A powerful flexor of the thigh at the hip joint (psoas major) and a weak flexor of the trunk and lumbar spinal column (psoas minor). Psoas is derived from the Greek ""psoa"", the plural meaning ""muscles of the loin"". It is a common site of infection manifesting as abscess (psoas abscess). The psoas muscles and their fibers are also used frequently in experiments in muscle physiology. [MeSH]