A representative model is necessary for the analysis of spine kinematics and dynamics during motion. Existing models, based on stationary imaging or cadaveric data, may not be accurate through the full range of spinal motion or for clinical populations. In this paper, we propose a functional method for estimating subject-specific spinal joint centers, generating a one-joint or two-joint kinematic model of the spine. These models are driven by the motion of the thorax and pelvis as observed by eight surface landmarks. We apply this method to experimental data from ten subjects performing flexion/extension and sit-to-stand motions. The recovered functional models are assessed against an allometric model though the analysis of marker residuals. We found that the functional models provide lower residuals than the allometric methods. Between the functional models, the two-joint model provided lower residuals with less sensitivity to the training action, while the one-joint model should be trained on the motion of interest.