This article describes a sagittal motion model that transforms skin marker coordinates into corresponding vertebral body coordinates. Coordinate data were obtained from 45 subjects with radiopaque skin markers over the vertebral spinous processes. Data of 30 subjects were used for model development while data of the other Fifteen subjects were used for model validation. The subjects were subjected to lateral spinal radiographs in positions of neutral, flexion, and extension. The model was developed by taking the non-linear difference between lumbar spinal profile and skin profile, the L(4) skin-fold, and the difference of L(1)-S(1) skin distraction into consideration. Regression analysis shows high coefficients of determination (range, 62.43-99.86%; mean, 86.41%) in comparing the estimated position data with the actual data in all vertebral body positions. Based on the estimated coordinates, both the intersegmental joint angles and intersegmental joint mobilities were calculated. The result of this study using the non-invasive protocol and the transformation matrices demonstrated that the intersegmental sagittal motion of lumbar spine can be obtained with error ranged from 0.05 to 0.56 degrees.