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

20% compression strains were imposed. Material constants for the matrix quasiincompressible
behaviour and fibre passive resistance were derived from [5,6], while
muscle peak stress was based on [8] (Table 1).
Table 1. Material parameters.
Description Parameter Value References
Stress in the DEV term G [MPa] 16.42 x 10 -4 Adapted from
Martins et al. (1998)
Stress in the VOL term K [MPa] 1.642 Blemker et al (2005)
Stress in the PE term A 4 Lu et al (2010),
Martins et al. (1998)
Stress in the PE and SE T0 [MPa] 0.46 Bogduk et al.
(1992b)
For the active fibre term, morphometry-based estimations for the Erector Spinae (ES)
(Longissimus thoracis pars lumborum and Longissimus thoracis pars thoracis, LT,
Iliocostalis lumborum pars lumborum, IL), the Multifidus (MF) and the Psoas Major
(PS) muscles based on [9] were used to calculate the parameter CCE (Eq. 5) and to
simulate muscle activation (Table 2). A parametric analysis was planned for further
exploration of the influence of CCE on muscle stress predictions in order to assess
possibilities of model calibration based on muscle activation criteria. Such exploration
was performed assuming that the proportion of activation from one muscle to another
due to the morphometric differences should be preserved.
In the end, the contribution of each stress term represented in Eq. (6) was analyzed in
order to conclude about the most critical term to consider for further model adjustments.
3. RESULTS
Table 2. Morphometry-based values for CCE for lumbar muscles.
MF LT IL PS
CCE 0.811 0.825 0.846 1.111
Stress analysis of the models indicated that the initial CCE values based on the literature
(Table 2) were not able to induce any active response of the muscle due to a systematic
overestimation of at any fibre stretch ratio. Therefore, further parametric analysis
showed that CCE values below 0.637 in traction and above 0.706 in compression were
necessary to predict non-zero active stresses at some point (Fig.1).
3