The bony prominences felt when sitting are commonly known as the “butt bones.” These structures serve as anchor points that bear the weight of the upper body whenever a person is seated. They are a fundamental part of the human skeletal design, allowing for stable and balanced posture. Understanding these weight-bearing structures requires looking into human anatomy.
The Common Name for Sitting Bones
The bony points felt beneath the buttocks are most frequently called the “sit bones” or, using the German translation, “sitz bones.” This common term directly describes their function: making contact with a surface when a person adopts a seated position. To locate these paired structures, sit on a firm surface and gently tilt the pelvis forward and backward. The distinct, hard protrusions felt pressing against the surface are the sit bones. When standing, these bones are typically covered by the large gluteal muscles and are not as easily felt.
The Anatomical Identification of the Ischium
The common “sit bone” has a precise anatomical name: the Ischial Tuberosity. This name specifies the exact point of contact rather than the entire bone. The Ischial Tuberosity is a large, roughened projection found on the lower and posterior aspect of the larger bone known as the Ischium. This specific bony feature is structurally adapted for its weight-bearing function, acting as a small tripod base when the body is seated.
The roughened surface of the tuberosity serves as a major attachment point for powerful leg muscles. The entire group of hamstring muscles, including the biceps femoris, semitendinosus, and semimembranosus, originate from this spot. The adductor magnus muscle and the sacrotuberous ligament also attach here, making the tuberosity a significant nexus for lower limb movement and pelvic stability.
The Ischium itself is a curved bone that constitutes the lower and back part of the hip bone. The development of this prominent area allows the weight of the torso to be supported directly on bone.
Context Within the Pelvic Girdle
The Ischium is one of three bones that fuse together to form the entire hip bone, formally called the Os Coxae or innominate bone. This large, irregular bone forms the side and front of the larger structure known as the Pelvic Girdle. The Ischium joins with the Ilium, the broad upper part of the hip, and the Pubis, which forms the front section.
The fusion of these three bones creates a robust, bowl-shaped ring when joined with the sacrum and coccyx at the back. The Pelvic Girdle’s overall role is to connect the axial skeleton (the spine) to the lower appendicular skeleton (the legs). The structure acts as a stable foundation for the torso and transfers the full weight of the upper body to the lower limbs during walking and standing.
The Pelvic Girdle also provides substantial protection for delicate internal organs located within the lower abdomen, such as the bladder and reproductive organs. It offers extensive surface area for the attachment of numerous muscles that control posture, hip movement, and the movement of the trunk.