The purpose of the study was to provide a guideline for the measurement of instantaneous axis of rotation (IAR) in the lateral bending and to compare the position and translation of IAR of normal persons and the patients with disc herniation. The X and Y axes were considered to be as the follows; X axis was a extension line passing bilateral the most upper margin of superior iliac crest, and Y axis was a perpendicular line to X axis passing L5 spinous process. 25 normal individuals and 24 patients with disc herniation were included in this study. There was no significant difference of the position and lateral translation of IAR of L2 vertebra body between in normal and patient groups. The results suggest that the support of the muscles and orientation of the facet joints of the lumbar spine would prevent a instability of the motion segments.
