Role of Rotated Head Postures on Volunteer Kinematics and Muscle Activity in Braking Scenarios Performed on a Driving Simulator

Abstract

Occupants exposed to low or moderate crash events can already suffer from whiplash-associated disorders leading to severe and long-lasting symptoms. However, the underlying injury mechanisms and the role of muscle activity are not fully clear. Potential increases in injury risk of non-nominal postures, i.e., rotated head, cannot be evaluated in detail due to the lack of experimental data. Examining changes in neck muscle activity to hold and stabilize the head in a rotated position during pre-crash scenarios might provide a deeper understanding of muscle reflex contributions and injury mechanisms. In this study, the influence of two different head postures (nominal vs. rotation of the head by about 63 ± 9° to the right) on neck muscle activity and head kinematics was investigated in simulated braking experiments inside a driving simulator. The braking scenario was implemented by visualization of the virtual scene using head-mounted displays and a combined translational-rotational platform motion. Kinematics of seventeen healthy subjects was tracked using 3D motion capturing. Surface electromyography were used to quantify muscle activity of left and right sternocleidomastoideus (SCM) and trapezius (TRP) muscles. The results show clear evidence that rotated head postures affect the static as well as the dynamic behavior of muscle activity during the virtual braking event. With head turned to the right, the contralateral left muscles yielded higher base activation and delayed muscle onset times. In contrast, right muscles had much lower activations and showed no relevant changes in muscle activation between nominal and rotated head position. The observed delayed muscle onset times and increased asymmetrical muscle activation patterns in the rotated head position are assumed to affect injury mechanisms. This could explain the prevalence of rotated head postures during a crash reported by patients suffering from WAD. The results can be used for validating the active behavior of human body models in braking simulations with nominal and rotated head postures, and to gain a deeper understanding of neck injury mechanisms.

Keywords: Driving simulator; Electromyography; Neck musculature; Rotated head posture; Volunteer testing.

Figure 1

Driving Simulator Setup. Left: Signal flows of the driving simulator and measuring hardware with coordinate systems of platform K_p and head K_h. Right: Subject inside the driving simulator with electrode placement on the muscles TRP (blue) and SCM (red), (c) University of Stuttgart/Uli Regenscheid. The gray reflective markers for motion capturing (right) are displayed as black circles in the left picture. Note: In the experiments presented here, the subject’s hands are not on the steering wheel, but on the legs.

Figure 2

Platform motion. Platform motion during braking event with longitudinal acceleration  ${\ddot{x}}_{\mathit{plat}}$ (left, gray), pitch rotational displacement ${\theta }_{\mathit{plat}}$ (right, red) and pitch rotational velocity  ${\dot{\theta }}_{\mathit{plat}}$ (right, blue). Mean values (dashed) ± 1 standard deviation (corridors). Note that deviations in platform motion despite similar driving scenarios appeared due to platform loading resulting from interindividual differences in subjects’ weight and height.

Figure 3

SEMG processing. Processing SEMG signals u_EMG (blue) to obtain base activity (base, black) and peak activation value (peak, green) exemplarily shown on a SCM_l signal displayed as percentage of MVC in a right shoulder check experiment (rot) with trigger signal u_TRG (red) used for synchronizing EMG with kinematic data. Visualization of the onset times of platform motion (plat onset (platform onset), black dashed) and muscle activation (mus onset (muscle onset), black dotted).

Figure 4

Kinematic Results. Head relative x-displacement ∆x_head displayed in the platform coordinate system K_p. Mean values (lines) and standard deviation for nominal (light gray area) and rotated head posture (dark gray area). The red shaded area marks the time range, where physiological reflexes are expected. t₀ = 0: platform motion onset.

Figure 5

Onset times. Motion and muscle onset times as mean values (bar plot) and standard deviation (lines). Red shaded area marks time corridor of potential reflexes triggered by platform motion or body segment displacements. t₀ = 0: platform motion onset.

Figure 6

SEMG amplitudes. EMG levels in % of MVC displayed as mean values (bars) and standard deviation (lines) for nominal (light gray) and rotated (dark gray) view. (a): base activation (left) (b): additional peak activation u_EMG,rflx − u_base (right). t₀ = 0: platform motion onset.

Figure 7

EMG signals of subject HU456 for nominal (blue) and rotated head posture (red), one trial each.

Figure 8

EMG signals of subject MT825 for nominal (blue) and rotated head posture (red), one trial each.

Figure 9

EMG signals of subject MZ731 for nominal (blue) and rotated head posture (red), one trial each.