A person uses seven muscles to do a single sit-up, according to azcentral.com. These muscles used for sit-ups are located in the stomach, pelvis and legs.
The rectus abdominis is the main muscle targeted when doing sit-ups. This muscle starts at the pelvis and connects to the fifth, sixth and seventh ribs. Other muscles like the iliopsoas, tensor fasciae latae, rectus femoris, sartorius and obliques assist the rectus abdominis. The last muscle used is the tibialis anterior, which stabilizes the different muscle groups during a sit-up. The combination of all seven muscles is what allows a person to complete one sit-up.