The ischial tuberosity (or tuberosity of the ischium, tuber ischiadicum), also known colloquially as the sit bones or sitz bones, or as a pair the sitting bones, is a large posterior bony protuberance on the superior ramus of the

ischium The ischium (; : is ...

. It marks the lateral boundary of the pelvic outlet. When

sitting Sitting is a basic action and resting position in which the body weight is supported primarily by the bony ischial tuberosities with the buttocks in contact with the ground or a horizontal surface such as a chair seat, instead of by the low ...

, the weight is frequently placed upon the ischial tuberosity. The

gluteus maximus The gluteus maximus is the main extensor muscle of the hip in humans. It is the largest and outermost of the three gluteal muscles and makes up a large part of the shape and appearance of each side of the hips. It is the single largest muscle in ...

provides cover in the upright posture, but leaves it free in the seated position.Platzer (2004), p 236 The distance between a

cyclist Cycling, also known as bicycling or biking, is the activity of riding a bicycle or other types of pedal-driven human-powered vehicles such as balance bikes, unicycles, tricycles, and quadricycles. Cycling is practised around the world fo ...

's ischial tuberosities is one of the factors in the choice of a

bicycle saddle A bicycle saddle, sometimes called a bicycle seat, is one of five contact points on an upright bicycle, the others being the two Bicycle pedal, pedals and the two handles on the Bicycle handlebar, handlebars. (A bicycle seat in the specific sense ...

Divisions

The tuberosity is divided into two portions: a lower, rough, somewhat triangular part, and an upper, smooth, quadrilateral portion. * The ''lower portion'' is subdivided by a prominent longitudinal ridge, passing from base to apex, into two parts: ** The outer gives attachment to the

adductor magnus The adductor magnus is a large triangular muscle, situated on the medial side of the thigh. It consists of two parts. The portion which arises from the ischiopubic ramus (a small part of the inferior ramus of the pubis, and the inferior ramu ...

** The inner to the

sacrotuberous ligament The sacrotuberous ligament (great or posterior sacrosciatic ligament) is situated at the lower and back part of the pelvis. It is flat, and triangular in form; narrower in the middle than at the ends. Structure It runs from the sacrum (the lowe ...

* The ''upper portion'' is subdivided into two areas by an oblique ridge, which runs downward and outward: ** From the upper and outer area the

semimembranosus The semimembranosus muscle () is the most medial of the three hamstring muscles in the thigh. It is so named because it has a flat tendon of origin. It lies posteromedially in the thigh, deep to the semitendinosus muscle. It extends the hip joint ...

arises ** From the lower and inner, the long head of the

biceps femoris The biceps femoris () is a muscle of the thigh located to the posterior, or back. As its name implies, it consists of two heads; the long head is considered part of the hamstring muscle group, while the short head is sometimes excluded from this ...

and the

semitendinosus The semitendinosus () is a long superficial muscle in the back of the thigh. It is so named because it has a very long tendon of insertion. It lies posteromedially in the thigh, superficial to the semimembranosus. Structure The semitendinosus, ...

Additional images

File:Tuberosity of the ischium.PNG, Muscles of the gluteal and posterior femoral regions, with ischial tuberosity highlighted in red File:Gray235.png, Right hip bone. External surface. File:Gray236.png, Right hip bone. Internal surface. File:Gray237.png, Plan of ossification of the hip bone File:Gray239.png, Diameters of inferior aperture of lesser pelvis (female) File:Gray339.png, Right hip-joint from the front File:Gray436.png, The

obturator externus The external obturator muscle or obturator externus muscle (; OE) is a flat, triangular muscle, which covers the outer surface of the anterior wall of the pelvis. It is sometimes considered part of the medial compartment of thigh, and sometime ...

File:Slide3ADA.JPG, Anterior view of the pelvis with the ischial tuberosity labelled in the lower part of the image

Divisions

Additional images

See also

Notes

References

External links