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.@en

Objective
To identify effects of a deviant motor drive in the autospectral power of dystonic muscles during voluntary contraction in cervical dystonia patients.
Methods
Submaximal (20%) isometric head-neck tasks were performed with the head fixed, measuring surface EMG of the sternocleidomastoid, splenius capitis and semispinalis capitis in CD patients and controls. Autospectral power of muscle activity, and head forces was analyzed using cumulative distribution functions (CDF). A downward shift between the theta/low alpha-band (3-10 Hz) and the high alpha/beta-band (10-30 Hz) was detected using the CDF10, defined as the cumulative power from 3-10 Hz relative to power from 3-30 Hz.
Results
CDF10 was increased in dystonic muscles compared to controls and patient muscles unaffected by dystonia, due to a 3-10 Hz power increase and a 10-30 Hz decrease. CDF10 also increased in patient head forces.
Conclusions
Submaximal isometric contractions with the head fixed provided a well-defined test condition minimizing effects of reflexive feedback and tremor. We associate shifts in autospectral power with prokinetic sensorimotor control.
Significance
Analysis of autospectral power in isometric tasks with the head fixed is a promising approach in research and diagnostics of cervical dystonia.@en

Background Effective sensorimotor integration is essential to modulate (adapt) neck stabilization strategies in response to varying tasks and disturbances. This study evaluates the hypothesis that relative to healthy controls cervical dystonia patients have an impaired ability to modulate afferent feedback for neck stabilization with changes in the frequency content of mechanical perturbations. Methods We applied anterior-posterior displacement perturbations (110 s) on the torso of seated subjects, while recording head-neck kinematics and muscular activity. We compared low bandwidth (0.2–1.2 Hz) and high bandwidth (0.2–8 Hz) perturbations where our previous research showed a profound modulation of stabilization strategies in healthy subjects. Cervical dystonia patients and age matched controls performed two tasks: (1) maintain head forward posture and (2) allow dystonia to dictate head posture. Findings Patients and controls demonstrated similar kinematic and muscular responses. Patient modulation was similar to that of healthy controls (P > 0.05); neck stiffness and afferent feedback decreased with high bandwidth perturbations. During the head forward task patients had an increased neck stiffness relative to controls (P 

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The human head-neck system requires continuous stabilization in the presence of gravity and trunk motion. We investigated contributions of the vestibulocollic reflex (VCR), the cervicocollic reflex (CCR), and neck muscle co-contraction to head-in-space and head-on-trunk stabilization, and investigated modulation of the stabilization strategy with the frequency content of trunk perturbations and the presence of visual feedback. We developed a multisegment cervical spine model where reflex gains (VCR and CCR) and neck muscle co-contraction were estimated by fitting the model to the response of young healthy subjects, seated and exposed to anterior-posterior trunk motion, with frequency content from 0.3 up to 1, 2, 4 and 8 Hz, with and without visual feedback. The VCR contributed to head-in-space stabilization with a strong reduction of head rotation (<8 Hz) and a moderate reduction of head translation (>1 Hz). The CCR contributed to head-on-trunk stabilization with a reduction of head rotation and head translation relative to the trunk (<2 Hz). The CCR also proved essential to stabilize the individual intervertebral joints and prevent neck buckling. Co-contraction was estimated to be of minor relevance. Control strategies employed during low bandwidth perturbations most effectively reduced head rotation and head relative displacement up to 3 Hz while control strategies employed during high bandwidth perturbations reduced head global translation between 1 and 4 Hz. This indicates a shift from minimizing head-on-trunk rotation and translation during low bandwidth perturbations to minimizing head-in-space translation during high bandwidth perturbations. Presence of visual feedback had limited effects suggesting increased usage of vestibular feedback. @en