Musculo-tendinous stiffness of head-neck segment in the sagittal plane: an optimization approach for modeling the cervical spine as a single-joint system.

Despite the multi-linked architecture of the cervical spine, all previous studies that have made estimations of mechanical properties of the neck have considered the head-neck segment as a rigid link, with a fixed center of rotation at C7. The aim of this study was to consider the head-neck segment as a changeable geometry system for locating the resultant center of rotation and for calculating the musculo-tendinous stiffness by the quick-release method. Head kinematics during quick-releases was analyzed by recording the trajectory of surface markers. With an optimization procedure, the position of the resultant center of rotation of the head-neck segment was estimated. Thereafter, the angular displacement and acceleration of the head, together with the isometric torque developed by the cervical muscles were used to calculate the segment's stiffness. The results showed a consistent center of rotation and a significant increase of the musculo-tendinous stiffness with increasing torque.