Neck postural stabilization, motion comfort, and impact simulation

The human head-neck system requires continuous muscular stabilization in the presence of gravity and trunk motion. This chapter presents experimental and modeling efforts, applying mechanical perturbations to seated subjects, evaluating trunk and head motion, to investigate postural stabilization. A detailed multisegment neck model has been developed including vestibular/visual and muscular feedback loops and cocontraction. Dynamic validation is presented in the frequency domain in all six motion directions. The neck model captures primary motion responses and interaction terms such as head rotation in response to seat translation. Results show major contributions of vestibular/visual feedback stabilizing the head in space while muscular feedback stabilizes the head on the torso. In addition, muscular feedback is essential to stabilize the individual vertebral joints and prevent neck buckling. The contribution of cocontraction is estimated to be minor in the neck. Validation in impact conditions shows that postural control parameters estimated that fitting the model to small-amplitude experimental data can predict postural responses in high-amplitude loading conditions reasonably well. This manuscript focuses on the neck but also includes experiments with combined stabilization of the complete spine, measuring trunk and head motion, with a perspective toward full spine and full-body modeling. Lumbar stabilization has been captured using a simplified model by assuming a virtual pivot around L4/L5. The model uniquely separates stabilizing contributions of intrinsic stiffness and damping (including muscle cocontraction) and muscle feedback (length, velocity, and acceleration). The model parameters allowed us to estimate the relative contributions of intrinsic and reflexive stabilization and showed intrinsic contributions, similar to or larger than reflexive contributions in lumbar stabilization with horizontal perturbations to the trunk or pelvis. Experiments with a rotating pelvis showed relevant contributions of vestibular and visual feedback, which are more effective to minimize head than trunk rotation. A full-body human model with multisegment spine was previously validated for impact and vertical vibration. Integrating the new detailed neck model in the full-body human model will enable simulation of full-body vibration and impact scenarios with realistic compliant seat models. Further experiments and modeling efforts will aim to capture sensory integration of visual and vestibular motion perceptions in relation to posture maintenance and motion sickness.

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