Importance of passive muscle, skin, and adipose tissue mechanical properties on head and neck response in rear impacts assessed with a finite element model

Objective The objective of this study was to improve head-neck kinematic predictions of a contemporary finite element (FE) head-neck model, assessed in rear impact scenarios (3-10 g), by including an accurate representation of the skin, adipose tissue, and passive muscle mechanical properties. The soft tissues of the neck have a substantial contribution to kinematic response, with the contribution being inversely proportional to the impact severity. Thus accurate representation of these passive tissues is critical for the assessment of kinematic response and the potential for crash induced injuries. Contemporary Human Body Models (HBMs) often incorporate overly stiff mechanical properties of passive tissues for numerical stability, which can affect the predicted kinematic response of the head and neck. Methods Soft tissue material properties including non-linearity, compression-tension asymmetry, and viscoelasticity were implemented in constitutive models for the skin, adipose, and passive muscle tissues, based on experimental data in the literature. A quasi-linear viscoelastic formulation was proposed for the skin, while a phenomenological hyper-viscoelastic model was used for the passive muscle and adipose tissues. A head-neck model extracted from a contemporary FE HBM was updated to include the new tissue models and assessed using head rotation angle for rear impact scenarios (3 g, 7 g, and 10 g peak accelerations), and compared to postmortem human surrogate (PMHS) data for 7 g impacts. Results The head rotation angle increased with the new material models for all three rear impact cases: (3 g: +43%, 7 g: +52%, 10 g: +71%), relative to the original model. The increase in head rotation was primarily attributed to the improved skin model, with the passive muscle being a secondary contributor to the increase in response. A 52% increase in head rotation for the 7 g impact improved the model response with respect to PMHS data, placing it closer to the experimental average, compared to the original model. Conclusions The improved skin, adipose tissue, and passive muscle material model properties, based on published experimental data, increased the neck compliance in rear impact, with improved correspondence to published PMHS test data for medium severity impacts. Future studies will investigate the coupled effect of passive and active muscle tissue for low severity impacts.