Cervical muscle response to head rotation in whiplash-type left lateral impacts.

STUDY DESIGN Twenty healthy volunteers were subjected to left lateral impacts, randomly looking either left or right. OBJECTIVES The purpose of this study is to determine the response of the cervical muscles to lateral impact whiplash-type perturbations when the head is rotated at the time of impact. SUMMARY OF BACKGROUND DATA A previous study of left lateral impacts with head in neutral posture suggests that the burden of impact is borne primarily by the splenius capitis muscles. In order 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 at the time of lateral impact. METHODS Bilateral electromyograms of the sternocleidomastoids, trapezii, and splenii capitis were recorded. Triaxial accelerometers recorded the acceleration of the sled, torso, and head of the participant. RESULTS For participants experiencing a left lateral impact, whether having the head rotated to the left or right at the time of impact, the muscle responses generally increased with increasing levels of acceleration (P < 0.05). The time of onset and peak electromyograph reading for most muscles progressively decreased with increasing acceleration. Overall, however, muscle responses were of low magnitude with the head rotated to the left or right. The sternocleidomastoid muscle responsible for head rotation (i.e., right sternocleidomastoid muscle when looking left) had a greater electromyograph response than its counterpart (P < 0.05). The contralateral splenius capitis and trapezius had the highest electromyograph responses. The remaining muscles showed similar levels of electromyograph responses, but even at the highest acceleration of 12.8 m/s, all generated less than 45% of their maximal voluntary contraction electromyograph reading. CONCLUSIONS Compared to a lateral impact with the volunteer's head in the neutral position, a lateral impact with head rotation to either right or left may reduce the muscle response. Further studies are needed to determine whether or not head rotation at the time of lateral impact reduces overall injury risk.