Anatomy of a Sit-Up

The Sit-Up that we often do in this lineage of yoga - starting flat on the back with the arms overhead and legs straight, then sitting all the way up and touching the head to the knees - is more complex than it looks. It requires strength in two major muscle groups: those of the abdomen, specifically the rectus abdominis (6-pack) and the psoas, which crosses the hips.

Rectus Abdominis

Psoas

RECTUS ABDOMINIS
This muscle (pictured on the left), best know as the 6-pack, is a powerful muscle in your abdomen. It moves the ribs closer to the pelvis and vice versa. Anytime you do a sit-up motion, this muscle needs to work. Your ribs will get closer to your pelvis and your low back will round. If you try to keep your low back flat, or if it stays flat due to tightness or some other reason, focus on engaging your rectus abdominis.

When your rectus abdominis is fully engaged, your arms, shoulders and upper back will come off the floor even though your hips don't move. You will end up in a position like the picture above. Notice that the ribs are close to the pelvis and the spine is rounded, but the hips are still fully extended. Flexing the hips is the next step.

PSOAS
The other main muscle that we need to do a Sit-Up is the psoas. This muscle flexes the hips, drawing the thigh bone closer to the spine. You can see in the picture to left that the psoas weaves through the pelvis to connect the lower spine and the legs. 

This is the muscle that we use to sit up the rest of the way, after the spine is bent. From the point pictured above, all that needs to happen is for the hips to flex, lifting the entire upper body and pelvis into the final position, pictured below.

INHALE OR EXHALE
The question often arises, "should I inhale or exhale while doing a sit-up?" The answer is clear: You should exhale.

In a recent New York Times article about the remarkable abs of USA olympian Adam Rippon, Rippon's trainer Steve Zimm said, "Breathing is everything when it comes to abs. If you want ripped abs, you need to allow them to contract. Before every move, breathe out. Pull your belly button into the spine and continue breathing out as you’re contracting, so your abdominal wall is sinking into you."

Not surprisingly, Bishnu Ghosh agrees. In his book Yoga Cure he instructs: "Start from lying position with arms stretched beyond your head, exhale and hold your breath and then raise your upper body..."