The hip is a large ball-and-socket joint where your thighbone meets your pelvis. It’s the joint that lets you walk, run, sit down, climb stairs, and rotate your torso. Along with the shoulder, it’s one of the most mobile joints in the body, but it’s built to be far sturdier, bearing enormous loads with every step you take.
Bones That Form the Hip
Two structures create the hip joint. The first is the femur, or thighbone, the longest and strongest bone in your body. At its top end, the femur has a rounded head about the size of a golf ball. That ball fits into a deep, cup-shaped socket in the pelvis called the acetabulum. The acetabulum itself is formed where three pelvic bones (the ilium, ischium, and pubis) fuse together during childhood at a piece of growth cartilage.
The depth of the acetabulum is what makes the hip so stable compared to the shoulder, which has a much shallower socket. The ball of the femur sits deeply inside the cup, giving the joint a natural resistance to dislocation while still allowing a wide range of movement.
How the Hip Moves
As a ball-and-socket joint, the hip can move in almost every direction. You use it to flex your leg forward (bringing your knee toward your chest), extend your leg behind you, swing your leg out to the side, bring it back in, and rotate it inward or outward. A healthy hip can flex anywhere from about 80 to 140 degrees and extend roughly 20 to 40 degrees behind the body. All of these movements combine to let you do everything from walking in a straight line to pivoting during sports.
Cartilage, Labrum, and Ligaments
Bone doesn’t slide against bone inside a healthy hip. Both the femoral head and the inside of the acetabulum are covered with a layer of smooth articular cartilage that reduces friction and absorbs shock. Surrounding the rim of the socket is a ring of tough, flexible tissue called the labrum. The labrum does three important things: it deepens the socket for extra stability, it seals joint fluid inside the joint space, and it distributes pressure so that no single spot on the cartilage takes too much force. By keeping fluid pressurized against the cartilage surface, the labrum essentially acts as a built-in cushioning system.
Wrapping around the outside of the joint are three strong ligaments that reinforce the joint capsule. The iliofemoral ligament runs across the front (it’s one of the strongest ligaments in the body), the pubofemoral ligament covers the lower front, and the ischiofemoral ligament reinforces the back. Together they prevent the femoral head from slipping out of the socket, especially when you stand upright or lean backward.
Muscles That Power the Hip
Some of the body’s largest and most powerful muscles cross the hip joint. They work in opposing groups to produce movement in every direction.
- Flexion (lifting the leg forward): The iliopsoas, a deep muscle connecting the spine to the femur, is the primary hip flexor. The rectus femoris, part of the quadriceps on the front of the thigh, assists.
- Extension (pushing the leg back): The gluteus maximus, the largest muscle in the body, drives this movement. The three hamstring muscles on the back of the thigh work alongside it.
- Abduction (moving the leg outward): The gluteus medius and gluteus minimus, sitting on the outer surface of the pelvis, handle this. They also stabilize your pelvis every time you stand on one leg.
- Rotation: A group of small, deep muscles behind the hip joint rotate the leg outward, while several inner thigh muscles help rotate it inward.
Weakness in any of these groups, particularly the gluteal muscles, can change walking patterns and contribute to pain in the hip, knee, or lower back.
Forces the Hip Handles
The hip bears remarkable forces even during everyday activities. Measurements taken from sensors implanted directly inside hip replacements show that walking at a moderate pace loads the joint with roughly 280 to 410 percent of a person’s body weight. At a brisk walk or light jog, that jumps to about 550 percent. A stumble can spike forces as high as 720 to 870 percent of body weight. For a 150-pound person, that means the hip absorbs over 1,200 pounds of force during a stumble. The joint’s combination of deep socket, thick cartilage, labral seal, and muscular support is what makes this possible day after day.
Where Hip Pain Actually Shows Up
One reason hip problems get misdiagnosed is that pain from the hip joint often doesn’t feel like it’s “in the hip” the way most people picture it. Research mapping pain patterns in patients with confirmed hip joint problems found that the most common location for hip-related pain is actually the buttock, reported by 71 percent of patients. Groin pain appeared in 55 percent, and thigh pain in 57 percent. About one in five patients felt pain below the knee, and a small number even felt it down in the foot. Pain along the outer side of the hip, where many people point when they say “my hip hurts,” is often coming from tendons or a fluid-filled sac (bursa) rather than the joint itself.
If your pain is primarily in the groin or deep in the buttock and gets worse with activities like getting in and out of a car or putting on shoes, the hip joint is a likely source. Pain on the outer thigh that flares when lying on that side at night points more toward tendon irritation than a joint problem.
Common Hip Conditions
Osteoarthritis is the most frequent hip joint problem, caused by gradual breakdown of the cartilage lining. As the cartilage wears, the joint space narrows, bone spurs develop, and movement becomes stiff and painful. Labral tears, often from repetitive motion or structural differences in the shape of the socket, are another common source of hip pain, particularly in younger and more active people.
Hip fractures, usually from falls in older adults with weakened bones, are among the most serious hip injuries. Bursitis (inflammation of the cushioning sacs around the joint) and tendinopathy (irritation of the tendons, especially the gluteal tendons on the outer hip) round out the conditions that bring most people to a doctor.
When cartilage loss becomes severe enough that pain and stiffness limit daily life, total hip replacement is one of the most reliable surgical solutions in modern medicine. In the United States alone, over 140,000 primary hip replacements were performed in 2023, and the numbers are similar in Germany and other developed countries. The procedure replaces the worn femoral head and damaged socket lining with metal and plastic components, and most people return to walking within days.