The precision and accuracy of techniques for analysis of lumbar spinal-segment motions were evaluated using a specially designed orthogonal axis fixture and video image processing. Triplicate validation tests were performed by monitoring precisely controlled motions of the triaxial measurement device. The errors of the overall measurements were found to be less than 0.5 mm for axial and 1 degree for rotational displacements. The system accuracy was within 0.230 mm based on the calculation of the points on a calibration frame. The geometry of the calibration frame was designed with the capability to analyze the 3-D motion properties of a swine lumbar spine. Statistical analyses of the validation test data showed that the precision and accuracy of this motion-analysis system offer new opportunities for the measurement of total-length spinal-motion profiles. The system was then applied utilizing fresh thawed swine spines to test the limits of relative-motion measurements for the centers of adjacent vertebrae in a coupled unit. Lateral-medial and anterior-posterior video images of spines and the triaxial measurement devices were made simultaneously while applying cyclic axial and torsional loads. The distribution of data from six spines illustrated biological variability of relative displacements for adjacent coupled vertebrae.