Figure 7

Data fitting in suboptimal load-deflection curves. a. the worst fit in the validation experiments of this study with porcine lumbar motion segments (r² = 0.976). The double sigmoid function (gray) is not able to follow the entire experimental load-deflection curve (black). Yet the fitted function allows assessing mathematically the deflection point of the curve and thereby the NZ. b. Load-deflection curve recorded from a human specimen from another study[12], showing a non-monotonous curve at 1.5 Nm extension load (probably due to local failure) and a non-linear range of the NZ. The double-sigmoid function (gray) fits reasonably well over the NZ and allows objective determination of its borders by the maximum and minimum of the second derivative.