The sit-up and the crunch are commonly performed exercises for targeting the rectus abdominis (RA) muscles, AKA 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 a crunch, it pulls its insertion points on the ribs and sternum 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 join the party in order to pull the trunk up any higher. This initial point of active hip flexion is the boundary between a crunch (AKA a trunk curl) and a sit-up. In other words, a crunch is just the beginning portion of a sit-up that requires trunk flexion only, whereas a sit-up involves trunk flexion plus subsequent hip flexion in order to pull the trunk higher off the ground.
