The Boundary Between a Sit-Up and a Crunch

The sit-up and crunch are commonly performed exercises that target the rectus abdominis (RA) muscles, a.k.a. the six-pack muscles. This makes sense because both exercises necessitate trunk flexion, which is a primary action of the RA.

When the RA contracts and shortens during a sit-up or crunch, it pulls its insertion points on the rib cage toward its origin points on the pelvis and curls the trunk upward. As the trunk curls upward, a position is reached where the hip flexors must kick in to pull the trunk any higher off the floor. This point of active hip flexion is the boundary between a crunch and sit-up. In other words, a crunch is the beginning portion of a sit-up that involves trunk flexion only, whereas a sit-up requires both trunk and hip flexion.