Your sit bones are the two bony knobs at the very bottom of your pelvis, and they bear a large share of your body weight every time you sit down. Anatomists call them the ischial tuberosities, but you can feel them yourself by sitting on a hard surface and rocking slightly side to side: the firm points pressing into the seat are your sit bones. They matter because they anchor major muscles, influence spinal posture, and are a common source of pain when something goes wrong with the surrounding soft tissue, nerves, or the bone itself.
Where Exactly They Are
The pelvis is made up of three fused bones on each side: the ilium (the broad “wing” you feel at your hip), the pubis (the bone in front), and the ischium (the bone at the back and bottom). The ischial tuberosity is the thick, rounded projection at the lowest point of each ischium. When you stand, these knobs are hidden under the gluteus maximus muscle and a layer of fat. When you sit, the gluteus slides upward and the tuberosities make direct contact with whatever surface is beneath you. That is why a hard bench feels so different from a padded chair: on a hard surface, nearly all your upper-body weight is focused on these two small contact patches.
The tuberosities also serve as attachment points for some of the body’s most powerful muscles. The hamstrings (semimembranosus, semitendinosus, and the long head of the biceps femoris) all originate here, making the sit bones the anchor for the muscles that bend your knee and extend your hip during walking, running, and jumping. The adductor magnus, a large inner-thigh muscle, also attaches to the ischial tuberosity. This dual role as both a weight-bearing surface and a muscular anchor is why the sit bones come up in conversations about everything from office chairs to sprint injuries.
Sit Bones and Spinal Alignment
When you transition from standing to sitting, your pelvis rotates backward. Research measuring this change found that pelvic tilt shifted dramatically: the sacrum rotated rearward and lumbar lordosis (the natural inward curve of your lower back) dropped by roughly half compared to standing.1PubMed. Sagittal lumbar and pelvic alignment in the standing and sitting positions In plain terms, sitting flattens your lower back. The degree of flattening depends on what angle your thighs make with your trunk and how much your knees are bent. A more reclined or slumped posture pushes the pelvis further back, flattening the lumbar curve even more.2Applied Human Science. The Relationships between Lumbar Curves, Pelvic Tilt and Joint Mobilities in Different sitting postures in Young Adult Males
This matters because the sit bones are the pivot point around which the whole chain moves. If you slouch backward, the tuberosities roll forward and the pelvis tips into what clinicians call posterior pelvic tilt. The lumbar spine follows, losing its curve. Over hours of desk work, this posture can load the intervertebral discs unevenly, contributing to stiffness and lower-back pain. A study on patients with lower-back pain found that sitting with pelvic support restored lumbar lordosis significantly compared to a habitual slumped position.3PubMed Central. The Effect of a Pelvic Support Chair on Sagittal Lumbosacral Alignment in Sitting Position in Patients with Lower Back Pain One ergonomic approach tested adjustable ischial support combined with a fitted lumbar backrest. Reducing the pressure under the sit bones while supporting the lower spine helped maintain spinal curves, lowered muscle activity, and increased disc height, all of which point toward a more back-friendly seated position.4Spine. Sitting with Adjustable Ischial and Back Supports: Biomechanical Changes
The practical takeaway is that how your sit bones contact the seat surface affects the entire posture chain above them. Chairs that let you sit with your thighs sloping slightly downward (knees a bit lower than hips) encourage the pelvis to tilt forward, which preserves more of your standing lumbar curve. If you spend long hours seated, paying attention to what is happening at the sit-bone level is at least as important as back support.
Why Sit-Bone Width Varies and Why It Matters for Cycling
Not everyone’s sit bones are the same distance apart. Ischial tuberosity width varies from person to person and is consistently wider, on average, in women than in men.5PubMed. Gender differences in bicycle saddle pressure distribution during seated cycling This width difference has real consequences for bicycle saddle fit. If a saddle is too narrow, the sit bones hang off the edges and body weight shifts onto the soft tissues between them, raising the risk of numbness, chafing, and chronic discomfort. If the saddle is too wide, it can chafe the inner thighs during pedaling.
Research on female cyclists found that a saddle whose width exceeds the cyclist’s ischial tuberosity width by about one centimeter distributes pressure more effectively and improves comfort.6PubMed Central. The Effect of Bicycle Saddle Widths on Saddle Pressure in Female Cyclists Many bike shops now measure sit-bone width using a simple gel pad or a piece of corrugated cardboard that the customer sits on. The impressions left behind reveal the distance between the two tuberosities, and that measurement guides saddle selection. It is a quick, low-tech process, but it can be the difference between enjoying a long ride and dreading one.
Cycling also raises the stakes for the area between the sit bones. When a rider leans forward onto drop handlebars, the pelvis rotates and more pressure shifts to the perineum (the soft-tissue area between the tuberosities). This is where blood vessels and the pudendal nerve run, and sustained compression of these structures can cause numbness in the groin. Finding a saddle that supports the sit bones properly is not just about comfort; it is about protecting the nerves and vessels that run beneath the pelvic floor.
Nerve and Vascular Compression Around the Sit Bones
The pudendal nerve passes close to the ischial tuberosity on its way from the sacral spine to the perineum. In some people, this nerve can become trapped or compressed in the bony and ligamentous structures near the sit bones. When that happens, the result is chronic, progressive perineal pain that characteristically gets worse with sitting. A study following patients who underwent surgical decompression for pudendal nerve entrapment reported that about 60 percent experienced meaningful pain relief afterward.7PubMed Central. Pudendal entrapment as an etiology of chronic perineal pain: Diagnosis and treatment
Not all sit-bone-area nerve pain is pudendal entrapment, though. The sciatic nerve, the thickest nerve in the body, runs just lateral to the ischial tuberosity under the piriformis muscle. When the hamstring tendons at the tuberosity become swollen or scarred, they can press on the sciatic nerve nearby. Surgeons repairing torn hamstring tendons at the sit bone routinely identify and protect the sciatic nerve during the procedure, sometimes needing to free it from scar tissue.8PubMed Central. Functional Outcomes of Proximal Hamstring Tendon Repair in Patients Over 40 Treated Without Postoperative Bracing – Section: Surgical Technique If you have deep buttock pain that shoots down the back of your leg and worsens when you sit, the source of the problem may be at the sit bone rather than in the lower spine.
Ischial Bursitis, or “Weaver’s Bottom”
A bursa is a small, fluid-filled sac that cushions the space between bone and overlying soft tissue. There is one between each ischial tuberosity and the gluteus maximus. When this bursa becomes inflamed, the condition is called ischial bursitis, and its older name is “Weaver’s bottom,” a nod to the weavers who once sat on hard benches for hours. A case report described a 35-year-old man with chronic bilateral pain over both sit bones. Imaging revealed well-defined fluid collections overlying both ischial tuberosities, surrounded by inflammatory swelling in the overlying gluteal muscle. Conservative treatment with anti-inflammatory medication and activity modification gradually relieved his symptoms.9PubMed Central. Typical MRI findings of bilateral ischial bursitis: bilateral Weaver’s bottom.
The condition is not limited to weavers, of course. Anyone who sits for prolonged periods on hard surfaces, including rowers, cyclists, and office workers with minimal seat padding, can develop it. The hallmark symptom is a dull ache right at the sit bone that worsens when sitting and eases when standing or lying down. Because the pain overlaps with hamstring problems and referred pain from the lower back, ischial bursitis is often misdiagnosed. If you press directly over one sit bone and it is tender, bursitis is worth considering.
Hamstring Tendon Problems at the Sit Bone
Because the hamstrings originate on the ischial tuberosity, the tendon-to-bone junction here is a common trouble spot. Proximal hamstring tendinopathy is an overuse condition that causes deep buttock pain right at the hamstring’s origin. It affects both athletes (sprinters, hurdlers, dancers) and people who are not particularly active but spend long periods sitting, which compresses the tendon against the bone.10Journal of Orthopaedic & Sports Physical Therapy. Proximal Hamstring Tendinopathy: Clinical Aspects of Assessment and Management The pain tends to be stubborn and can limit everyday activities like driving or climbing stairs.
When the tendon does not just degenerate but actually tears away from the bone, the injury is a proximal hamstring avulsion. This can happen during a sudden forceful stretch, such as a waterskiing fall or an aggressive hamstring stretch with the knee locked. If the tear is complete or the tendon retracts significantly, surgical repair is the standard treatment. The current worldwide approach involves re-anchoring the torn tendon complex (semitendinosus, biceps femoris long head, and semimembranosus) to the ischial tuberosity using suture anchors.11Journal of ISAKOS. Proximal hamstring tendon avulsion: state of the art The procedure requires careful handling because the sciatic nerve lies just next to the surgical field.
Sit-Bone Injuries in Adolescents
In children and teenagers, the ischial tuberosity is not yet fully fused to the rest of the pelvis. It remains connected through a cartilaginous growth plate (apophysis) until the late teens or early twenties. This makes it vulnerable to a category of injury that adults do not typically face. Sports involving sudden accelerations and decelerations, like sprinting, soccer, and gymnastics, can stress the growth plate enough to cause inflammation (apophysitis) or even a fracture where the apophysis separates from the bone.12PubMed Central. Apophysitis of the Ischial Tuberosity: A Case Report
The symptoms mimic a hamstring strain: pain in the lower buttock that worsens with running or kicking. Because x-rays in young athletes can be tricky to read (the growth plate already looks like a gap in the bone), the injury is sometimes missed on initial assessment. When the bony fragment pulls away by more than about 1.5 centimeters, surgical refixation is generally recommended. A review of surgical outcomes in adolescents found that timely intervention produced good-to-excellent results and high return-to-sport rates, while delayed treatment of chronic cases was less promising.13PubMed Central. Surgical repair techniques, functional outcome, and return to sports after apophyseal avulsion fractures of the ischial tuberosity in adolescents If a young athlete has persistent buttock pain after a sudden pulling sensation, imaging sooner rather than later can make a real difference.
Pressure Injuries for Wheelchair Users
For people who use wheelchairs full-time, the ischial tuberosities become a critical pressure point. Unlike someone who can shift weight, stand up, or walk around, a person with a spinal cord injury may sit on the same two bony prominences for most of the day without the sensory feedback to know when pressure is building. Records from 600 spinal-injury patients demonstrated that maintaining low pressure beneath the ischial tuberosities is a key indicator of wheelchair cushion suitability.14Nature. Pressure sore prevention for the wheelchair-bound spinal injury patient
Pressure injuries (sometimes still called pressure sores or decubitus ulcers) over the sit bones can be deep and slow to heal because the tissue between the bone and the skin is relatively thin. The consequences extend beyond skin breakdown: infections of these wounds can reach the bone itself, leading to osteomyelitis and lengthy hospital stays. Specialized wheelchair cushions, pressure-mapping technology, and regular weight-shifting routines are all built around the goal of distributing force away from the ischial tuberosities. If you help care for someone in a wheelchair, checking the skin over the sit bones regularly is one of the most impactful things you can do.
How to Find Your Own Sit-Bone Width at Home
You do not need a clinic visit to estimate your sit-bone width. The simplest method is to sit on a piece of corrugated cardboard placed on a hard, flat surface. Sit upright for about 30 seconds, then stand and look at the cardboard. The two deepest impressions correspond to your ischial tuberosities, and the center-to-center distance between them is your sit-bone width. Most adults measure somewhere between 9 and 15 centimeters, with considerable variation that does not correlate neatly with overall body size. A slim person can have wide sit bones, and a larger person can have narrow ones.
This measurement is most commonly used for bicycle saddle fitting, but it is also helpful if you are shopping for a meditation cushion, a rowing seat, or even a stadium seat pad. Any surface that is narrower than your sit-bone width will force the load onto soft tissues instead of bone, and any surface that is much wider may cause friction during movement. Knowing your number puts you ahead of most people who choose seats based on how they look rather than how they match the anatomy underneath.
When Sit-Bone Pain Is Not Actually Coming from the Sit Bone
One of the trickier aspects of sit-bone pain is that several conditions share the same real estate. Ischial bursitis, proximal hamstring tendinopathy, pudendal nerve entrapment, sciatic nerve irritation, and referred pain from the lumbar spine can all present as deep aching in the lower buttock. Even a stress fracture of the ischial tuberosity itself, though rare in adults, can mimic a soft-tissue problem. The location of tenderness matters: pain right on the bone points toward bursitis or a bony issue, pain slightly lateral and down the leg suggests the sciatic nerve, and pain that worsens specifically with hamstring contraction points to the tendon.
Imaging helps sort things out. MRI can distinguish a swollen bursa from a degenerative tendon, reveal bone-marrow edema in a stress fracture, or show fluid around a compressed nerve. Ultrasound, which is cheaper and faster, can identify tendon tears and bursitis in real time. If you have had sit-bone pain for more than a few weeks and it has not responded to rest and anti-inflammatories, getting a specific diagnosis is worth the effort. Treating the wrong structure rarely helps and can prolong the problem.