What Bones Are in Your Butt? Anatomy of the Pelvic Girdle

The bones you sit on and feel beneath the flesh of your buttocks belong to the pelvic girdle, a ring of bone formed by two large hip bones at the front and sides and the sacrum and coccyx (tailbone) at the back. Each hip bone is itself made of three bones that fuse together during adolescence: the ilium, the ischium, and the pubis. The bony prominences you can feel when you sit on a hard surface are the ischial tuberosities, the lowest points of the ischium, which bear most of your seated body weight. But these “sit bones” are just the most noticeable part of a structure that does far more than give you something to sit on.

The Three Bones That Form Each Hip Bone

Each side of your pelvis contains what anatomists call the os coxae, or hip bone. It looks and feels like a single piece of bone in adults, but it starts life as three separate bones: the ilium, the ischium, and the pubis. These three gradually fuse together during childhood and adolescence.1PubMed. Ontogeny of the Human Pelvis Since you have a left and a right hip bone, that means six separate bones eventually become two.

The ilium is the broad, wing-shaped bone at the top. When you put your hands on your hips, your palms rest on the iliac crests, the curved upper edges of each ilium. The ilium provides a massive surface area for muscle attachment and forms the back part of the hip joint where it meets the sacrum.

The ischium sits at the bottom and back of the hip bone. Its most prominent feature is the ischial tuberosity, the thick, knobby projection that supports your weight when you sit. People sometimes call these the “sit bones,” and if you have ever spent too long on a wooden bench, you know exactly where they are.

The pubis is at the front. The left and right pubic bones meet at the midline in a cartilaginous joint called the pubic symphysis. Together, the pubis and ischium frame a large opening called the obturator foramen, through which nerves and blood vessels pass into the thigh.

All three bones converge at the acetabulum, the deep cup-shaped socket of the hip joint. Each contributes a portion of the socket wall, and the Y-shaped growth plate where they meet, called the triradiate cartilage, is a landmark pediatric radiologists use to track skeletal development in children.

The Sacrum and Coccyx Complete the Ring

The hip bones alone do not make a complete ring. At the back, the sacrum wedges between them like a keystone in an arch. The sacrum is a triangular bone formed from five fused vertebrae, and it connects to each ilium at the sacroiliac joints. Below the sacrum hangs the coccyx, a small chain of three to five fused or semi-fused vertebral remnants. Together, the two hip bones, the sacrum, and the coccyx form the bony pelvis.2PubMed. Basic science Anatomy of the pelvis

The sacrum is not just filling a gap. It channels the entire weight of your upper body down through the pelvis and into your legs. The sacroiliac joint transfers large compression loads to the lower extremities, and the sacrum resists sliding forces because it is tightly wedged between the hip bones and held in place by an extensive network of ligaments.3PubMed Central. Biomechanics of the Sacroiliac Joint: Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of Pain The result is a joint that barely moves, which is exactly the point. You want a stable platform between your spine and your legs, not a flexible one.

Why You Feel Your “Sit Bones” and Not Everything Else

When you drop into a chair, your weight funnels to a surprisingly small area: the ischial tuberosities, those bony bumps at the base of each ischium. The soft tissue between the bone and the chair surface is thinner than most people assume. Researchers using MRI have measured the layers of muscle, fat, and skin directly over the ischial tuberosity and found that tissue thickness changes with hip angle and the load of sitting.4PubMed. Use of MRI images to measure tissue thickness over the ischial tuberosity at different hip flexion When you flex your hips to a seated position, the gluteal muscles slide upward and to the side, leaving less padding between bone and seat. That is why the ischial tuberosities are so easy to feel when sitting, even if you have thick gluteal muscles.

This matters clinically, because the tissue right over the ischial tuberosity consistently shows the lowest oxygen perfusion and the highest carbon dioxide levels across every chair design tested, meaning that area is always the most compressed and least well-supplied with blood when you sit.5PubMed. The effect of chair designs on sitting pressure distribution and tissue perfusion This is why prolonged sitting on hard or poorly designed surfaces leads to discomfort, pressure sores, and sometimes a painful condition called ischial bursitis (also known as “weaver’s bottom”), where the bursa over the ischial tuberosity becomes inflamed.

The Ligaments Holding It All Together

Bones get the credit, but the pelvis depends heavily on ligaments to stay intact under load. Two ligaments at the back deserve special attention: the sacrotuberous ligament and the sacrospinous ligament. Both run from the sacrum to the ischium. The sacrotuberous ligament connects the sacrum to the ischial tuberosity (your sit bone), while the sacrospinous ligament connects the sacrum to the ischial spine, a smaller bony projection just above the tuberosity. These two ligaments resist rotational forces and help divide the opening at the back of the pelvis into two passageways for nerves and blood vessels.6PubMed. Putative proprioceptive function of the pelvic ligaments: biomechanical and histological studies

Beyond mechanical stability, these ligaments appear to have a sensory role. Histological studies have identified nerve endings within them that may help the body sense the position and movement of the pelvis, similar to the proprioceptive sensors in knee ligaments. When a pelvic ligament is damaged, the loss of this sensory feedback may contribute to ongoing instability and pain even after the torn tissue heals.

The Muscles That Anchor to Your Pelvic Bones

The pelvis serves as a hub for some of the largest and most powerful muscles in the body. The gluteus maximus, the muscle that gives the buttock its shape, originates from a surprisingly wide area: the ilium, the sacrum, the coccyx, the thoracolumbar fascia, and several ligaments including the sacrotuberous and dorsal sacroiliac ligaments.7PubMed. Anatomy and biomechanics of gluteus maximus and the thoracolumbar fascia at the sacroiliac joint Its fibers converge into a thick tendon that inserts on the femur and the iliotibial band along the outside of the thigh. This muscle is the primary driver of hip extension, the motion that propels you forward when walking, running, or climbing stairs.

Deeper and slightly higher sit the gluteus medius and gluteus minimus. The gluteus medius originates from three distinct sites on the outer surface of the ilium and the iliac crest, while the gluteus minimus arises from the ilium’s gluteal fossa and attaches to the hip joint capsule itself.8PubMed. The anatomy of the hip abductor muscles These two muscles are the main hip abductors, pulling your leg out to the side, and they are critical for stabilizing the pelvis when you stand on one leg. A weakness in the gluteus medius is one of the most common contributors to a waddling gait and is frequently implicated in hip and knee pain.

Other muscles using the pelvic bones as an anchor include the hip flexors (iliacus originating from the inner ilium), the adductors of the inner thigh (originating from the pubis and ischium), the hamstrings (originating from the ischial tuberosity), and the pelvic floor muscles (spanning the space enclosed by the bony ring). The pelvis is, in effect, a central switching station for force transmission between the trunk and the legs.

Why Male and Female Pelvises Look Different

The human pelvis is one of the most sexually dimorphic structures in the skeleton. While overall pelvis size is similar between men and women, the shape differences are pronounced enough that forensic scientists can determine sex from a pelvis with high accuracy. Women’s pelvises tend to be wider relative to their height, have a broader subpubic angle (the angle beneath the pubic symphysis), and have hip sockets that are smaller and spaced farther apart. In one study, there was almost no overlap between female and male pelvis shapes when plotted together.9PubMed. Allometry and Sexual Dimorphism in the Human Pelvis

Some of these differences are simply proportional: a wider pelvis scales predictably with body proportions. But the dimensions most relevant to childbirth, like the subpubic angle and the spacing of the hip sockets, do not follow that scaling pattern. These birth-relevant features appear to be shaped primarily by sex hormones during puberty rather than by body size, which is why they differ so consistently between men and women regardless of stature.9PubMed. Allometry and Sexual Dimorphism in the Human Pelvis

During childbirth, the pelvic bones are not as rigid as they normally feel. Hormonal changes during pregnancy loosen the ligaments of the pelvis, allowing slight movement at the sacroiliac joints and pubic symphysis. The sacrum can tilt forward (nutation) or backward (counternutation) by small amounts, which expands different planes of the birth canal at different stages of labor.10PubMed. Movements of the pelvic bones of expectant mothers during vaginal delivery Counternutation widens the pelvic inlet (the top of the birth canal), while nutation widens the outlet (the bottom). The mother’s posture during labor can influence these movements, which is one reason birth positions vary across cultures and clinical settings.

When the Bones Fuse and Why It Matters

In a newborn, the pelvis is mostly cartilage with scattered islands of bone. The ilium, ischium, and pubis ossify from separate centers, and throughout childhood they remain connected by the triradiate cartilage at the acetabulum. Studies tracking fusion on imaging show that the secondary ossification centers around the acetabulum close at a median age of roughly 12 to 14 in girls and 13 to 14 in boys, with most centers in girls closing about two to three years earlier than in boys.11PubMed. Analysis of Acetabular Ossification From the Triradiate Cartilage and Secondary Centers

This timeline matters for youth athletes and pediatric orthopedics. Until the triradiate cartilage closes, the acetabulum is still developing its final shape and depth. An injury to this growth plate during adolescence can alter how the hip socket forms, potentially leading to a shallow socket (dysplasia) or an overly deep one. Pediatric surgeons and sports medicine physicians pay close attention to triradiate cartilage status when deciding how to manage hip problems in teenagers.

How the Pelvis Evolved for Upright Walking

If you compare a human pelvis to that of a chimpanzee, the differences are dramatic. In our earliest upright ancestors, the pelvis underwent fundamental changes compared to non-human primates in order to support bipedal walking.12PubMed Central. The evolution of the human pelvis: changing adaptations to bipedalism, obstetrics and thermoregulation The iliac blades shortened and flared outward, repositioning the gluteal muscles so they could stabilize the pelvis during single-leg stance, something that happens with every step you take. The sacrum widened and tucked between the hip bones, creating a more bowl-shaped structure that supports the abdominal organs from below.

These changes created an engineering tension that paleoanthropologists have debated for decades: a pelvis optimized for efficient walking is narrow and deep, but a pelvis optimized for giving birth to large-brained infants is wide and shallow. This “obstetric dilemma” is probably not as stark as older textbooks suggested, since recent work has highlighted that thermoregulation, trunk stability, and locomotor efficiency all exerted selection pressures on pelvic shape. Still, it is striking that the human pelvis manages to serve such competing demands as well as it does.

Common Sources of Pain in the Buttock Region

Pain felt in the buttock area can originate from the bones, joints, or soft tissues of the pelvis. Understanding which structure is involved helps make sense of why certain positions or activities hurt.

Higher-energy pelvic fractures from falls or motor vehicle crashes follow predictable patterns depending on the direction of force. Orthopedic surgeons classify these using a system based on the force vectors involved: front-to-back compression, side-to-side compression, and vertical shearing. The severity and stability of the fracture determine whether treatment is conservative or surgical.16PubMed. Pelvic ring fractures: what the orthopedic surgeon wants to know Isolated fractures of the front part of the pelvic ring (the pubic rami) are generally stable and heal without surgery, while fractures involving the posterior ring, near the sacroiliac joint, are considered unstable and often require surgical fixation.17PubMed Central. Pelvic ring fractures in the elderly

Pelvic Tilt and How It Changes Joint Mechanics

The pelvis does not just sit passively beneath your spine. It tilts forward and backward constantly as you move, and even small changes in tilt angle affect how your hip joints function. When the pelvis tilts backward (posterior tilt), the hip sockets face slightly more forward, giving the femur more room to rotate before bone contacts bone. In people with femoroacetabular impingement, a condition where the hip joint pinches during deep flexion, posterior pelvic tilt can substantially increase the range of internal rotation before impingement occurs.18PubMed. Posterior Pelvic Tilt Allows for Increased Hip Motion, While Anterior Pelvic Tilt Decreases Joint Stress: A Systematic Review of Biomechanical and Motion Analyses

The flip side is that anterior tilt (tilting forward) appears to reduce joint contact stress by increasing how much of the femoral head is covered by the socket. In people with hip dysplasia, where the socket is too shallow, anterior tilt improved femoral head coverage and lowered contact pressures compared to posterior tilt.18PubMed. Posterior Pelvic Tilt Allows for Increased Hip Motion, While Anterior Pelvic Tilt Decreases Joint Stress: A Systematic Review of Biomechanical and Motion Analyses This means the ideal pelvic position depends on the individual’s hip anatomy: someone with impingement benefits from a bit of posterior tilt, while someone with a shallow socket may do better with a neutral or slightly anterior tilt. Physical therapists and surgeons increasingly consider pelvic tilt as a variable in treatment planning rather than assuming one “correct” posture for everyone.

Seat Design and Your Pelvic Bones

Given that the ischial tuberosities bear most of your seated weight and the tissue over them gets the least blood flow, chair design has a measurable impact on comfort and tissue health. Research comparing different cushion shapes found that a bowl-shaped (center-trough) cushion produced the lowest average interface pressure at roughly 42 mmHg, compared to about 57 mmHg for a flat cushion.19Ergonomics in Design: The Quarterly of Human Factors Applications. Contours of Comfort: Mapping Pressure Landscapes Across Anthropometric Seating Interfaces The curved shape redistributes load away from the ischial peaks and spreads it across a wider area of the thighs and buttocks.

Some chairs address the problem differently by using adjustable ischial supports that can be lowered or relieved. When the support beneath the ischial tuberosities is reduced, the center of pressure shifts forward toward the thighs and more of the body’s weight transfers to the backrest.20Spine. Sitting with Adjustable Ischial and Back Supports: Biomechanical Changes This strategy can be useful for people recovering from ischial bursitis or pressure injuries, since it unloads the exact spot where tissue compression is worst. For anyone who spends long hours seated, the practical takeaway is that the shape of the surface under your sit bones matters more than the firmness of the cushion. A contoured seat that allows your ischial tuberosities to settle into slight depressions, rather than perching on a flat or peaked surface, tends to reduce peak pressures and improve blood flow to the compressed tissues underneath.