(3.56)

Now let’s move on to look at the muscles that produce movement at the hip joint. There are five muscle groups to look at: the short rotators, the flexors, the adductors, the abductors, and the extensors. We’ll look at the six short rotator muscles first, because they lie deep to all the others.

The short rotator muscles arise from four different bony surfaces, the inner surface of the sacrum, both the inner surface and the outer surface of the area around the obturator foramen,and from the outer surface of the ischium. They all converge on this small area on the back of the femur, just medial to the greater trochanter.

The one that arises from the sacrum is piriformis. Here it is. Piriformis arises from here on the sacrum. It leaves the pelvis by passing through the greater sciatic foramen, along with the sciatic nerve. We’ll go right round to the back to see where it comes out. Here’s piriformis emerging. It inserts here, high up on the medial aspect of the greater trochanter.

Piriformis is an important muscle to understand, because it’s closely related a very important structure, the sciatic nerve. Now we’ll look at the two obturator muscles, obturator internus and externus. They lie on each side of the obturator membrane.

Here’s obturator externus. It arises from the obturator membrane, and from the adjoining edge of the ischio-pubic ramus.

Obturator externus passes backwards just beneath the neck of the femur. Here’s its tendon, passing laterally, to reach its insertion here, below piriformis.

Next we’ll see obturator internus, which comes around the back of the ischium here, from inside the pelvis. We’ll go around this way to see where it arises. Here’s obturator internus. It arises from this wide area on the ilium and ischium, and from the obturator membrane. It leaves the pelvis through the lesser sciatic foramen. As it passes round the back of the ischium, obturator internus does a 90° turn.

As it makes the turn, it’s joined above and below by these two little muscles, gemellus superior, and gemellus inferior. They arise from the ischium here, and here. Along with obturator internus, they’re inserted here.

The last of the short rotators is quadratus femoris. Quadratus femoris arises from the ischial tuberosity here, and inserts on the femur here, on the intertrochanteric crest.

All these short rotator muscles have the same action: by pulling the back of the femur medially, they produce this movement: lateral rotation at the hip joint. When the foot is on the ground, the same muscle action produces what appears to be a different movement: rotation of the trunk to the opposite side. So contraction of these muscles has two possible effects, depending on whether the limb is free to move, or whether it’s fixed. As we’ll see, that’s holds true for every muscle that produces movement at the hip joint.