Jean-Sébastien Blouin1, J Timothy Inglis2*, and Gunter P. Siegmund3
1 School of Human Kinetics, University of British Columbia
2 School of Human Kinetics, ICORD, Brain Research Centre, University of British Columbia
3 School of Human Kinetics, University of British Columbia, MEA Forensic Engineers & Scientists
* To whom correspondence should be addressed. E-mail: firstname.lastname@example.org .
The human startle response produces muscle contractions throughout the body but the most brisk and synchronized contractions appear in the neck muscles. This response, which is greatest with the first exposure to a startling stimulus, could produce excessive and inappropriately directed muscle contractions that could explain the higher incidence of whiplash injuries in people who are unprepared for the collision. This study seeks neurophysiological evidence of startle responses in the neck muscles of 120 healthy subjects exposed to between 1 and 16 rear-end impacts or forward perturbations of different speeds. Startle responses were quantified by the synchronous electromyographic (EMG) activity between 10 and 20 Hz in bilaterally-homologous sternocleidomastoid, scalene and cervical paraspinal neck muscles. Coherence analyses of EMG from the left and right muscles were used to estimate synchrony for: i) the first unexpected trial, ii) subsequent habituated trials and iii) the superposition of habituated trials and a loud acoustic stimulus (40 ms, 124dB sound). The peak in coherent EMG activity between contralateral muscle pairs in the 10 to 20 Hz bandwidth was related to startle. Synchrony in this bandwidth was observed between the left and right muscles during the first impact or whiplash-like perturbation. This synchrony decreased significantly in the habituated trials, but reappeared when the loud acoustic stimulus was introduced. Its presence in the first trial indicates that startle is part of the neuromuscular response to an unexpected rear-end impact. This startle component of the neuromuscular response could play a role in the aetiology of whiplash injuries.
Key words: Neck • posture • Reflex