The reflex response of human neck muscles to whiplash-like perturbations

Most whiplash injuries are sustained in isolated rear-end collisions which occur without warning. Most studies of whiplash injury, however, have used multiple tests of subjects aware of the imminent perturbation. This thesis examined how multiple exposures and subject awareness of the presence, timing and amplitude of a whiplash-like perturbation affected the activation and amplitude of the neck muscle response and the peak kinematic response of the head and torso. In Experiment 1, the malleability of neck muscle reflexes was examined in 20 subjects (9F, 11M) who performed ballistic flexion and rotation head movements in a warned, simple reaction-time protocol. When a loud startling sound (124 dB) replaced the 'go' tone (76 dB), a hypermetric version of the reaction-time movement was evoked at the startle reflex onset latency. This result indicated that a reflex neck muscle response could be altered by mental preparation of a movement. In Experiment 2, 66 seated subjects (35F, 31M) underwent multiple perturbations with a peak forward acceleration of 1.5g. To their first perturbation, subjects who were deceived and unexpectedly perturbed responded differently than subjects given either exact or inexact information regarding perturbation timing. Advance warning of the perturbation appeared to produce anticipatory facilitation of the sensorimotor system mediating the reflex response. Subjects exposed to ten more perturbations exhibited a rapid habituation of their muscle response and complex changes in their kinematic response. Thirty-six of the 66 subjects (20F, 16M) then underwent 72 more perturbations interspersed with high (2.2g) or low (0.8g) acceleration perturbations. Response differences were not observed between warned and unwarned presentation of these different perturbations, which suggested that advance knowledge of acceleration amplitude did not affect subject responses. The remaining 30 subjects (15F, 15M) were exposed to seven different perturbations which showed that neck muscle and kinematic responses were graded to both perturbation acceleration and velocity. These experiments demonstrated that subject awareness of an imminent perturbation and habituation of the muscle response to multiple perturbations produced complex changes in the kinematic response, and suggested that neck muscle and kinematic responses of unprepared occupants in real whiplash collisions were different than human subject responses observed in most whiplash injury studies.Education, Faculty ofKinesiology, School ofGraduat