Looking away from whiplash: effect of head rotation in rear impacts.

STUDY DESIGN Twenty healthy volunteers in a laboratory were subjected to rear-end impacts 4.4, 7.9, 10.9, and 13.1 m/s acceleration, with head rotation to the right and left. OBJECTIVE The purpose of this study was to determine the response of the cervical muscles to increasing low-velocity rear impacts when the head is rotated at the time of impact. SUMMARY OF BACKGROUND DATA A previous study of rear impacts with head in neutral posture suggests that the burden of impact is borne primarily by the sternocleidomastoid muscles bilaterally. To improve automobile designs to prevent whiplash injury, we need to understand the response of the cervical muscles to whiplash-type perturbations in less-than-ideal conditions, such as when the head is rotated to the right and left at the time of rear-end impact. METHODS Triaxial accelerometers recorded the acceleration of the sled, torso at the shoulder level, and head of the participant, while bilateral electromyograms of the sternocleidomastoids, trapezii, and splenii capitis were also recorded. RESULTS For participants having a rear-end impact, whether having the head rotated to the left or right at the time of impact, the muscle responses increased with increasing levels of acceleration (P < 0.01). The time to onset and time to peak electromyogram for all muscles progressively decreased with increasing levels of acceleration (P < 0.01). Which muscle responded most to a whiplash-type neck perturbation was determined by the direction of head rotation. With the head rotated to the left, the right sternocleidomastoid generated 88% of the maximal voluntary contraction electromyogram (at least triple the response of other muscles). In comparison, the left sternocleidomastoid, both trapezii, and the splenii capitis generated on average only 10% to 30% of the maximal voluntary contraction electromyogram with head rotated to the left. On the other hand, with the head rotated to the right, the left sternocleidomastoid generated 94% of the maximal voluntary contraction electromyogram (again, at least triple the response of other muscles). CONCLUSIONS If the head is rotated out of neutral posture at the time of rear impact, the injury risk tends to be greater for the sternocleidomastoid muscle contralateral to the side of rotation. Measures to prevent whiplash injury may have to account for the asymmetric response because many victims of whiplash are expected to be looking to the left or right at the time of collision.