An observational electromyography study of the effect of trunk flexion in low-velocity frontal whiplash-type impacts.

OBJECTIVE

To examine the effect of forward and lateral trunk flexion on the cervical electromyogram and head kinematic response to whiplash-type frontal impacts.

DESIGN

Observational study of sled impacts.

SETTING

Laboratory.

PARTICIPANTS

Twenty healthy volunteers.

INTERVENTION

Twenty volunteers were subjected to increasing low-velocity (<8km/h) frontal impacts of 4.4, 7.6, 10.3, and 13.3m/s(2) acceleration with trunk forward flexed by 45 degrees and laterally flexed to the right and left by 45 degrees .

MAIN OUTCOME MEASURES

Bilateral electromyography of the sternocleidomastoids, trapezii, and splenii capitis and acceleration of the sled, torso, and head were recorded.

RESULTS

With either direction of lateral trunk flexion at impact, the trapezii electromyographic activity increased with increasing acceleration (P<.05). With the trunk flexed to the left, the left trapezius generated 39% of its maximal voluntary contraction (MVC) electromyographic activity, while the right trapezius generated 31% of its MVC electromyographic activity. The left splenius (ipsilateral to leftward trunk flexion) generated 24% of its MVC electromyographic activity, with all other muscles generating 15% or less of this measure. With the trunk flexed to the right, the right trapezius generated 38% of its MVC electromyographic activity, while the left trapezius generated 32% of this value. Again, the ipsilateral (to trunk flexion) splenius capitis generated 27% of its MVC electromyographic activity, and all other muscles 11% or less of this measure.

CONCLUSIONS

When subjects sit with trunk flexed out of neutral posture at the time of frontal impact, the cervical muscle response is low and unlikely to be injurious.