Figure 4

Determination of the NZ from experimental data. Load-deflection curve of a porcine lumbar segment before (a) and after a seven-hour period of axial compression under 250N (b). The NZ as determined by the method of Wilke (NZW) underestimates the NZ before the compression test, because the range of minimal stiffness is clearly larger than the angulation difference at zero load (a). The NZ as determined by the double sigmoid function (NZef, NZfe) appear to present a better estimation. After a seven-hour axial compression, the range of motion (ROM) of the segment has decreased and hysteresis has increased (b). The angulation difference at zero load (NZW) is larger than the NZ determined by the double sigmoid function. Note that in this case the NZ for extension to flexion (NZef) is smaller than the NZ from flexion to extension (NZfe).