Neck muscle responses of driver and front seat passenger during frontal-oblique collisions

Abstract

Background: Low-velocity motor vehicle crashes often lead to severe and chronic neck disorders also referred to as whiplash-associated disorders (WAD). The etiology of WAD is still not fully understood. Many studies using a real or simulated collision scenario have focused on rear-end collisions, whereas the kinematics and muscular responses during frontal-oblique collisions have hardly been investigated. In particular for rear-end collisions, drivers were shown to have a higher WAD risk than front seat passengers. Yet, independently from the impact direction, neither the muscular nor the kinematic responses of drivers and front seat passengers have been compared to date, although some findings indicate that the neck muscles have the potential to alter the head and neck kinematics, and that the level of neck muscle activity during impact may be relevant for the emergence of WAD.

Objective: In this study, we quantitatively examined the subjects' neck muscle activity during low-velocity left-frontal-oblique impacts to gain further insights into the neuromuscular mechanism underlying whiplash-like perturbations that may lead to WAD.

Methods: In a within-subject study design, we varied several impact parameters to investigate their effect on neck muscle response amplitude and delay. Fifty-two subjects experienced at least ten collisions while controlling for the following parameters: change in velocity Δv (3 / 6 km/h), seating position (driver / front seat passenger), and deliberate pre-tension of the musculature (tense / relaxed) to account for a potential difference between an expected and an unexpected crash. Ten of the 52 subjects additionally ran the same experimental conditions as above, but without wearing a safety belt.

Findings: There were significant main effects of Δv and muscle pre-tension on the reflex amplitude but not of seating position. As for the reflex delay, there was a significant main effect of muscle pre-tension, but neither of Δv nor of seating position. Moreover, neither the safety belt nor its asymmetrical orientation had an influence on the reflexive responses of the occupants.

Conclusion: In summary, we did not find any significant differences in the reflex amplitude and delay of the neck musculature between drivers and front seat passengers. We therefore concluded that an increased risk of the driver sustaining WAD in frontal-oblique collisions, if it exists, cannot be due to differences in the reflexive responses.

Fig 1. Crash scenario.

Scenario of the collision with the towing vehicle drawing the crash vehicle with the help of the towing mechanism. Four motion capture cameras (Qualisys, Sweden) and two video cameras (GoPro, USA) were mounted to a metal frame at the front of the crash vehicle. The pendulum impacted the bumper from a left-oblique direction.

Fig 2. Subjects.

(A) Anterior view of subjects sitting in driver and FSP position. Infrared-reflective markers were attached at anatomical landmarks. The LED-flash (at the middle-top of the figure) was triggered when the pendulum hit the bumper. The steering wheel was trimmed to provide a better view for the motion capture cameras. (B) Posterior (C) and lateral view of the subjects. The EMG electrodes were placed at the sternocleidomastoid, trapezius, and paraspinal muscles.

Fig 3. Reflex amplitude.

Mean and 95%-confidence interval of the reflex amplitude of cervical paraspinal (PARA), sternocleidomastoid (SCM), and trapezius (TRA) muscles in driver and front seat passenger position following a Δv of 3 (left) and 6 km/h (right), and in relaxed (bottom) and tense condition (top).

Fig 4. Reflex delay.

Mean and 95%-confidence interval of the reflex delay of cervical paraspinal (PARA), sternocleidomastoid (SCM), and trapezius (TRA) muscles in driver and front seat passenger position following a Δv of 3 (left) and 6 km/h (right), and in relaxed (bottom) and tense condition (top).

Fig 5. Effect of the safety belt on the EMG.

Mean and 95%-confidence interval of the reflex amplitude (top) and the reflex delay (bottom) of the cervical paraspinal (PARA), the sternocleidomastoid (SCM), and the trapezius (TRA) muscles in the belted and not belted condition.

Fig 6. Kinematics.

Mean and 95%-confidence interval of the path length of the head in relation to the torso (left), the maximum velocity of the head in relation to the torso (middle), and the maximum acceleration of the head in relation to the torso (right) of 52 subjects in driver and front seat passenger position following collisions with a change in velocity of 6 km/h.