HIP AND THIGH
The hip joint is a ball-and-socket type joint and is formed where the thigh bone (femur) meets the pelvis. The femur has a ball-shaped head on its end that fits into a socket formed in the pelvis, called the acetabulum. Large ligaments, tendons, and muscles around the hip joint hold the bones (ball and socket) in place and keep it from dislocating.
Normally, a smooth cushion of shiny white hyaline (or articular) cartilage about 1/4 inch thick covers the femoral head and the acetabulum. The articular cartilage is kept slick by fluid made in the synovial membrane (joint lining). Synovial fluid and articular cartilage are a very slippery combination—3 times more slippery than skating on ice and 4 to 10 times more slippery than a metal on plastic hip replacement. Synovial fluid is what allows us to flex our joints under great pressure without wear. Since the cartilage is smooth and slippery, the bones move against each other easily and without pain.
The stability of the hip is increased by the strong ligaments that encircle the hip (the iliofemoral, pubofemoral, and ischiofemoral ligaments). These ligaments completely encompass the hip joint and form the joint capsule. The iliofemoral ligament is considered by most experts to be the strongest ligament in the body.
The muscles of the hip, thigh and lower back work together to keep the hip stable, aligned and moving. The hip muscles are divided up into three basic groups based on their location: anterior muscles (front), posterior (back), and medial (inside). The muscles of the anterior thigh consist of the quadriceps (or quads): vastus medialis, intermedius, lateralis and rectus femoris muscles. The quads make up about 70% of the thigh’s muscle mass. The main functions of the quads are flexion (bending) of the hip and extension (straightening) of the knee.
The gluteal and hamstring muscles, as well as the external rotators of the hip are located in the buttocks and posterior thigh. The gluteal muscles consist of the gluteus maximum, gluteus medius, and gluteus minimus. The gluteus maximus is the main hip extensor and helps keep up the normal tone of the fascia lata or iliotibial (IT) band, which is the long, sheet-like tendon on the side of your thigh. It helps with motion of the hip, but perhaps more importantly, acts to help stabilize the knee joint.
Gluteus medius and minimus are the main abductors of the hip —that is, they move the leg away from the midline of the body (using the spine as a midline reference point). They also are the main internal rotators of the hip (i.e. turn the foot inwards). The gluteus medius and minimus are also important stabilizers of the hip joint and help to keep the pelvis level as we walk.
The tensor fascia lata (TFL) is another abductor of the hip, which, along with the gluteus maximus, attaches to the IT band. The IT band is a common cause of lateral (outside) hip, thigh, and knee pain.
The medial muscles of the hip are involved in the adduction of the leg i.e. bringing the leg back towards the midline. These muscles include the adductors (adductor magnus, adductor longus, adductor brevis, pectineus, gracilis).
The thigh has three sets of strong muscles: the hamstring muscles in the back of the thigh, the quadriceps muscles in the front, and the adductor muscles on the inside. The quadriceps and hamstrings work together to straighten (extend) and bend (flex) the leg. The adductor muscles pull the legs together.