Cervical muscle response during whiplash: evidence of a lengthening musclecontraction

Objective. To assess the potential for cervical muscle injury from a rear-e nd automobile collision. Design. Experimental design in which human subjects were exposed to low-spe ed rear-end collisions. The influence of independent variable (gender, spee d change, muscle group, and motion phase) on dependent variables (kinematic response, muscle onset and muscle activation level) was examined using rep eated-measures analysis of variance. Background Injuries to various tissues of the cervical spine have been prop osed, yet little attention has been focused on the cervical muscles as a si te of injury, Methods. 42 subjects (21 males, 20-40 yr) were exposed to collisions of 4 a nd 8 km/h speed change while measuring kinematic response of the head and t orso and electromyography of the sternocleidomastoid and cervical paraspina l muscles. Results. Muscle activation occurred earlier in females and in the 8 km/h sp eed change. Sternocleidomastoid onset preceded paraspinal onset. Muscle act ivation level varied significantly with speed change, motion phase and musc le group, Initial rearward retraction of the head relative to the torso res ulted in lengthening of the activated sternocleidomastoid, consistent with a contraction-induced muscle injury. Conclusions. The cervical muscles contract rapidly in response to impact an d the potential exists for muscle injury due to lengthening contractions. Relevance The Clinician should recognize the role of cervical retraction in the mecha nism of whiplash injury and avoid aggressive motion in that plane during di agnosis and treatment. An understanding of whiplash injury mechanisms shoul d improve patient education and preventative measures. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.