Influence of muscle contraction on whiplash kinematics.
It is unclear whether reflexive muscle contraction in unaware occupants can alter spinal kinematics to mitigate injury in the unaware occupant subjected to whiplash loading. Whiplash injury likely occurs during the non-physiologic S-curvature phase of spinal kinematics, present during the first 100 msec after the initiation of T1 acceleration. Experimental investigations using human volunteers have reported 45 to 60 msec delays prior to electrical activity of the sternocleidomastoid. The effects of reflexive contraction of the neck muscles were investigated using a validated head-neck computational model consisting of head, cervical spine, and first thoracic vertebra. Intervertebral discs. spinal ligaments, and facet joints were modeled using discrete elements. Passive and active musculature were incorporated using the Hill-type muscle model. The computational model was subjected to 2.6 m/sec rear impact velocity, applied to T1. Reflexive muscle contraction in the unaware occupant model was incorporated using a 54-msec muscle delay, 13-msec electromechanical delay, and an 81-msec muscle rise time. Results of the unaware occupant model were compared to the model exercised without muscle contraction. Reflexive muscle contraction altered segmental angulations by less than 10% and facet joint capsular ligament distractions by less than 16% during the time of maximum S-curvature. At the C5-C6 and C6-C7 levels, muscle contraction increased capsular ligament distractions. Due to the nominal affect of reflexive muscle contraction on segmental angulations and facet joint capsular ligament distractions during S-curvature, it is unlikely that this contraction can alter the cervical kinematics responsible for whiplash injury.