(4.54)

Now we'll move on to look at the four muscles that extend the hip. The first three, which are known collectively as the hamstring muscles, act at the hip and at the knee. The fourth one, gluteus maximus, acts only at the hip. We’ll come to it last because it lies outside the others and covers them up.

Here’s the back of the thigh as we saw it last. Here’s gluteus medius, here are all the short rotators, here’s adductor magnus, and here’s the back of quadriceps.

The three hamstring muscles all arise from the ischial tuberosity. Here they are. Two of them run to the medial aspect of the leg, one runs to the lateral aspect. The lateral one is biceps femoris. The two medial ones are semimembranosus, and semitendinosus.

Here’s semimembranosus by itself. It has a long flat membrane-like tendon of origin, which arises from here on the ischial tuberosity. It’s inserted here on the back of the tibia.

Here’s semitendinosus, lying behind semimembranosus. It has a long cord-like tendon of insertion. It arises from here, next to semimembranosus. It inserts down here on the medial aspect of the tibia, close to two other muscles that we’ve seen already, sartorius and gracilis.

Now that we’ve seen the two "semi" muscles, we’ll add biceps femoris to the the picture. Biceps femoris has two heads of origin, a long head, and a short head.

The long head arises from here on the ischial tuberosity, along with semitendinosus. The short head arises from almost the whole length of the linea aspera, and from this supracondylar line.

The two heads of biceps femoris join together, forming a tendon that runs down behind the lateral aspect of the knee, then runs forward to insert here, on the head of the fibula.

We’ll be taking another look at the hamstring muscles, and their actions at the the knee, in the next section. We’ll look at their action at the hip in a minute, but before we do that we’ll add the last and largest of the hip extensors, gluteus maximus, to the picture.

Here’s the upper end of the hamstring muscles, here, overlying them, is gluteus maximus. It’s a thick, flat sheet of muscle. Gluteus maximus arises from here on the back of the ilium, and from the side of the sacrum, and from the sacrotuberous ligament.

The upper three quarters of gluteus maximus inserts into the ilio-tibial tract. The lower one quarter of gluteus maximus passes more deeply, and inserts here on the back of the femur, on the gluteal tuberosity.

Now let’s look at the actions of the hip extensor muscles, starting with the hamstring muscles. Contraction of the hamstring muscles can produce both knee flexion, and hip extension. When knee flexion is held in check by the action of quadriceps, the hamstrings just produce extension at the hip, which is the action that propels us forward in normal walking.

Gluteus maximus isn’t used in the gentle action of normal walking. It comes into play when a powerful action is needed, especially an action that opposes the force of gravity. The action of gluteus maximus extends the hip from a position of full flexion, as in climbing stairs, or rising from a squatting or sitting position. The same action, balanced against the force of gravity, controls the rate of hip flexion, as we sit down.

Gluteus maximus is one of the two big anti-gravity muscles of the lower extremity. The other one, acting at the knee, is quadriceps as we’ll see in the next section.

Gluteus maximus has yet another set of actions when the lower extremity is fixed and upright. Then the action of gluteus maximus pulls the back of the pelvis downward, raising the body from a forward bend at the hips, or, when balanced against gravity, controlling the rate of bending forward.

Now we’ve looked at all the muscles that produce hip movement. As we’ve seen, several of them also have important actions at the knee, and we’ll be looking at these a second time in the next section.