The effect of trunk flexion in healthy volunteers in rear whiplash-type impacts.

STUDY DESIGN: Twenty young, 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 right and left.

OBJECTIVES: 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 other conditions that mimic road collisions, 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 (EMGs) of the sternocleidomastoids, trapezii, and splenii capitis were also recorded on 20 subjects (10 males and 10 females, mean age of 23.6 +/- 3 years)

RESULTS: For participants experiencing 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 EMG 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 its maximal voluntary contraction EMG (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 their maximal voluntary contraction EMG with head rotated to the left. On the other hand, with the head rotated to the right, the left sternocleidomastoid generated 94% of its maximal voluntary contraction EMG (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 whiplash victims are expected to be looking to the left or right at the time of collision.