Turning away from whiplash. An EMG study of head rotation in whiplash impact

To determine the response of the cervical muscles to whiplash-type perturbations through low-velocity frontal impacts when the head is rotated to the right and left.

Twenty healthy volunteers were subjected to increasing acceleration in low-velocity frontal impacts, randomly with head rotated either left or right. Bilateral EMG of the sternocleidomastoids, trapezii, and splenii capitis and acceleration of the sled, torso, and head were recorded.

With either direction of head rotation at the time of impact, the muscle responses increased with increasing levels of acceleration (p < 0.01). The time to onset and peak electromyogram for all muscles progressively decreased with increasing levels of acceleration. With the head rotated to the left, the left trapezius generated 77% of its maximal voluntary contraction (MVC) EMG (more than double the response of other muscles). In comparison, the right trapezius generated only 33% of its MVC. The right sternocleidomastoid (25%) and left splenius muscles (32%), the ones responsible for head rotation to the left, were more active than their counterparts (the left sternocleidomastoid generated only 5% of its MVC EMG and the right splenius 9%). On the other hand, with the head rotated to the right, the right trapezius generated 71% of its MVC EMG, while the left trapezius generated only 30% of this value. Again, the left sternocleidomastoid (27% of its MVC EMG) and right splenius (28% of its MVC EMG), being responsible for head rotation to the right, were more active than their counterparts (the right sternocleidomastoid generated only 4% of its MVC EMG and the left splenius 13%).

Frontal impacts tend to generate the most muscle activity in the ipsilateral trapezius muscle, increasing the risk of their injury.