Cervical muscle response to whiplash-type right anterolateral impacts

Abstract

Frontal impacts are a common cause of whiplash injury. Yet, volunteer studies of the cervical muscular response and head-neck kinematics to frontal impacts are uncommon, and specifically, the effect of an offset (anterolateral) frontal impact on the resultant muscle responses is unknown. The purpose of this study was to determine the response of the cervical muscles to increasing low-velocity frontal impacts offset by 45°to the right, and to compare the quantitative effects of expected and unexpected impact. Ten healthy volunteers were subjected to frontal impacts, offset by 45°to the subject's right, of 5.1-, 8.7-, 12-, and 13.7-m/s 2 peak 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. At a peak acceleration of 13.7 m/s 2 , with an unexpected impact, the contralateral trapezius (i.e., left trapezius in a right anterolateral impact) generated 83% of its maximal voluntary contraction electromyogram, whereas all other muscles generated 50% or less of this variable. Although it generated less EMG, the splenius capitis muscle also tended to show an asymmetric EMG response, with the left (contralateral) splenius capitis generating a higher percentage (46%) of its maximal voluntary contraction electromyogram than the ipsilateral (right) splenius capitis. In comparison, the sternocleidomastoid muscles behaved symmetrically and generated 25% or less of this variable under all impact conditions. Similarly, the times to onset and times to peak electromyogram for the contralateral (left) splenius capitis and (left) trapezius progressively decreased with increasing levels of acceleration (p<0.01). Subjects exhibited lower levels of their maximal voluntary contraction electromyogram when the impact was expected (p<0.01). The kinetic variables and the electromyographic variables regressed significantly on the acceleration (p<0.01). In response to right anterolateral impacts, muscle responses were greater with higher levels of acceleration, and more specifically, when a frontal impact is offset to the subject's right, it results in not only increased EMG generation in the contralateral trapezius, but the splenius capitis contralateral to the direction of impact also bears part of the force of the neck pertubation. Expecting or being aware of imminent impact plays a role in reducing muscle responses in low-velocity anterolateral impacts.

The role of sternocleidomastoid muscle in simulated low velocity rear-end impacts

“…The present study did not identify more rapid SCM muscle response with greater impact velocity. This finding is not in agreement with those of Kumar et al [6,7,9], and Brault et al [2]. Despite the relatively large sample size in the present study, there were large variances of the timing variables resulting in low statistical power (%0.1).…”

“…Despite the relatively large sample size in the present study, there were large variances of the timing variables resulting in low statistical power (%0.1). The Kumar et al [9] (seven subjects) [6,7] (ten subjects) studies were based on a smaller sample size and although the variance appears smaller than the present study, they did not report statistical power. The Brault et al [2] study was based on a large sample size Time is presented as raw data Time is presented in milliseconds with onset of chair movement recorded as time zero.…”

“…It has been reported that awareness of an impact influences the muscle response in a simulated whiplash event [9,21]. Kumar et al [6,7,9,10], reported SCM EMG magnitude reduced with expectancy of impact. Likewise, Kumar et al [9] reported reduced EMG latency in expected impacts.…”

“…Siegmund et al [23] and Brault et al [2] used the bumper contact between the two cars as time zero. Kumar et al in all his studies [6][7][8][9][10] used the firing of the solenoid as time zero; therefore, the actual movement of the chair varies for each impact. It remains unknown as to what extent the finding of reduced onset of head movement and muscle activity with increased impact magnitude is due to the significantly reduced onset of chair movement.…”

“…Kumar et al [6][7][8][9][10] used a molded plastic chair rather than an automobile seat. Siegmund et al [21] suggest that the use of a molded plastic chair may not allow direct comparison between these studies and those that use a car seat.…”