Effect of trunk flexion on the occupant neck response to anterolateral whiplash impacts.

OBJECTIVE The purpose of this study was to determine the response of the cervical muscles to increasing low-velocity anterolateral impacts with the volunteer's trunk flexed to the right and left. METHODS A total of 20 healthy volunteers were subjected to left anterolateral impacts of 4.0, 7.6, 10.7, and 13.4 m/sec and, sequentially, with trunk flexed either left or right. Bilateral electromyograms (EMGs) of the sternocleidomastoids, trapezii, and splenii capitis were recorded. DESIGN At an acceleration of 13.4 m/sec, with the trunk flexed left, the left trapezius generated 48% of its maximal voluntary contraction EMG, whereas the right trapezius (contralateral to the left anterolateral impact) generated 38% of this variable. All other muscle generated </=23% of their maximal voluntary contraction EMG, a significant difference from the trapezii (P = 0.005). Similarly, with the trunk flexed to the right under these same conditions, the left trapezius generated 26% and the right trapezius 35% of their maximal voluntary contraction EMG. Again, all other muscles generated significantly less EMG activity, </=22% (P = 0.009). Overall, the EMG responses were of low magnitude compared with known data with the trunk in neutral posture in this direction of impact. CONCLUSIONS When the subject sits with trunk flexed out of neutral posture at the time of an anterolateral impact, the cervical muscle response is reduced compared with anterolateral impacts with the trunk in neutral posture.