During locomotion, the SI joints flex/extend in unison with the corresponding hip joint.

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During locomotion, movement of the hip and the sacroiliac (SI) joints is tightly coordinated to keep the pelvis and spine aligned as the limb travels through its arc. The SI joints permit small motions in the sagittal plane (nutation and counternutation) that occur as the pelvis tilts with hip flexion and hip extension. When the leg flexes at the hip, the pelvis tends to tilt in a way that nutates the sacrum (the sacral base moves anteriorly). When the leg extends, the pelvis tilts the other way and the sacrum counternutates (posterior base movement). Because gait involves alternating hip flexion and extension, the SI joints adjust in sync with the hip to accommodate these changes, even though the joint motions are small. So the SI joints flex/extend in a coordinated manner with the corresponding hip joint during locomotion.