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.