility. When I talk with one of these folks, I often think I should just ask him to try Paripurna Navasana (Full Boat Pose). He'd quickly discover how much it works his quadriceps, abdominals, back muscles, and hip flexors (the muscles that draw the front of the thigh and the front of the torso toward each other). Of course, he'd discover that Navasana requires a good bit of flexibility, too.
In Paripurna Navasana, your torso and legs form a V (like the prow of a boat when you look at it head-on). Your arms reach straight across toward your knees, parallel to the floor, like the deck of a ship on calm seas.
To sustain this V-shape, your muscles must hold the weight of your torso and legs up against the pull of gravity. Strength in your iliopsoas muscle, a hip flexor, is a key to resisting gravity in this pose. The psoas portion of the iliopsoas originates on the sides of the lumbar (lower back) vertebrae, and the iliacus portion originates on the inner bowl of the pelvis; together, they run across the floor of the pelvis and attach to the inner rear surface of the upper femur (thighbone). When the iliopsoas contracts, it pulls the thigh and torso closer together. Once you're in Paripurna Navasana, the muscle continues to contract isometrically, working but not changing length. It acts like a wire cable that runs between the two sides of a ship's hull, keeping them from bulging out.
Along with the iliopsoas, your abdominal and back muscles also contract strongly in Navasana. These sets of muscles work in opposition to each other, and ideally that work holds your torso in a straight line from hip to shoulder to ear in Paripurna Navasana.
To get a sense of how your abdominals work in this pose, sit toward the front edge of a chair, drawing your posture up straight. About two to three inches on either side of your navel, press your fingers into your abdomen. Then gradually lean your torso toward the back of the chair without touching it. You should feel your abdominal muscles contract to help your hip flexors hold your torso up against gravity.
Since the abdominals flex your spine, pulling the pubic bones and the front of the rib cage closer together, they will pull the torso into a C-shaped slump unless you oppose their action with your erector spinae, the long muscles that run up each side of the back, parallel to the spine. You have to engage your erector spinae strongly in order to cancel out the slump and create the straight torso position of Paripurna Navasana.
Finally, all four quadriceps muscles must work hard to straighten your knee. In Paripurna Navasana, the challenge to the quads is so intense that some practitioners may feel them cramping. You may need even more effort from your quadriceps if you have tight hamstrings, because the hamstrings directly oppose the actions you're trying to do with your quads. Here's where the primary flexibility challenge of the pose comes into play.
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