Hip and Pelvis Anatomy and Physiology

The hip and the pelvis are two distinct but entirely interrelated parts of the human anatomy. The pelvis is a large semicircular bone complex that forms the base on which the torso and upper body are positioned. The pelvis, which is a rigid and inflexible portion of the skeleton, is built to provide a foundation of the movement of other parts of the anatomy, particularly the back and the legs. The pelvis also permits the weight of the entire upper body to be evenly distributed to the legs, which are connected to the pelvis through the hip joints.

The pelvis is comprised of three bones arranged in a ring: the ilium, which is formed in the shape of a wing, rising on each side of the pelvis; the ischium, which forms the middle portion of the pelvis; and the pubis, the bone at the base of the pelvic structure. The pelvis is connected to the skeleton of the upper body by way of the sacroiliac, a fused joint at the connection between the lower portion of the spinal column and the pelvic bones. The sacrum and the coccyx (the tailbone) are the bottom portion of the spine that make the connection to the pelvis; the presence of a ligament connecting the sacrum to the pelvis is not a typical joint, as the pelvic structure is capable of very little flexion or extension on its own. The pelvis also protects the lower organs of the abdomen, particularly those of the renal and intestinal tracts. Also important is the rigid and supportive structure of the pelvis, an essential aspect of the ability of the body to move dynamically through the legs. If the pelvis were less stable, the legs would not be able to generate either propulsion or their range of motion.

The hip joint is a structure of four bones, forming a ball and socket joint between the pelvis and the femur (thigh). The hip joint variously provides the body with stability for weight-bearing activities involving the legs, as well as mobility through the nature of the hip structure. The hip joint is also the mechanism by which the forces brought to bear upon the body are transmitted from the upper body, though the hip joints, to the lower legs.

The portion of the femur of particular importance to joint movement is the head of the femur, which fits into the acetabulum, the cup-shaped portion of the pelvis that creates the ball and socket hip joint. The stability of the hips in relation to the function of the pelvis is secured by a ligament that connects the femur to the acetabulum across the notch, or gap, created between the two bony surfaces. The snugness of the fit between the head of the femur and the acetabulum is achieved through the presence of the labrum, which covers its surface. Where the surface of the acetebulum makes contact with the head of the femur is a cartilage, a smooth, fibrous material that assists in the movement of the joint. The thinning or other reduction of this cartilage will result in various forms of arthritis. The acetabulum labrum is material that eliminates excess space between the bones of the joint, a circumstance that would tend to make the joint overly loose and inefficient in movement.

The hip joint is capable of a remarkable range of motion, due to the construction of the joint, supported through the presence of the four sets of muscles and connecting tendons that operate together with machine-like precision. The hip flexors, extensors, adductors, and external rotators combine to provide a 360° range of motion. The importance of the hip joint is not confined to the range of motion that it permits the upper leg, but also through the considerable muscular power and endurance that is delivered in concert with the motion.

The hip joint flexor supports the process of flexion, the movement of the hip joint that produces a bend, which helps propel the legs forward and upward. Extension is the hip action that straightens the leg. Rotation is the ability of the hip joint to direct the femur and the upper thigh through the 360° range of motion. Adduction is movement of the hip muscles that draws the femur and upper thigh toward the body; the adductors are important stabilizing muscles in running.

The hips and pelvis are directed through a number of nerve endings, the most important of which is the sciatic nerve. The pelvis is not capable of independent movement as it is not a joint. However, it is a structure with a reasonable degree of flexibility, absent the weakening of the pelvis of hip bones due to osteoarthritis or osteoporosis, two similar and degenerative bone conditions that can cause fractures. The female pelvis is slightly wider and shallower than that of the male, to facilitate the female in childbirth.

see also Back anatomy and physiology; Hip and groin injuries; Thigh and upper leg injuries.