The hip joint is a ball and socket joint: head and socket fit together precisely. The hip socket is part of the pelvis. The hip head is the ball of the joint and is part of the femur. Both the socket ('acetabulum') and head are covered with cartilage that acts as the joint's sliding layer. In combination with the joint (lubricating) fluid, this ensures that the hip joint moves almost without friction. At the edge of the hip socket is a cartilage ring ("labrum"), which increases the bearing surface of the hip socket but mainly functions as a sealing ring to keep the synovial fluid in the joint. Around the hip joint is a very strong joint capsule. This ensures that the head remains in the socket. On the inside of the capsule is a thin layer of mucous membrane that produces the synovial fluid and lubricates the joint. Surrounding the hip joint are many muscles.
In addition to moving the leg, these actively contribute to the stability of the joint. The main hip muscle groups are: extensors, abductors and exorotators. Extensors are upper leg extensors. Especially the large gluteus muscle or gluteus maximus. You use the extensors particularly when rising from a seated position such as rising from a chair, car or toilet. The abductors swing the upper leg outward. Especially the middle gluteus muscle or gluteus medius. The abductors stabilize the pelvis when walking. A weakness of these muscles results in a typical and annoying waddling gait pattern (Trendelenburg gait). The exorotators (which turn the upper leg outward) a.o. piriformis muscle. The exorotators also stabilize the hip.