Analysis of low velocity frontal impacts.

OBJECTIVES The objectives of the study were to determine the phasic recruitment of cervical muscles with increasing magnitudes of low velocity frontal impacts, and to determine quantitative effects of awareness of impending impact in comparison to being impacted unawares. BACKGROUND Biomechanics of low velocity frontal impact is poorly understood and requires more work. METHODS Ten healthy young adults were subjected to frontal impacts causing accelerations of 5.3, 8.6, 11.0 and 14.0 m/s(2) (0.54, 0.88, 1.12 and 1.43 gs) while the subjects were unaware of the impending impact and after being told that they were going to be impacted. Electromyograph from sternocleidomastoids, splenius capitis and upper trapezius was recorded bilaterally. Triaxial accelerometers recorded the acceleration of the sled, torso and head of the subjects. RESULTS The normalized electromyograph magnitude progressively rose with the level of acceleration whereas the time to onset generally decreased. At 14 m/s(2) sled acceleration the trapezius muscle generated 79% of their maximal voluntary contraction whereas the sternocleidomastoids generated 32% of their maximum voluntary contraction. The normalized peak electromyograph, the time to onset, the time of the peak electromyograph were significantly affected by the level of acceleration (P<0.01), the expectation of impact (P<0.01) and muscle group studied (P<0.01). The subject gender did not have a significant effect. The kinematic variables and the electromyograph regressed significantly on acceleration (P<0.01). CONCLUSIONS The muscle responses were greater with higher levels of acceleration, particularly the trapezius in frontal impacts. Since the muscular components play a significant and central role in head/neck complex motion abatement at higher levels of acceleration, it may be a primary site of injury at low velocity whiplash phenomenon. RELEVANCE An understanding of the pattern of biomechanical loading may assist in a more specific treatment of the patient injured in a low velocity frontal impact.