The pain results because the psoas borders the peritoneal cavity, so stretching (by hyperextension at the hip) or contraction (by flexion of the hip) of the muscles causes friction against nearby inflamed tissues. In particular, the right iliopsoas muscle lies under the appendix when the patient is supine, so a positive psoas sign on the right may suggest appendicitis. A positive psoas sign may also be present in a patient with a psoas abscess.