Direct impact loading experiments on human volunteer’s head were conducted at low impact force levels in frontal collisions simulating impact pattern of airbag deployments, for quantitative analysis on segmental motions of intervertebral disc and facet joints during impact.

Under forehead rearward impact, the head extension relative to the torso was reduced by neck muscle tension. However, the local motion of C5 relative to C6 that was large under the relaxed condition of neck muscles was not reduced. Further, the compressive strains of rear edge of the C5/C6 intervertebral disc and the C5/C6 facet joint were increased. Under chin rearward impact, the head flexion relative to the torso and C7 during neck S-shape curve was reduced by neck muscle tension. The flexion of C1~C3 relative to each lower vertebra, which was large under the relaxed neck muscles condition, were reduced too. However, the inflection point of cervical spine S-shape curve between C3 and C4 occurred when the relative flexion between C3 and C4 was reduced to less than that of C1 and C2. Moreover, there was no reduction effect on the tensile strain of rear edge of the C3/C4 facet joint.

The kinematics of the head and neck was reduced by neck muscle tension during low-severity impact. On the contrary, the influence of neck muscle tension on the intervertebral motion was negligible. The study indicated that such local motion could not be sufficiently evaluated by existing neck injury criteria that are calculated from the kinematics of the head and neck. Thus, it is imperative to clarify the mechanism of minor neck injuries based on intervertebral motion for the investigation of minor neck injury criteria.