The insertion of the iliopsoas as a design favouring lateral rather than medial rotation at the hip joint.

A study of cadaveric material undertaken to investigate a possible anatomical basis for the nature of rotary influence of the iliopsoas at the hip joint shows that the winding course around the femoral neck with extension of tendon insertion towards the greater trochanter, and the spiralling course of the tendon both appear to be a design for causing external rotation during muscular contraction by a process of unwinding around the neck and untwisting of the spiral tendon under tension.