ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

4. DISCUSSION
The limited number of parameters, i.e. 5, needed in this model is advantageous
compared to the minimum number of 11 parameters involved in other models [11].
Indeed this model is more easily falsifiable and the parameters can be more easily
determined empirically. Parametric analysis of active fibre parameter CCE showed that
the stress results obtained were in good agreement with other studies reported in
literature. However, a phenomenological calibration of the parameter CCE may be
needed out of the studied strain range in order to achieve more realistic stress
predictions. Thus, clinical explorations of the strain range of activation of lumbar
muscles, e.g. through MRI explorations [12], could provide realistic calibrations for
further implementation of the equations in a lumbar spine model, opening the way also
to patient specific calibrations. Such explorations would be particularly necessary for
the Psoas Major, since results showed an unrealistic stress response over = 0.85
compression (Fig. 2a), where the active stress accounted for more than 90% of the total
stress. On the contrary, under traction, Psoas Major volumetric stress response was
prominent (Fig. 2b), suggesting the need to assess its dependence upon muscle fibre
activation. Indeed, the model sensitivity was related to the bulk modulus K for which, a
ten-fold increase would allow a more realistic approximation of the incompressible
muscle behaviour. However, although previous studies reported 15% maximum muscle
activation simulating the biceps brachii behaviour [6], literature still lacks references for
the possible strain range of activations that the Psoas Major can undergo during
occupational and athletic activities.
5. CONCLUSION
A new constitutive model for the lumbar spine muscles was proposed and tested. The
stress predictions of the muscle active response were very sensitive to variations of the
parameter CCE. For the MF, LT and IL models, similar stress-strain dependencies were
observed for the studied range of strains. Relative stress terms contributions analysis
pointed out the significance of realistic simulations of muscle activation for all muscles.
Nevertheless, PS strongly indicated possible overestimations of the muscle resistance to
volumetric deformations under traction and of the active stress term under compression.
This suggests that the mechanisms of activation for this very specific muscle may have
to be explored independently on the other lumbar muscles simulated.
6. ACKNOWLEDGEMENTS
Financial funding from the European Commission (MySpine FP7-ICT-269909) is
acknowledged.
7. REFERENCES
1. Zander, T., Rohlmann, A. and Bergmann, G., Estimation of muscle forces in the
lumbar spine during upper-body inclination, Clinical Biomechanics, 2001, Vol. 16,
S73-80.
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