Ischial tuberosities

The ischial tuberosities are the two bony bumps at the bottom of the pelvis, better known as the sit bones. Each one is part of the ischium, the lower back part of the hip bone, and when you sit it is these tuberosities that take your body weight (TeachMeAnatomy).

Where are the ischial tuberosities?

The hip bone on each side is made of three parts, the ilium, the pubis and the ischium, which only start to fuse at around 15 to 17. The tuberosity is the lower back part of the ischium. A ligament, the sacrotuberous ligament, runs from the sacrum to the tuberosity (TeachMeAnatomy).

They are also an anchor point for the hamstrings. The biceps femoris (long head), semitendinosus and semimembranosus all start on the ischial tuberosity (TeachMeAnatomy).

What is between your sit bones and the chair?

Less than most people think. Standing up, the gluteus maximus covers the tuberosity. As the hip bends, the muscle and the fat around it slide aside, so when you sit the bone bears weight more directly. A bursa, a small sac that reduces friction, forms between the bone and the muscle from the rubbing of everyday movement and helps the two glide (Johnson & Varacallo, 2024).

An upright MRI study looked at this in 35 people sitting on foam, 29 of them full-time wheelchair users. The soft tissue under the sit bones was between 5.6 and 32.1 mm thick, mostly fat, and thickest in the people who didn’t use wheelchairs. Most participants had little or no gluteus muscle over the tuberosity, whatever group they were in (Sonenblum et al., 2020). The study was designed to understand pressure ulcers: wheelchair users who had had one tended to have thinner tissue with a more peaked shape at the bone.

For a desk worker, it means much of the cushioning under your sit bones comes from the seat rather than from muscle. Seat shape and height are covered in how to set up your office chair.

Why do my sit bones hurt?

Ischial bursitis

When you sit, the bursa is pressed between the bone and the seat, so constant weight-bearing can make it inflamed. Ischial bursitis has the old names “weaver’s bottom” and “tailor’s bottom”, from trades in which people sat on hard surfaces for most of the day (Johnson & Varacallo, 2024).

Proximal hamstring tendinopathy

This is a problem with the hamstring tendons where they attach to the tuberosity. It typically causes deep buttock pain at that spot, affects people who play sport and people who don’t, and is often long-standing and limiting in daily life. Evidence on how best to assess and manage it is limited, and no randomised trials of rehabilitation had been published at the time of a 2016 clinical review (Goom et al., 2016).

Sciatica

Pain felt around the sit bone isn’t always from the bone or tendons. Sciatica usually affects the bottom and the back of one leg, often with tingling, numbness or weakness, and can be worse when moving, sneezing or coughing (NHS).

Sitting time and your sit bones

Your weight rests on the tuberosities whenever you sit, so standing up is the simplest way to take the load off them. Ideas for building that into the day are in microbreaks, and the sitting time calculator shows how many hours a week you spend in a chair.

When to see a doctor

See a GP if buttock or sit bone pain hasn’t improved after a few weeks, is getting worse or is stopping your normal activities. Go to A&E or call 999 if you have sciatica on both sides, severe or worsening numbness or weakness in both legs, numbness around your genitals or bottom, or changes in bladder or bowel control (NHS).