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This study evaluated the biofidelity of both the Hybrid III and the THOR-NT anthropomorphic test device (ATD) necks in quasistatic tension-bending and pure-bending by comparing the responses of both the ATDs with results from validated computational models of the living human neck. This model was developed using post-mortem human surrogate (PMHS) osteoligamentous response corridors with effective musculature added (Chancey, 2005). Each ATD was tested using a variety of end-conditions to create the tension-bending loads. The results were compared using absolute difference, RMS difference, and normalized difference metrics. The THOR-NT was tested both with and without muscle cables. The THOR-NT was also tested with and without the central safety cable to test the effect of the cable on the behavior of the ATD. The Hybrid III was stiffer than the model for all tension-bending end conditions.

A computational head-neck model was developed to more efficiently study dynamic responses of the head and neck to near-vertex head impact. The model consisted of rigid vertebrae interconnected by assemblies of nonlinear springs and dashpots, and a finite element shell model of the skull. Quasi-static flexion-extension characteristics of ten human cadaveric cervical spines were measured using a test frame capable of applying pure moments. The cadaveric motion segments demonstrated a nonlinear stiffening response without a no-load neutral zone. Computational model parameters were based upon these measurements and existing data reported in the literature. Geometric and inertial characteristics were derived from three-dimensional reconstructions of skull and vertebral CT images. The model reproduced the shape and timing of the cervical spine buckling deformations observed in high speed video of cadaveric studies of near-vertex head impact [1].

Unlike other modes of loading, the tolerance of the human neck in tension depends heavily on the load bearing capabilities of the muscles of the neck. Because of limitations in animal models, human cadaver, and volunteer studies, computational modeling of the cervical spine is the best way to understand the influence of muscle on whole neck tolerance to tension. Muscle forces are a function of the muscle's geometry, constitutive properties, and state of activation. To generate biofidelic responses for muscle, we obtained accurate three-dimensional muscle geometry for 23 pairs of cervical muscles from a combination of human cadaver dissection and 50th percentile male human volunteer magnetic resonance imaging and incorporated those muscles into a computational model of the ligamentous spine that has been previously validated against human cadaver studies.

The absence of constitutive data on muscle has limited the development of models of cervical spinal dynamics and our understanding of the forces developed in the cervical spine during impact injury. Therefore, the purpose of this study is to characterize the structural and material properties of skeletal muscle. The structural responses of the tibialis anterior of the rabbit were characterized in the passive state using the quasi-linear theory of viscoelasticity (r = 0.931 ± 0.032). In passive muscle, the average modulus at 20% strain was 1.75 ± 1.18, 2.45 ± 0.80, and 2.79 ± 0.67 MPa at test rates of 4, 40, and 100 cm·s-1, respectively. In stimulated muscle, the mean initial stress was 0.44 ± 0.15 MPa and the average modulus was 0.97 ± 0.34 MPa. These data define a corridor of responses of skeletal muscle during injury, and are in a form suitable for incorporation into computational models of cervical spinal dynamics.

The passive combined flexion and axial loading responses of the unembalmed human cervical spine were measured in a dynamic test environment. The influence of end condition (the degree of constraint imposed on the head by the contact surface) was varied to determine its effect on observed column stiffness and on failure modes of the cervical spine. Multi-axis load cells were used to completely describe the forces and moments developed in the specimen. Twenty three specimens were studied. The Hybrid III neckform performance was assessed to determine its suitability as a mechanical simulator of the neck during head impact. Changes in end condition produced significant changes in axial stiffness in both the Hybrid III neckform and the cadaver neck. The mode of injury also varied as a function of end condition in a repeatable fashion. Separation of injuries based upon imposed end condition identified groups with significantly different axial load to failure.

The aim of this study was to describe and explain the variation of neck muscle strength along the cervical spine. A three-dimensional model of the head-neck complex was developed to test the hypothesis that the moment-generating capacity of the neck musculature is lower in the upper cervical spine than in the lower cervical spine. The model calculations suggest that the neck muscles can protect the lower cervical spine from injury during extension and lateral bending. The maximum flexor moment developed in the lower cervical spine was 2 times higher than that developed in the upper spine. The model also predicted that the neck musculature is 30% stronger in the lower cervical spine during lateral bending. Peak compressive forces (up to 3 times body weight) were higher in the lower cervical spine. These results are consistent with the clinical finding that extension loading of the neck often leads to injuries in the upper cervical spine.