When settling into a chair, most people feel a distinct bony prominence. This bone bears the body’s weight during sitting and is important for understanding posture and movement.
Identifying Your Sitting Bone
The bone you feel when sitting is the ischium (pronounced “ISK-ee-um”). Often called the “sit bone” or “sitz bone,” it is a paired bone forming the lower, back part of the hip. It is one of three major bones that fuse to form the pelvis. The ilium forms the upper, blade-like portion, and the pubis is located at the front. These three bones develop separately in children and fully fuse during puberty.
The ischial tuberosity is a large, rounded bony prominence on the ischium’s lower, posterior aspect. This structure makes contact with a sitting surface. It withstands body weight pressure and serves as an attachment site for muscles and ligaments.
How Your Sitting Bone Functions
The ischium, particularly its tuberosity, primarily bears the body’s weight when seated. Body weight transfers through the spine to the pelvis, resting directly on these ischial tuberosities. This provides a stable base for the torso and upper body, allowing for balance and maintaining an upright posture.
Beyond weight-bearing, the ischium serves as an attachment point for muscles and ligaments important for movement and stability. Hamstring muscles originate from the ischial tuberosity, aiding hip extension and knee flexion. The adductor magnus and some gluteal muscles also attach here, contributing to thigh movement and hip stability. Ligaments, such as the sacrotuberous ligament, connect to the ischium, reinforcing the pelvis.
Understanding Common Related Discomfort
Discomfort in the “sitting bone” area often stems from issues with the ischium and its surrounding structures. Prolonged sitting, especially on hard surfaces, can lead to localized pain. A frequent cause is ischial bursitis, an inflammation of the ischial bursa. This fluid-filled sac cushions the ischial tuberosity, but excessive pressure or repetitive stress can cause inflammation, leading to pain and tenderness.
Another source of discomfort is hamstring origin pain or tendinopathy, involving inflammation or degeneration of hamstring tendons where they attach to the ischial tuberosity. This condition presents as deep, localized pain in the lower buttock, worsening with activities like running, lunging, or prolonged sitting. These conditions typically resolve with rest, activity modification, and other conservative treatments.