The region commonly referred to as the buttock or gluteal region is supported by a complex skeletal structure that provides stability and mobility to the lower body. The foundation for this part of the torso is formed by a large, basin-shaped bony ring known as the pelvic girdle. This structure is responsible for bearing the weight of the upper body and transferring it downward, making it one of the strongest skeletal areas. Understanding the components of this bony ring reveals the specific names for the structures people feel when they sit down.
The Pelvic Girdle Foundation
The pelvic girdle is composed of two large, symmetrical hip bones, medically known as the coxal bones, which join together in the front. These two structures connect posteriorly to the lower, fused sections of the spine. The central connection point is the wedge-shaped sacrum, which is made up of five fused vertebrae, and the small tailbone, or coccyx, located beneath it.
Each individual coxal bone begins development as three separate bones that eventually fuse together during adolescence and early adulthood. The largest of these three components is the ilium, which forms the broad, flaring structure often felt at the sides of the hips. This expansive bone provides a wide surface area for the attachment of large muscles that govern hip movement.
Beneath the ilium are the other two components: the pubis, located toward the front, and the ischium, positioned toward the rear. The pubis bones meet at the front of the pelvis to form a joint called the pubic symphysis, completing the anterior connection of the ring. Together, the ilium, ischium, and pubis contribute to forming the deep socket, the acetabulum, where the head of the femur articulates to form the hip joint.
The Sitting Bones
When a person refers to the specific bones they feel pressing against a chair, they are referring to a part of the ischium called the ischial tuberosities. The ischium is the lower, posterior section of the hip bone, and the tuberosities are large, thickened, and roughened projections extending from its lower border.
In the standing position, the body’s weight is primarily distributed through the legs and feet. Upon sitting, the body’s mass is channeled through the pelvic girdle and directly onto these two bony points. The ischial tuberosities act as the main points of contact between the skeleton and the supporting surface, which is why they are commonly nicknamed the “sitting bones.”
The presence of bursae and soft tissue padding around these structures helps to cushion the pressure against external surfaces. Furthermore, these rough surfaces serve as the origin points for some of the powerful hamstring muscles in the back of the thigh.
Key Roles and Functions
Beyond providing a stable base for sitting, the entire pelvic structure performs several major functional roles for the body. The large surfaces of the ilium and the ischium provide extensive attachment sites for the powerful gluteal muscles. These bony anchors are necessary to generate the force required for activities like walking, running, and climbing.
The basin shape of the pelvic girdle also serves an important protective function. It forms a sturdy enclosure for delicate internal structures, including the urinary bladder and the lower digestive tract. In females, this bony ring also surrounds and protects the reproductive organs from external forces.
Perhaps the most fundamental mechanical role is the efficient transfer of body weight from the upper torso to the lower limbs. The sacrum acts as a bridge, accepting the weight from the vertebral column and distributing it laterally to the two hip bones at the strong sacroiliac joints. This distribution system allows the body to maintain balance and move upright with maximum stability.