Figure 2

The fitted double sigmoid function. a. Load-deflection data (gray) are fitted with a double sigmoid function (black). The first derivative indicating the compliance is shown as a dotted line. At the maximum and minimum of the second derivative (dashed line) vertical lines are drawn delineating NZ. Goodness of fit (b.) and residuals plot (c.) show the quality of the double sigmoid model for describing the mechanical behaviour of the spinal motion segment. The data in both lower panels were down-sampled for clarity.