Electromyographic and kinematic exploration of whiplash-type rear impacts: effect of left offset impact.

BACKGROUND CONTEXT Although there are some volunteer collision studies reporting the effects of rear impacts on head and neck kinematics, there are few studies detailing the cervical muscle electromyogram response. Moreover, the effect of a rear impact offset to the left on the resultant muscle responses is unknown. PURPOSE 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 left, and to compare the quantitative effects of expected and unexpected impact. STUDY DESIGN/SETTING Nine healthy volunteers were subjected to rear impacts, offset by 45 degrees to the subject's left, of 4.6, 8.7, 11.0 and 14.5 m/s2 acceleration, at two levels of expectation: expected and unexpected. METHODS Bilateral electromyograms of the sternocleidomastoids, trapezii and splenii capitis were recorded. Triaxial accelerometers recorded the acceleration of the chair, torso at the shoulder level and head of the participant. RESULTS At an acceleration of 14.5 m/s2, the left sternocleidomastoid generated 71% and the right sternocleidomastoid 82% of their maximal voluntary contraction electromyogram in the unexpected impact conditions. Under these conditions, the right splenius capitis (contralateral to the left offset rear impact) also generated 71% of its maximal voluntary contraction, whereas the left splenius capitis generated only 20% of this variable. The trapezii generated only 25% of their maximal voluntary contraction. Subjects exhibited lower levels of their maximal voluntary contraction electromyogram when the impact was expected. Electromyographic variables were significantly affected by the levels of acceleration and expectation (p<.001). The time to onset and time to peak electromyogram for all muscles progressively decreased with increasing levels of acceleration, in the unexpected condition. The kinetic variables and the electromyographic variables regressed significantly on the acceleration (p<.001). In response to rear impacts offset to the subject's left, muscle responses were greater with higher levels of acceleration, greater with unexpected impact conditions and greatest for both sternocleidomastoids and for the splenius capitis muscle contralateral to the side of impact. CONCLUSIONS Because the muscular component of the head-neck complex plays a role in the abatement of impact at higher acceleration levels, they are likely a primary site of injury in the whiplash phenomenon in rear collisions. More specifically, when a rear impact is offset to the subject's left, it results in not only increased electromyographic generation in both sternocleidomastoids, but the splenius capitis contralateral to the direction of impact offset also bears part of the force of the neck perturbation. Expecting or being aware of imminent impact also plays a role in reducing muscle responses in low-velocity offset rear impacts.