A powerful muscle which arises from the front of the vertebral column in the lumbar region, and passes down, round the pelvis and through the groin, to be attached to the inner side of the thigh-bone not far from its upper end. The act of sitting up from a recumbent posture, or that of bending the thigh on the abdomen, is mainly accomplished by the contraction of this muscle. Disease of the lumbar spine may produce a psoas ABSCESS which lies within the sheath of this muscle and makes its way down to the front of the thigh.