Sit on a hard wooden bench and rock gently side to side. The two hard points you feel taking your weight are the ischial tuberosities — the sit bones, also called sitting bones or sitz bones. Almost everything that makes sitting uncomfortable traces back to what these two structures do.
Where they are and what they are
Your pelvis is a ring of bone. At the bottom of each side, where the ring curves under, there is a thickened, rounded knob of bone. That is an ischial tuberosity. You have two, one under each half of your pelvis, typically 4 to 6 inches apart in adults, measured center to center, according to the published measurements. A cushion built around exactly those two points is described on the ischial tuberosity cushion page.
They are not delicate. They are among the densest, sturdiest structures in the pelvis, built to bear load. The hamstring tendons anchor to them, which is why they matter to runners and cyclists as well as to office workers.
Why they hurt when nothing is wrong with them
Standing up, your weight travels down through your legs and spreads across two feet — a large, well-cushioned surface area. Sitting down changes the route entirely. The load now runs down your spine, into your pelvis, and out through those two knobs of bone.
The problem is arithmetic. Pressure is force divided by area. The same body weight that spreads comfortably across two feet gets funneled into two contact patches you could cover with a pair of coasters. Peak pressure under a sit bone can be many times what any other part of your seated body experiences.
That is not a defect. It is what the structures are for. It only becomes a problem when the sitting goes on for hours and the surface underneath does nothing to help.
The one-line version
Sit-bone pain is usually not an injury. It is a small contact area carrying a large load for a long time.
What sits between the bone and the chair
Not much, and less than most people assume. There is skin, a layer of fat, and a small fluid-filled sac called a bursa that reduces friction between the bone and the tendons running past it. That is the entire buffer.
Two things change how much buffer you have. Body composition affects the fat layer, which is why people who lose weight quickly often start noticing hard chairs for the first time. And sustained load compresses whatever tissue is there — the buffer you have at minute one is thinner by hour three, which is why a chair that felt fine after lunch can be unbearable by five.
Why posture moves the pain
Sit upright with your pelvis neutral and your weight lands squarely on the sit bones. Slump backwards and the pelvis rotates, rolling the load rearward onto the sacrum and coccyx. Perch forward on the front edge of a seat and the load concentrates further onto the sit bones.
This is worth knowing because where the pain sits is a clue, not a diagnosis. Two symmetrical sore points suggest the sit bones. One central sore point at the base of the spine suggests the tailbone, which is a different problem needing a differently shaped solution — telling the two apart takes about a minute.
What this means for a seat
Once you understand that the issue is concentration, the options narrow to three:
- Spread the load. Increase the contact area so pressure per square inch falls. This is what every padded cushion attempts.
- Interrupt the load. Stand up regularly. Sustained compression is the aggravating factor, and standing removes it entirely. This costs nothing and is the most effective thing on the list.
- Remove the contact. Arrange for nothing to be under the sit bones at all, so the load has to travel around them to the thighs and glutes.
The first option has a known failure mode: soft foam compresses fastest exactly where pressure is highest, so it bottoms out under the sit bones first and quietly stops working while still feeling soft everywhere else. Why firm beats soft covers that in detail.
Remove the contact instead of spreading the load
Two openings positioned where the sit bones land, so they rest in open space while firm foam carries the load across the thighs and glutes. $99.99 on Amazon, 4.3 stars across 4,500+ ratings, 100-day trial, free returns.
Your spacing is your own
Sit-bone width varies with pelvic structure, and it does not track with how large or small someone looks. Two people of the same height and build can measure an inch apart. This matters the moment you consider any seat designed around fixed geometry — a bike saddle, a cutout cushion — because the design only helps if it lines up with your anatomy.
It takes about sixty seconds to find out: the cardboard method gives you the number, and the fit checker tells you plainly whether that number works with the Socket Seat, including when the answer is no.
When to stop reading and see someone
Everything above describes ordinary load mechanics. It does not describe pain that radiates down a leg, comes with numbness, tingling or weakness, follows a fall, wakes you at night, or simply keeps getting worse regardless of what you sit on. Several conditions produce pain in this area and telling them apart requires an examination. That is a clinician’s job, not a cushion’s.
The short version
- Two dense knobs of bone at the base of the pelvis carry most of your seated weight.
- They concentrate a large load into a small area, which is why hard chairs hurt in two specific places.
- The tissue buffer between bone and chair is thin, and it compresses over hours.
- Posture moves the load between sit bones and tailbone, so where it hurts tells you something.
- Spreading, interrupting, or removing the load are the only three levers. Standing up is the free one.
- Your spacing is individual, and any fixed-geometry seat only works if it matches it.