The hip is a very stable ball and socket type joint with an inherently large range of motion. The hip contains some of the largest muscle in the body as well as some of the smallest. Most people lack mobility due to a relatively sedentary lifestyle. Periods of prolonged sitting results in tightness of the hip flexors and hamstrings. Tightness in the muscles and ligaments can created joint forces that result in arthritis, postural problems, bursitis, and mechanical back pain.
The iliopsoas is another powerful hip flexor that begins in two distinct regions proximally. The iliacus has a broad origin, arising from the inner table of the iliac wing, the sacral alae, and the iliolumbar and sacroiliac ligaments. The psoas originates at the lumbar transverse processes, the intervertebral discs, and the adjacent bodies from T12 to L5, in addition to the tendinous arches between these points. Distally, the two large muscular bodies converge to become one distinct structure—the iliopsoas—and subsequently jointly insert at the lesser trochanter of the proximal femur. The nerve to the iliopsoas (i.e., the anterior division of L1 to L3) supplies the iliopsoas muscle.
The hip flexors help balance the posterior pelvic muscles. Three key muscles often become tight and shortened as a result of activities of daily living. These are the iliacus, psoas major, and the rectus femoris. The iliacus and the psoas major are often referred to as the iliopsoas because they share the same insertion at the lesser trochanter of the femur. The psoas minor inserts on the superior ramus of the pubis bone and mainly supports the natural lordotic curvature of the spine, but is only found in about 40% of the population. The psoas major originates on the anterior surface of the lumbar vertebrae and runs over the pubis bone and inserts into the lesser trochanter of the femur. This muscle not only helps to flex the hip, but also has an effect on the lordotic curvature of the lumbar vertebrae. The rectus femoris has a proximal attachment at the acetabulum and inserts into the tibial tuberosity. This long muscle plays a role in both hip flexion and leg extension (Figure 9-4).