Biomechanical Mechanisms of Whiplash Injury: Effects of Impact Severity, Initial Posture, and Muscle Activation on Cervical S-shaped Deformation in Rear-End Collisions

Abstract Although rarely life-threatening, whiplash-associated disorders caused by rear-end collisions frequently lead to chronic symptoms, and their underlying mechanisms of injury remain incompletely understood. In particular, the mechanical relationship between the characteristic S-shaped deformation of the head-neck complex during the early phase of rear-end collisions and cervical soft tissue injury has not been fully clarified. Herein, rear-end collision simulations were performed using a human body model incorporating autonomous cervical muscle control. Moreover, the effects of impact severity, initial head-neck posture, and cervical muscle activation on whiplash kinematics and facet joint capsule loading were systematically investigated. Specifically, S-shaped deformation was quantified using the maximum rotational difference (S-θmax) between the head and cervical vertebrae, and its association with facet joint capsule strain was examined. The results showed that increasing impact severity resulted in larger S-θmax values and earlier peak deformation occurrence. Under high-severity impact conditions, pronounced S-shaped deformation was observed during the early postimpact phase, accompanied by increased tensile strain in the facet joint capsules, particularly at the C2-C3 and C4-C5 levels. By contrast, simulations with a mildly flexed initial head-neck posture combined with preactivated cervical muscles exhibited more stable kinematic responses, reduced S-shaped deformation, and decreased facet joint capsule strain. Taken together, initial head-neck posture and cervical muscle activation during the early phase of rear-end collisions play an important role in regulating cervical deformation patterns and the associated risk of soft tissue injury.