The startle response during whiplash: a protective or harmful response?

Journal of Applied Physiology August 15, 2012; Vol. 113; No. 4, pp. 532-540 Daniel WH Mang, Gunter P Siegmund, J Timothy Inglis, JS Blouin KEY POINTS FROM THIS STUDY: 1) During whiplash collisions, initially relaxed occupants exhibit brisk,stereotypical muscle responses consisting of postural and startle responses thatmay contribute to the injury. 2) The neck neuromuscular response to a rear-end impact consists of a posturalresponse and a startle response elicited by a multisensory stimulus(somatosensory, acoustic, and vestibular) associated with the vehicle impact. 3) During a rear-end collision, afferents from the somatosensory, acoustic, andvestibular systems are activated and trigger a startle response in the neckmuscles. 4) These authors sought to determine if the startle response elicited during arear-end collision contributes to head stabilization or represents a potentiallyharmful, overreaction of the body. In the context of a rear-end collision, it is notclear whether these reflex actions are protective and thus beneficial or potentially injurious and therefore harmful. 5) Three experiments were performed using 33 different subjects. The analysisincluded the use of surface electromyography, and head accelerations weremeasured using an accelerometer array. 6) The startle response represents an overreaction that increases the kinematicsin a way that potentially increases the forces and strains in the neck tissues. [Key] 7) Neck muscle activity begins about 50 – 100 ms after vehicle accelerationonset, early enough to influence peak head and neck kinematics. There is evidencethat neck muscles are a contributor to other neck tissue injuries during whiplashmechanism. 8) The time period over which the neck muscles are active overlaps the timeperiod during which peak acceleration and displacement of the head and neckoccur. This overlap suggests that muscle-induced strains and motion-inducedstrains in the posterior neck tissues potentially coincide and cause more severewhiplash injury and related symptoms following a rear-end collision. 9) In experimental rear-end collisions, subjects who reported temporary neckpain exhibited larger startle responses in their posterior neck muscles than didsubjects who did not report neck symptoms. 10) Based on prior studies, injuries sustained during a rear-end car collisioncould be exacerbated by a startle response that increases neck muscle activityduring a time when the posterior neck tissues may be vulnerable. 11) During whiplash, the startle response inhibits muscle tone; therefore, thestartle response is potentially harmful and ill-adapted for whiplash collisionexposures. 12) The startle response during a rear-end collision decreased the whiplash-evokedneck muscle response by 16 – 29%. [Muscles protect joints. Jointinjuries (facet/disc) are primarily responsible for chronic whiplash pain.Startle reduces muscle protection of joints, increasing whiplash injury andchronicity]. 13) These kinematic differences suggest that the startle response evoked by arear-end collision may be more harmful than protective. 14) Our findings suggest that the startle response evoked by a rear-end collisionmay increase the risk of certain whiplash injuries. 15) The cervical facet joints are a source of neck pain in 40 – 68% of patients withchronic whiplash injuries, and excess strain in the facet joint capsule can occurduring whiplash exposures. The neck multifidus muscles insert directly onto the[facet] capsule. 16) Posterior neck muscle activity, and multifidus muscle activity in particular,elicited by the collision may exacerbate cervical facet capsular ligament strain at amoment when the ligament is already being strained by the collision-inducedintervertebral motion. 17) Our results provide additional support for the potential role of the startleresponse in exacerbating certain whiplash-related neck injuries. 18) Although startle responses are generalized body reactions to intense stimuliand are generally thought to protect the body from potential injury by drawing inthe limbs and stiffening the body, during whiplash they tend to inhibit cervicalmuscle tone and increase cervical spine injury. COMMENTS FROM DAN MURPHY This article may help explain why some patients can be injured in very low impactscenarios; they are injured as a consequence of the startle response, caused by acervical neuromuscular multisensory response from somatosensory, acoustic, andvestibular afferents associated with the vehicle impact.