Now let’s move on to look at the muscles which produce flexion at the hip joint. There are four, two that act only at the hip, and two that act at the hip and also at the knee. The first two are the most important hip flexors - they’re called iliacus, and psoas major.
Iliacus arises here from the iliac crest, psoas major from all the way up here, on the lumbar spine.
Here’s psoas major. Psoas major arises from the transverse processes of all five lumbar vertebrae, and from the sides of the intervertebral discs and the adjoining vertebral bodies. We’ll see its insertion in a moment.
Here’s iliacus. Iliacus arises from almost all of the inner aspect of the wing of the ilium.
As they pass downward together, iliacus and psoas major pass over the superior pubic ramus and under a structure here that we haven’t been introduced to yet, the inguinal ligament.
The two muscles pass downward and backward, and insert together down here, on the lesser trochanter. Contraction of the iliacus and psoas major produces flexion of the hip joint.
When the limb is free to move, flexion brings the thigh forward. When the limb is fixed, as it is here, flexion of both hips brings the body upright.
The other two muscles which help in hip flexion are rectus femoris, and sartorius. They’re more important for their actions at the knee, than for their actions at the hip. We’ll look at them briefly here, and in more detail in the next section. Rectus femoris is part of a huge muscle with four heads called quadriceps which we’ll see in a moment.
First, let's briefly review the muscles we’ve seen already: psoas major and iliacus, pectineus, adductors brevis, longus, and magnus, and gracilis. Now let’s add quadriceps to the picture.
All this is quadriceps. It’s the main muscle that extends the knee. The only part of quadriceps which acts as a hip flexor is this part, rectus femoris. Its the only part that arises from above the hip joint, which is here.
Rectus femoris arises by two heads, from here and here, just above the acetabulum. Its final insertion, along with the other three heads of quadriceps, is right down here on the tibia. Rectus femoris is quite a weak hip flexor.
Now we’ll add the last hip flexor, sartorius. Sartorius is a very long narrow muscle that lies outside all the others. It runs in a spiral, starting here on the anterior superior iliac spine, and ending up all the way down here, on the tibia. Sartorius helps to flex the hip. It can also produce lateral rotation at the hip.