An electromyographic study of low-velocity rear-end impacts.

STUDY DESIGN: Experimental analysis was performed to determine the effects of low-velocity rear-end impacts on phasic and magnitudinal activity of the cervical muscles in expected and unexpected conditions.

OBJECTIVES: To determine the phasic response of the cervical muscles to increasing low-velocity rear-end impacts, and to compare the quantitative effects of awareness and unawareness of impending impact.

SUMMARY OF BACKGROUND DATA: 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 low-velocity impacts are rare.

METHODS: Seven healthy volunteers were subjected to rear-end impacts of 4.9, 8.8, 10.8, and 13.7 m/s2 acceleration at two levels of expectation: expected and unexpected. 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 13.7 m/s2, the sternocleidomastoids generated up to 179% of their maximal voluntary contraction electromyogram, whereas the splenii and trapezii did not exceed 35% of their maximal voluntary contraction in most of the experimental conditions. 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 progressively decreased with increasing levels of acceleration. The kinetic variables and the electromyographic variables regressed significantly on the acceleration (P < 0.01), explaining from 96% to 100% of the variability.

CONCLUSIONS: Muscle responses were greater with higher levels of acceleration. 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.