Kinematics of head movement and onset of sternocleidomastoid muscle activity in simulated low-velocity rear-end impacts

Abstract

Objectives. Rear-end impacts and whiplash injuries are highly associated. More than a million whiplash injuries occur each year in the US.1Malleson A. Chronic whiplash syndrome. Psychosocial epidemic.Canadian Family Physician. 1994; 40: 1906-1909Google Scholar The kinematics of head movement in simulated rear-end impacts has been described. However, the influence of impact awareness remains controversial. In addition, the suggestion that cervical muscles activate too late in rear-end impacts may be incorrect.2Siegmund G.P. Brault J.R. Wheeler J.B. The relationship between clinical and kinematic responses from human subject testing in rear-end automobile collisions.Accident Analysis & Prevention. 2000; 32: 207-217Google Scholar The goal of this study was to analyze the kinematics of head movement in rear-end impacts related to 2 impact magnitudes, impact awareness and onset of sternocleidomastoid (SCM) muscle response. Study design. The Human Research Ethics Board at the University of Alberta approved this study. Thirty subjects completed the study. Each subject underwent 3 impacts: 1 slow unexpected (4.5 m/sec/sec), 1 fast unexpected (10.0 m/sec/sec), and 1 fast expected impact of the same magnitude as the fast unexpected impact. Kinematics of head movement was simultaneously recorded with a custom accelerometer board and the use of video cameras. Angular head displacement, and angular and linear head acceleration were recorded. Normalized SCM surface electromyography (EMG) was obtained. Onset and peak time for the EMG and the kinematics were determined. Onset time was defined as the time in which 5% of the magnitude value of the peak occurred. Peak time was defined as the time in which the maximum value of the variable was reached. Repeated-measures statistical method was used to analyze the head movement behavior associated with impact velocity and expectation. The same statistical method was used to determine whether muscle activity or head movement was initiated first. Results. The video camera and accelerometer data presented a good agreement. Subjects presented an initial rearward movement in which the head moved backward followed by a forward movement of the head. The rearward angular head displacement almost doubled (∼13 degrees) with increased impact magnitude (P < .05). The peak magnitude of rearward and forward angular acceleration increased 2 to 3 times with increased impact magnitude (P < .05). Rearward and forward linear displacement was 2.5-3.5 times higher for the fast than the slow unexpected impacts (P < .05). There were no significant differences in the magnitude of angular head acceleration, angular head displacement, and linear head acceleration regarding awareness (P>0.05). The onset time of SCM peak EMG ranged from 78-114 ms later than peak of linear head acceleration for all groups (P < .05). The onset of the SCM peak EMG ranged from 136-188 ms earlier than the peak rearward angular head displacement in all impacts (P < .05). Onset time of peak SCM EMG was significantly earlier (30 ms) than the peak angular head acceleration for the fast unexpected impact (P < .05). Conclusions. Kinematics of head movement increased with increased impact magnitude. Magnitude of angular head displacement was within physiologic limits and do not support the hyperextension theory. Temporal and amplitude awareness did not produce different magnitude in kinematics of head movement. The temporal relationship between the SCM and angular head acceleration is different from the temporal relationship between the SCM and linear head acceleration.