Head postures during naturalistic driving.

OBJECTIVE: A rotated head posture at the time of a rear-end impact is associated with a higher risk of acute and chronic whiplash injury. The objective of this study was to quantify the amplitude and duration of rotated head postures observed in drivers during naturalistic driving.

METHODS: Twenty volunteers (14 males: 36 ± 12 years, 6 females: 27 ± 5 years) drove a 2010 Subaru Impreza on public roads while their 3D head angular position relative to the car was recorded using inertial measurement units. An experimenter rode in the passenger seat (right side) and logged when subjects performed one of 6 head movements: Bilateral shoulder and side mirror checks, looking at the rearview mirror, and looking at the front seat passenger. Video of the subjects was used to confirm the logged head movements and identify movements that the experimenter missed. The duration and amplitude of all 6 head movements were tabulated and then compared between periods when the car was moving and when the car was stationary.

RESULTS: During a 68 ± 5-min drive, subjects performed a median (range) of 15 (5-39) left shoulder checks, 82.5 (29-167) left mirror checks, 40.5 (10-168) rearview mirror checks, 27.5 (3-113) right mirror checks, 60 (0-185) passenger looks, and 12.5 (1-28) right shoulder checks. Peak yaw angles of the head relative to the vehicle for these 6 movements averaged -81.5°, -34.3°, 16.2°, 42.1°, 58.2°, and 84.3°, respectively. Drivers spent a larger proportion of time in nonneutral postures when the vehicle was stopped (17.5%) compared to moving (8.2%) (Z = 3.92, P < .0001). Drivers also moved their head further from neutral during the movements when the car was stationary compared to moving (t19  = 5.90, P < .0001).

CONCLUSIONS: Drivers use larger and longer duration head movements when stationary than when driving. Given an increased risk of whiplash injury for initially rotated head postures, these findings provide a possible explanation for why drivers are more likely to be injured when hit from behind while their vehicle is stationary. Further, the head postures characterized in this study can be used as initial conditions in volunteer and computational studies to improve our understanding of why nonneutral head postures are associated with increased whiplash injury risk.