The iliopsoas is a pretty cool muscle.
It’s one of the most complex muscles in the body, and it is the only lower back and hip muscle to attach to the front of the spine.
In fact, the iliopsoas is actually two muscles in one: the psoas muscle and the iliacus muscle.
The psoas muscle attaches along the lumbar spine and the intervertebral discs then descends obliquely to attach at the upper inner thigh bone. The iliacus muscle attaches to the upper two-thirds of the iliac fossa then descends to join the psoas major tendon, with some of its fibers attaching directly to the femure near the lesser trochanter.
The primary function of both of these muscles is hip flexion. In other words, these muscles work to lift the knee and take your next step while walking.