Cervical muscle response to trunk flexion in whiplash-type lateral impacts

The purpose of this study was to determine the response of the cervical muscles to increasing low-velocity, whiplash-type lateral impacts when the occupant is seated out of the recommended driving position (neutral posture). Twenty healthy volunteers were subjected to left lateral impacts of 4.1, 7.7, 10.5, and 13.7 m/s(2) acceleration, with their trunk flexed by 45 degrees and laterally flexed to the right and left also by 45 degrees at the time of impact. Bilateral electromyograms of the sternocleidomastoids, trapezii, and splenii capitis were recorded. Under these conditions of trunk-flexed postures, in a left lateral impact, muscle responses were of generally low magnitude with the trunk flexed to either the left or right. Even at the highest acceleration of 13.7 m/s(2), all muscles generated less than 37% of their known maximal voluntary contraction electromyogram. Also, in these left lateral impacts, the right splenius capitis showed a greater EMG response than the left splenius capitis regardless of whether the subject was flexed to the right or left at the time of impact. The right splenius capitis (the one contralateral to the left lateral impact direction) was more active than its counterpart. Compared to what is known for EMG responses with an occupant in the neutral posture, the right sternocleidomastoid (usually the most active muscle in a left lateral collision) was significantly less-active with trunk flexion than with neutral posture conditions (P<0.01). In the absence of bodily impact, the flexed trunk posture does not produce a biomechanical response that would increase the likelihood of cervical muscle injury in low velocity lateral impacts, and may lessen the risk of injury for some muscles.