As with any joint, the SIJ is more stable against forces that are perpendicular to its surfaces. The SIJ is almost vertical, so a horizontal force is most stabilizing. When standing, force comes up from the ground, through the legs, bending with the neck of the femur bone, into the pelvic socket (acetabulum) at an angle that is about 45 degrees to the SIJs surface. Then, the actions of the upper leg and pelvic muscles change the force stream to an almost horizontal plane, perpendicular to the SIJ, enhancing stability. However, when sitting, the legs are removed from the situation and all the upper body weight goes on the ischia (sit bones). Now, the force stream changes to a vertical plane, and becomes destabilizing to the SIJ. Accordingly, sitting can be considered a repetitive motion that places consistent stress on the SIJ ligaments over the course of one’s lifetime. Eventually, micro tears may occur, which trigger an avoidance reaction that may be expressed as tightness and pain. For this reason, wearing the Serola Belt during sitting may help relieve stress on the SIJ ligaments and enhance comfort.
