Seven healthy volunteers were subjected to left lateral impacts of 4.8, 7.9, 11, and 13.7 m/s acceleration at two levels of expectation: expected and unexpected. The purpose of this study was to determine the response of the cervical muscles to increasing low-velocity left lateral impacts, and to compare the quantitative effects of expected and unexpected impact. The literature contains little information on the etiology of whiplash injuries. Animal and cadaver studies have yielded some insight into the phenomenon. However, in vivo studies of the cervical muscular response and head-neck kinematics to lateral impacts are rare. Bilateral electromyograms of the sternocleidomastoids, trapezii, and splenii capitis were recorded bilaterally. Triaxial accelerometers recorded the acceleration of the chair, torso at the shoulder level, and head of the participant. At an acceleration of 13.7 m/s, the sternocleidomastoids and trapezii generated approximately <or=50% of their maximal voluntary contraction electromyogram in both the expected and unexpected impact conditions. Study participants exhibited lower levels of their maximal voluntary contraction electromyogram when the impact was expected. The splenius capitis behaved similar to these muscles in the expected condition, but when the impact was unexpected, the splenius capitis muscle contralateral to the impact (i.e., right splenius in a left lateral impact) generated 94% of its maximal voluntary contraction electromyogram. Electromyographic variables were significantly affected by the levels of acceleration and expectation (P < 0.001). The onset time and peak electromyogram time for the sternocleidomastoid, splenii capitis, and trapezii on the side contralateral to the side of impact progressively decreased with increasing levels of acceleration. The onset time and peak electromyogram time for the sternocleidomastoid, splenii, and trapezii on the same side of impact progressively increased with increasing levels of acceleration. The kinetic variables and the electromyographic variables regressed significantly on the acceleration (P < 0.01). In response to left lateral impacts, muscle responses were greater with higher levels of acceleration, greater with unexpected impact conditions, and greatest for the splenius capitis muscle contralateral to the side of impact. Because the muscular component of the head-neck complex plays a central role in the abatement of higher acceleration levels, it may be a primary site of injury in the whiplash phenomenon in lateral collisions. Expecting or being aware of imminent impact may play a role in reducing muscle responses in low-velocity impacts. (Author/publisher)
Abstract