UBC Theses and Dissertations
Are inversion, posture, motion and muscle effects important to spinal alignment? Newell, Robyn
Rollover accidents are dynamic and complex events in which head contacts with the vehicle interior can cause catastrophic neck injuries through head-first impact. Ex vivo cadaver tests are valuable for studying these mechanisms of head-first axial loading neck injuries; however, they lack a biofidelic representation of neuromuscular control, postural stability, and overall spine posture. Computational modeling can be used to evaluate changes in the risk of neck injury under the influence of muscle forces, yet the exact muscles and levels of forces that are involved leading up to a head-first impact are unknown. Knowing the state of the neck prior to impact is critical to improving cadaveric and computational models of neck injury. Four human volunteer experiments were conducted to determine whether inversion, head position, muscle tensing, and dynamic motion influence the cervical spine alignment. These four studies included: (1) static inversion, (2) muscle tensing, (3) moment generation, (4) dynamic flexion/extension. For each experiment, cervical alignment was captured using fluoroscopy and muscle activity was captured using electromyography. The inverted posture and muscle activations were found to be different than the upright relaxed posture and the differences depend on the position of the head (study 1). Actively tensing the neck muscles in a free unconstrained task (study 2) and in generating flexion and extension forces with head constraint (study 3) resulted in different cervical alignment compared to the initial resting spine. Not only do these neck muscle contractions induce postural changes, they also provide a substantial stiffening effect to the neck. Finally, dynamically arriving at the neutral position did not result in the same cervical alignment as static neutral and the alignment depended on the direction that neutral is approached from (full flexion or full extension). These findings suggest that it may not be sufficient to replicate the upright resting posture in cadaveric and computational models of neck injury. Adopting in vivo postures and muscle activations, relevant to head-first impact, in the laboratory may help in replicating the spectrum of injuries observed in real life rollovers, an important step toward injury prevention.
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