The Effect of a 3-Point Harness Restraint and Car Seat in Whiplash-Type Lateral Impacts

Study Design. Seventeen healthy volunteers were subjected to right and left lateral impacts 5.0, 6.8, 9.2, and 16.8 m/s2 acceleration while positioned in a Volvo car seat with lap and shoulder seat belt restraint in laboratory setting. Objectives. The purpose of this study was to determine the effect of using a standard 3-point lap and shoulder seat belt and Volvo car seat on the response of the cervical muscles to increasing low-velocity lateral impacts. Summary of Background Data. A previous study of lateral impacts in a 5-point harness restraint with head and trunk in neutral posture suggests that the burden of impact is borne primarily by the splenius capitis muscle contralateral to the direction of impact. That study, however, used a nonstandard harness for automobiles, and other studies suggest that a lap-and-shoulder seat belt may increase the risk of whiplash injury. Methods. Triaxial accelerometers recorded the acceleration of the 1) sled, 2) torso at the shoulder level, and 3) head of the participant, while bilateral electromyograms of the sternocleidomastoids, trapezii, and splenii capitis were also recorded. Results. For participants experiencing a right or left lateral impact, the muscle responses increased with increasing levels of acceleration (P < 0.05). The time to onset and time to peak electromyogram for most muscles also showed a trend to progressively decrease with increasing levels of acceleration. The peak head accelerations relative to the sled ranged from 2.5 to 10.6 m/s2. When the impact was a right lateral impact, at the highest sled acceleration, the left splenius capitis generated 47% of its maximal voluntary contraction (MVC), and the left trapezius also 46% of its MVC; the left and right sternocleidomastoid, right splenius capitis, and right trapezius generated 29% or less of their MVC. For the highest level of acceleration in a left lateral impact, the right splenius capitis generated 48% of its MVC and the right trapezius 57% of the MVC, the left and right sternocleidomastoid, left splenius capitis, and left trapezius generated 29% or less of their MVC. In both directions of impact, the contralateral splenius capitis and trapezius showed a statistically significant difference in the EMG response compared with other muscles (P < 0.05). For participants experiencing a frontal impact, whether straight-on, right or left anterolateral, the muscle responses increased with increasing levels of acceleration (P < 0.05). The time to onset and time to peak electromyogram for most muscles also showed a trend to progressively decrease with increasing levels of acceleration. In a straight-on frontal impact, the trapezii (TRPs) muscles showed the greatest EMG response compared with the remaining muscles (P < 0.05). Conclusions. Compared with previously reported impact studies with a 5-point harness and rigid seat, the use of a 3-point lap and shoulder seat belt with car seat does not appear to adversely affect the cervical muscle response. In very-low- and low-velocity impact experiments, seat belt and seat type may thus not be particularly relevant to cervical EMG and kinematics.