Research into the biomechanics of low speed rear impacts has focused primarily on the kinematic responses of anthropometric dummies and human subjects. Occupant muscular activity during low speed rear impacts remains largely unquantified however. The current study enhances the existing database of human subject test exposures with an emphasis on electromyographic activity before, during, and after low speed rear impact. This information may provide insight into injury mechanisms, occupant mathematical modeling, and aspects of seat and head restraint design.

Low speed rear impacts using instrumented human subjects were conducted. Ten nominal 16 km/h closing speed car-to-car impacts were conducted using male and female subjects aged 22-54 years, with struck vehicle velocity changes of up to 10 km/h. Two head restraint conditions were studied. One was a standard seat integrated head restraint. For the second condition the integrated head restraint was modified by adding 2 inches of padding to the existing head restraint, thus reducing the initial head-to-head restraint horizontal distance.

Accelerometers were affixed to the target vehicle static center of gravity and the occupant's head, cervical spine, and lumbar spine. Accelerations at the head static center of gravity were obtained via a 9-accelerometer headgear array and algorithm. Anterior paracervical, posterior paracervical, trapezius, and paralumbar muscle activity was monitored using surface mounted electrodes. Kinematics in the sagittal plane were obtained via high speed video.

No injuries were sustained by any occupant. In all cases the subjects exhibited pre-impact muscle activity commensurate with that of a relaxed seated posture, indicating that the test protocol inhibited the subjects from bracing in anticipation of the impacts. Initial muscle activity typically occurred approximately 100 to 125 milliseconds after the moment of bumper contact, during the initial phase of impact as the occupant's cervical spine was extending. Full muscle tension likely did not develop until the cervical spine was flexing. Cervical flexor, cervical extensor and lumbar paraspinal musculature demonstrated similar activity onset times. A centrally generated response was thus hypothesized for initial onset of muscle activity. This response was consistent with being triggered by lumbar spine acceleration, and typically occurred approximately 90-120 milliseconds after onset of lumbar spine acceleration.

No significant differences were noted between muscle response times for the two head restraint conditions. Decreases in rearward head displacement, cervical spine extension, and head acceleration were found for the modified head restraint.