Objective:​ To assess the potential for cervical muscle injury from a rear-end automobile collision.

Design:​ Experimental design in which human subjects were exposed to low-speed rear-end collisions. The influence of independent variable (gender, speed change, muscle group, and motion phase) on dependent variables (kinematic response, muscle onset and muscle activation level) was examined using repeated-measures analysis of variance.

Background:​ Injuries to various tissues of the cervical spine have been proposed, yet little attention has been focused on the cervical muscles as a site of injury.

Methods:​ 42 subjects (21 males, 20–40 yr) were exposed to collisions of 4 and 8 km/h speed change while measuring kinematic response of the head and torso and electromyography of the sternocleidomastoid and cervical paraspinal muscles.

Results:​ Muscle activation occurred earlier in females and in the 8 km/h speed change. Sternocleidomastoid onset preceded paraspinal onset. Muscle activation level varied significantly with speed change, motion phase and muscle group. Initial rearward retraction of the head relative to the torso resulted in lengthening of the activated sternocleidomastoid, consistent with a contraction-induced muscle injury.

Conclusions:​ The cervical muscles contract rapidly in response to impact and the potential exists for muscle injury due to lengthening contractions.

Relevance:​ The clinician should recognize the role of cervical retraction in the mechanism of whiplash injury and avoid aggressive motion in that plane during diagnosis and treatment. An understanding of whiplash injury mechanisms should improve patient education and preventative measures.