Fig. 2.

Illustration, in a pressure-displacement (P-x) curve, of the parameter estimation process. The result of a correct estimation of both stiffness $K$ and reference position $x_0$ is represented by drawing a tangent line (red line) for the P-x curve at the final measurement point, reaching the horizontal line of $P=0$ (essentially analogous to deflating from the end diastolic state to zero pressure for the LV model). The amount of active tension is proportional to the length of the 5 horizontal dashed lines at the 5 measurement points, because that is the amount of force needed to compress the spring from the red line (pure passive behavior) to the corresponding positions of the P-x curve. The blue line represents the scenario of an incorrect estimation of the reference position, where the AT estimated at measurement point 4 (denoted by the dash vertical line) will be negative (note that the blue line falls to the left of the P-x curve, i.e., negative AT is needed to stretch the spring to match the P-x curve)