Effect of muscle deactivation in the kinematic responses of cervical spine under emergency ejection - Finite element simulation.

The cervical spine is highly vulnerable to injury during high-acceleration emergency ejection because of its flexibility, head-torso inertial lag, and dependence on coordinated muscle function. This study investigated the biomechanical effects of selective neck muscle deactivation on cervical spine kinematics under vertical acceleration loading using a validated C0-T1 finite element model based on a Hill-type muscle model. A 10-G vertical acceleration was applied, and individual muscle deactivation was simulated by reducing muscle activation to 0.005 at 80 ms. The results showed all muscular effects became prominent after 100 ms. Among the 12 neck muscle deactivations, longus capitis, semispinalis capitis, trapezius, splenius capitis, and scalenus anterior produced substantial alterations in upper cervical (C0-C2) and global cervical (C0-C7) motion patterns. Rotation analysis showed C0-C3 flexion and C3-C7 extension, forming S-shaped and C-shaped curvatures during 0-100 ms and 100-150 ms, respectively. Except under longus capitis deactivation, rapid motion reversal of flexion to extension at approximately 135 ms, the entire cervical C0-C7 segments formed two unique S- and C-shaped curvatures in the loading phase, with a 27.5% reduction in overall C0-C7 flexion of overall C0-C7 segment. These motion patterns caused stress concentration and altered load transfer at the C2-C3 and C6-C7 intervertebral discs and elongation of ligaments. The findings highlight the critical role of coordinated neck muscle function in cervical spine stability during high-G ejection and may contribute to the development of targeted muscle training and protective cockpit systems.