The hip is a ball-and-socket joint, one of only two in the human body (the shoulder is the other). A rounded ball at the top of your thigh bone (the femur) fits snugly into a cup-shaped socket in your pelvis called the acetabulum. This design allows movement in multiple directions while keeping the joint stable enough to bear your full body weight.
How a Ball-and-Socket Joint Works
Ball-and-socket joints belong to the synovial joint family, meaning they have a fluid-filled capsule that allows free movement between bones. What makes them special is their shape: a spherical head on one bone rotates inside a concave socket on another. This geometry permits motion along all three planes, forward and back, side to side, and rotationally. No other joint type offers that full range.
The hip’s formal anatomical name is the acetabulofemoral joint, named after the two bones that form it. The acetabulum is the deep, circular socket on each side of the pelvis, and the femoral head is the ball-shaped top of the thigh bone that sits inside it.
Why the Hip Is More Stable Than the Shoulder
The shoulder is the body’s other ball-and-socket joint, but the two behave very differently. The shoulder socket (the glenoid cavity) is shallow, giving it a wider range of motion at the cost of stability. That’s why shoulder dislocations are relatively common. The hip socket, by contrast, is deep. The acetabulum covers most of the femoral head, which makes the hip far more resistant to dislocation but somewhat less mobile than the shoulder.
This tradeoff makes sense functionally. Your shoulder needs to reach, throw, and rotate freely. Your hip needs to support your body weight during walking, running, and jumping, tasks where stability matters more than extreme flexibility.
Structures That Keep the Hip in Place
The ball-and-socket design alone doesn’t explain the hip’s stability. Several layers of tissue reinforce it. A ring of tough cartilage called the labrum lines the rim of the acetabulum, deepening the socket and creating a seal around the femoral head. This seal helps keep joint fluid inside the joint, ensuring smooth, low-friction movement. It also helps hold the bones in alignment as you move.
Three major ligaments wrap around the outside of the joint capsule. The iliofemoral ligament sits at the front, shaped like a Y, and prevents your hip from extending too far backward. The pubofemoral ligament reinforces the front and bottom of the capsule, limiting excessive outward spreading of the leg. The ischiofemoral ligament spirals around the back of the joint, further resisting hyperextension and helping hold the femoral head firmly in its socket.
Inside the capsule, a thin synovial membrane produces synovial fluid. This fluid does two jobs: it lubricates the joint surfaces to minimize friction, and it delivers nutrients to the articular cartilage covering the bones. Since cartilage has no blood supply of its own, this fluid is its only source of nourishment.
Range of Motion at the Hip
Because the hip is a ball-and-socket joint, it can move in six directions: flexion (bringing your knee toward your chest), extension (moving your leg behind you), abduction (stepping your leg out to the side), adduction (crossing your leg toward the midline), and internal and external rotation (twisting your leg inward or outward).
Flexion has the largest range. CDC reference data shows adults aged 20 to 44 typically flex the hip to about 130 to 134 degrees. Extension is much smaller, around 17 to 18 degrees in the same age group. Both flexion and extension gradually decrease with age. Adults aged 45 to 69 average about 127 to 131 degrees of flexion and 13 to 17 degrees of extension. Children are notably more flexible, with flexion values reaching 131 to 141 degrees and extension around 26 to 28 degrees.
Common Problems With This Joint Type
The hip’s structure makes it vulnerable to specific types of injury and disease. Osteoarthritis is one of the most common, especially in older adults. Over time, the cartilage covering the femoral head and acetabulum wears down, leading to pain, stiffness, and reduced mobility. Because the hip bears so much load, it’s one of the joints most affected by this wear-and-tear process.
Hip dysplasia is a condition where the acetabulum is too shallow, leaving the femoral head poorly covered and prone to slipping out of place. It can be present from birth or develop over time, and in severe cases it leads to dislocations. Femoroacetabular impingement occurs when extra bone grows along the femoral head or acetabular rim, causing the bones to pinch against each other during movement. Labral tears often accompany impingement, since the abnormal contact can damage the cartilage ring lining the socket.
Fractures are another significant concern, particularly in older adults with osteoporosis. Weakened bone around the hip can break from a fall that would be harmless in a younger person. Bursitis, an inflammation of the fluid-filled sacs cushioning the outside of the joint, and muscle strains from overuse or sports are also common sources of hip pain.