Head and neck control varies with perturbation acceleration but not jerk: implications for whiplash injuries

Abstract

Recent studies have proposed that a high rate of acceleration onset, i.e. high jerk, during a low-speed vehicle collision increases the risk of whiplash injury by triggering inappropriate muscle responses and/or increasing peak head acceleration. Our goal was to test these proposed mechanisms at realistic jerk levels and then to determine how collision jerk affects the potential for whiplash injuries. Twenty-three seated volunteers (8 F, 15 M) were exposed to multiple experiments involving perturbations simulating the onset of a vehicle collision in eyes open and eyes closed conditions. In the first experiment, subjects experienced five forward and five rearward perturbations to look for the inappropriate muscle responses and 'floppy' head kinematics previously attributed to high jerk perturbations. In the second experiment, we independently varied the jerk ( approximately 125 to 3 000 m s(-3)) and acceleration ( approximately 0.65 to 2.6 g) of the perturbation to assess their effect on the electromyographic (EMG) responses of the sternocleidomastoid (SCM), scalene (SCAL) and cervical paraspinal (PARA) muscles and the kinematic responses of the head and neck. In the first experiment, we found neither inappropriate muscle responses nor floppy head kinematics when subjects had their eyes open, but observed two subjects with floppy head kinematics with eyes closed. In the second experiment, we found that about 70% of the variations in the SCM and SCAL responses and about 95% of the variations in head/neck kinematics were explained by changes in perturbation acceleration in both the eyes open and eyes closed conditions. Less than 2% of the variation in the muscle and kinematic responses was explained by changes in perturbation jerk and, where significant, response amplitudes diminished with increasing jerk. Based on these findings, collision jerk appears to have little or no role in the genesis of whiplash injuries in low-speed vehicle crashes.

Figure 1

Photo showing the test setup and reference frame

Figure 2. Sled pulses used in this experiment

A, five forward and five rearward pulses used in the forward/rearward experiment and highlights the typical pulse repeatability. B, the 12 forward pulses used for the acceleration/jerk experiment grouped together as the four low acceleration pulses (top), the four medium acceleration pulses (middle) and four high acceleration pulses (bottom). C, the peak acceleration and average jerk of all pulses. The open circle represents the pulses from the forward/rearward experiment and the filled circles represent the 12 pulses from the acceleration/jerk experiment. The dashed circle denotes the pulse used for normalization in the acceleration/jerk experiment, and the grey lines connect the six conditions plotted in Fig. 5.

igure 5. Sample raw EMG and kinematic data of a single subject exposed to three pulses of increasing acceleration and similar jerk (A) contrasted with four pulses of similar acceleration and increasing jerk (B)

These pulses are highlighted by the grey line in Fig. 2C. Note that the third pulse in A is the same as the first pulse in B. Note also that all of the dependent variables increase in amplitude with increasing acceleration (A) but not with increasing jerk (B). See Fig. 3 caption for explanation of abbreviations.

Figure 4. Pooled data from the forward/rearward experiment

A–C, the peak RMS EMG of the SCM, SCAL and PARA muscles (expressed as a multiple of pre-trial RMS amplitude) plotted against head angles for the forward perturbations (resulting in positive head angles) and rearward perturbations (resulting in negative head angles). D, the average of each subject's peak head angle in the forward and rearward directions for the initial experiment. E, a similar plot for the follow-up experiment highlighting the effect of eyes open/closed. The lines join the eyes open and eyes closed data for the two floppy subjects.

Figure 3. Typical raw EMG and kinematic response data for a single subject with eyes open (two left panels) and eyes closed (two right panels) from the forward/rearward experiment

These data are from the floppiest follow-up subject, but the eyes-open responses are typical of subjects from both the initial and follow-up experiments. SCM and PARA scales are arbitrary, but consistent across trials. SCM, sternocleidomastoid; PARA, paraspinal; L, left; R, right; a_x, horizontal acceleration; α_y, angular acceleration about the y-axis; Θ_y, flexion/extension angle.

Figure 8. Normalized peak head acceleration (top row) and normalized peak RMS EMG for the SCM muscles (bottom row) plotted as a function of normalized sled acceleration (left column) and normalized sled jerk (right column)

The grey lines represent data from individual subjects and the black lines show the means and standard deviations. Note the consistency of the responses across all subjects.

Figure 7. Mean and standard deviation of the normalized kinematic variables as a function of normalized sled acceleration (left column) and normalized sled jerk (right column)

See caption to Fig. 6 for additional details.

Figure 6. Mean and standard deviation of the normalized muscle variables as a function of normalized sled acceleration (left column) and normalized sled jerk (right column)

Data are shown for each of the 12 pulses. The diagonal line represents a unity gain between the normalized variables. Note the near linear dependence of the muscle variables on sled acceleration and the independence of the responses to sled jerk. Marker shade reflects the six levels of jerk in the left column and the three levels of acceleration in the right column (white = lowest level; black = highest level).