**5.7 Validation of the detailed spine model**

To validate the detailed spine model presented above, two approaches are used and presented as follows:


In the first approach, a gradually increasing horizontal force was applied onto the vertebra T7 of the spine model from posterior to anterior in the sagittal plane. From this force, axial and shear forces as well as the moment about the L5-S1 disc were calculated. Zee's model estimated an axial force of 4520 N and shear force of 639 N in the L5-S1 disc at a maximum extension moment of 238 Nm. Meanwhile, to obtain the same extension moment, the external force that needs to be applied in the present model is 1260 N. Corresponding with this force, axial and shear forces obtained in the model were 4582 N and 625 N, respectively. This is in accordance with the results presented by McGill et al. (1987) who found axial forces in the range of 3929–4688 N and shear forces up to 650 N.

In the second approach, a comparison was made with in-vivo intradiscal pressure measurements of the L4–L5 disc as reported by Wilke et al. (2001). They measured a pressure of 1.8 MPa in the L4–L5 disc while the subject (body mass: 70 kg; body height: 1.74 m) was holding a full crate of beer (19.8 kg) 60 cm away from the chest. The disc area was 18 cm2 and based on this the axial force was calculated to be 3240 N. The same situation was simulated using the spine model in this research. The estimated axial force was 3161.6 N. This is a good match considering the fact that no attempt was made to scale the model to the subject in this study. Body mass and body height of the subject in this study are quite similar to the body mass and height used in the model.
