Purpose <p>To quantify helical axis of motion (HAM) orientation, center of rotation (COR), and the bilateral symmetry, sex differences, and differences in HAM COR and dispersion between periods of hip flexion/extension during gait in healthy young adult hips.</p> Methods <p>Synchronized biplane radiographs of 46 hips were collected at 50 images/sec during treadmill gait. Each subject received a CT scan, which was segmented to create subject-specific pelvis and proximal femur bone models. Bone motion was determined with submillimeter accuracy by matching digitally reconstructed radiographs created from the bone models to the distortion-corrected radiographs using a validated volumetric model-based tracking technique. The femoroacetabular HAM was calculated for every three-degree interval of hip flexion/extension and described by the COR location and the HAM orientation relative to the sagittal, transverse, and coronal planes of the pelvis. Symmetry was measured by absolute side-to-side differences (SSD_A).</p> Results <p>The SSD_A in HAM orientation was 31.3° or less, and the SSD_A in COR was 2.3&#xa0;mm. The COR was located at neither the center of the acetabulum nor the femur head center; the COR in women was located more anteriorly and superiorly during extension compared with men; the instantaneous axis of rotation was more dispersed throughout extension than flexion.</p> Conclusion <p>The hip COR moves throughout gait and is not located at the center of the acetabulum in healthy young adults. The observed SSD_A, sex differences, and flexion/extension differences in COR location and HAM dispersion have implications for patient evaluation, computational modeling, and hip joint replacement design.</p>