The sit-up is an abdominal endurance training exercise commonly performed to strengthen and tone the abdominal muscles. It is similar to a crunch (crunches target the rectus abdominus and also work the external and internal obliques), but sit-ups have a fuller range of motion and condition additional muscles. Sit-ups target the hip flexors, rectus abdominus and also work the iliopsoas, tensor fasciae latae, rectus femoris, sartorius, and, to a very small degree, the obliques.