Cervical muscle response to whiplash-type right lateral impacts

Nine healthy volunteers were subjected to right lateral impacts of 4.9, 8.8, 10.8, and 13.7 m/sec acceleration at two levels of expectation: expected and unexpected.

To determine the response of the cervical muscles to increasing low-velocity right lateral impacts and compare the quantitative effects of expected and unexpected impact.

The literature contains little information on the etiology of whiplash injuries. In particular, in vivo studies of the cervical muscular response and head-neck kinematics to lateral impacts are rare. A previous study of left lateral impacts suggests that the burden of impact is borne primarily by the muscles contralateral to the side of impact.

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.

At an acceleration of 13.7 m/sec2, both sternocleidomastoids, both trapezii, and the right splenius capitis generated on average around 40% or less of their maximal voluntary contraction electromyogram in both the expected and unexpected impact conditions. The left splenius capitis (i.e., contralateral to the right lateral impact), however, generated 84% of its maximal voluntary contraction electromyogram (double the response of other muscles). Electromyographic variables were affected significantly by the levels of acceleration (P < 0.01). The time to onset and time to peak electromyogram time for all muscles decreased progressively with increasing levels of acceleration. The kinetic variables and the electromyographic variables regressed significantly on the acceleration (P < 0.01), and the head acceleration was affected significantly by expectation (P < 0.01). In response to right lateral impacts, muscle responses were generally greater, with higher levels of acceleration, and greatest for the splenius capitis muscle contralateral to the side of impact (P < 0.01).

Because the muscular component of the head-neck complex likely 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, and specifically, the splenius capitis muscle contralateral to the side of impact is most likely to be injured. Being aware of an impact may reduce the degree of head perturbation.