The action of the iliopsoas muscle in the newborn.

The standard anatomical texts are unanimous that the primary action ofthe psoas muscle is flexion of the hip joint. At the same time it is usually credited with some secondary rotary action, although there is less agreement about the direction in which this rotation occurs. Basmajian (1958) reviewed the literature in relation to this long-standing controversy and found opinion to be almost equally divided as to whether the secondary action was one of medial or of lateral rotation at the hip. Most of the English texts favour medial rotation (Grant 1948, Gray 1949, Cunningham 1956, Last 1963). It is usually pointed out that although the posterior and medial location of the lesser trochanter might suggest that the pull of the muscle would result in lateral rotation, the axis of rotation of the femur passes through the head and the condyles of the bone and not along its long axis (Fig. 9) so that it lies medial to the insertion of the tendon. Shortening of the muscle must therefore give rise to medial rotation, although it is unlikely that such movement will be very powerful. Basmajian (1958) attempted to assess the functional role of the iliacus muscle by electromyography but he was