Cervical muscle response to posterolateral impacts--effect of head rotation.

Background: Rear impacts may be offset, and may also occur with the head rotated at impact. The purpose of this study was to determine the response of the cervical muscles to increasing low-velocity rear impacts offset by 45 degrees to the subject's right, but also with the head rotated either right or left.

Methods: Twenty healthy volunteers underwent right posterolateral impacts of 4.6, 8.0, 11.0, and 13.1 m/s2 acceleration with head turned right or left. Electromyograms of the sternocleidomastoids, trapezii, and splenii capitis were recorded, as were acceleration of the sled, torso, and head of the participant.

Findings: At an impact acceleration of 13.1 m/s2, with head rotated to the right, the left sternocleidomastoid and the contralateral (to rotation) trapezius and splenius capitis generated more electromyogram activity than their counterparts (P < 0.05). Following impacts with the head rotated to the right, the left sternocleidomastoid generated 67% of its maximal voluntary electromyogram, and the right sternocleidomastoid 5% of the maximal voluntary electromyogram, whereas the remaining muscles did not generate more than 31% of the maximal voluntary contraction electromyogram. In impacts with head rotated left, the right sternocleidomastoid generated 53% of its maximal voluntary electromyogram, and the left splenius capitis and left trapezius showed more electromyogram activity than their counterparts (P < 0.05).

Interpretation: Head rotation in a right posterolateral impact modifies the cervical response mainly by generating an asymmetry in the paired sternocleidomastoid electromyograms. This may asymmetrically affect the risk of injury to the sternocleidomastoids. An understanding of the muscular response to rear-impacts of different types and the effect of head rotation at the time of impact is relevant to understanding the mechanism of acute whiplash injury and may be helpful to develop targeted treatments and preventative measures.