Segmental spinal instability is a known cause of back pain, but no method of accurately quantifying instability exists. The movement of complex joints with rotational and translational components (such as the lumbar motion segment) is tracked by a pathway of instantaneous centers of rotation, or a centrode. Instability, ie, excessive and/or erratic movement, is reflected by increased centrode length in cadaver studies. This study describes an in vivo method that precisely determines the centrode pattern and reports the results for 21 normal male volunteers who were studied at the L4-5 and L5-S1 levels. Each volunteer underwent lateral radiographs of the lumbar spine. High speed films were used and six positions, from full extension to full flexion, were recorded. Acetate tracings and contour matching techniques recorded the relative positions of the vertebral bodies on each film. Multiple tracings of each radiograph, combined with a digitizer and computer, were used to improve precision in the calculated centrode patterns. Centrode lengths measured 43.7 mm and 55.9 mm, respectively, for the L4-5 and L5-S1 levels. This study demonstrates that precise centrode pattern analysis for sagittal plane motion of the lumbar spine is possible in vivo. Studies are under way to determine whether this technique will be useful as a clinical test in diagnosing early segmental instability of the lumbar spine in patients with low-back pain.