'muscles of the loins') is a long fusiform muscle located in the lateral lumbar region between the vertebral column and the brim of the lesser pelvis.

The iliopectineal bursa separates the tendon of the iliopsoas muscle from the external surface of the hip-joint capsule at the level of the iliopubic eminence.

One study using autopsy data found that the psoas major muscle is substantially thicker in men of African descent than in Caucasian men, and that the occurrence of the psoas minor is also ethnically variant, being present in most of the white subjects and absent in most of the black subjects.

[8] In addition, attachment to the lesser trochanter, located on the posteromedial aspect of the femur, causes lateral rotation and weak adduction of the hip.

[9] A hypertonic and inflamed psoas can lead to irritation and entrapment of the ilioinguinal and the iliohypogastric nerves, resulting in a sensation of heat or water running down the front of the thigh.

The appearance of a protruding belly can visually indicate a hypertonic psoas, which pulls the spine forward while pushing the abdominal contents outward.