The effect of seat belt use on the cervical electromyogram response to whiplash-type impacts.

OBJECTIVE: The objective of this study was to determine the effect of a standard 3-point lap-and-shoulder seat belt and car seat on the electromyogram (EMG) response of the cervical muscles to increasing low-velocity impacts in comparison with that of a rigid seat and 5-point restraint. METHODS: Seventeen healthy volunteers were subjected to rear, frontal, right and left lateral and bilateral anterolateral, and posterolateral impacts with an acceleration varying from 4.4 to 16.8 m/s(2) while in a car seat with lap-and-shoulder seat belt. RESULTS: For rear-end impacts, whether straight on, right posterolateral, or left posterolateral, all muscles generated 50% or less of the maximal voluntary contraction (MVC) EMG. In straight-on rear impacts, the sternocleidomastoid was symmetrically the most active; however, in posterolateral impacts, the sternocleidomastoid contralateral to impact direction was more active than its counterpart. For a right lateral impact, at the highest acceleration, the left splenius capitis generated 47% of its MVC and the left trapezius did 46% of its MVC. In a left lateral impact, the right splenius capitis generated 48% of its MVC and the right trapezius did 57% of its MVC. In a straight-on frontal impact, the left trapezius generated 35% of its MVC and the right trapezius did 48% of its MVC. In a left anterolateral impact, the right splenius generated 60% of its MVC and the right trapezius did 66% of its MVC. Similarly, in a right anterolateral impact, the contralateral splenius muscle increased its activity to 52% of its MVC and the left trapezius was at 52% of its MVC. CONCLUSIONS: Compared with previously reported impact studies with a rigid seat and 5-point harness, the use of a 3-point lap-and-shoulder seat belt with a standard car seat did not appear to adversely affect cervical muscle response. In very-low-velocity and low-velocity impact experiments, seat belt and seat type may not significantly alter cervical EMG and kinematics.

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

OBJECTIVE: To determine the electromyogram (EMG) response of the cervical muscles to a right lateral impact whiplash-type perturbation when the head is rotated. METHODS: Twenty healthy volunteers were subjected to right lateral impacts of 4.2, 8.1, 10.3, and 12.5 m/s2 and were looking either left or right. Bilateral EMGs of the sternocleidomastoid, trapezius, and splenius capitis muscles were recorded. Triaxial accelerometers recorded the acceleration of the chair, torso at the shoulder level, and head of the participant. RESULTS: In a right lateral impact, muscle responses were of low magnitude with the head rotated to either the left or the right. At the highest acceleration of 12.5 m/s2, all generated less than 39% of their maximal voluntary contraction EMG. The sternocleidomastoid muscle showed a greater EMG response than its counterpart and the muscles contralateral to the direction of impact had higher EMG responses. The time to onset of the EMG for the splenii capitis and trapezii generally decreased with increasing levels of acceleration. As anticipated, an increase in applied acceleration resulted in an increase in accompanying head accelerations (P < .05), and when the head acceleration increased, so too did the force equivalent exertions by the various muscles. CONCLUSIONS: Overall, a right lateral impact with head rotation to either right or left appears to reduce the activity and thus the risk of muscle injury, perhaps because of "bracing" by muscles actively producing rotation or because of greater spinal stability from other structures when the head is in the rotated position.