The hip is a ball-and-socket joint where the femur attaches to the pelvis in a socket called the acetabulum. The joint is surrounded by articular cartilage, a rubbery substance that acts a shock absorber and facilitates ease of movement. Ligaments surrounding the acetabulum and femoral head work to stabilize the hip. A tendon called the iliotibial band connects the hip muscles to the lower leg. Muscles in the buttocks keep the pelvis stable while walking. Muscles in the front of the hip joint include the iliopsoas, a deep muscle which runs from the lower spine to the inside edge of the pelvis, and the rectus femorus, which operates as part of the quadriceps.