Assessing dynamic spinal stability using maximum finite-time Lyapunov exponents
Graham, Ryan Bevan
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The objective of this work was threefold: 1) to assess how local dynamic spinal stability is affected by various factors including: the personal lift-assist device (PLAD), different loads when lifting, and prolonged repetitive work; 2) to establish the between-day reproducibility of local dynamic stability and kinematic variability measures; and 3) to directly compare local dynamic spinal stability to quasi-static mechanical spinal stability. The first study was an investigation into the effects of the PLAD on local dynamic spinal stability during repetitive lifting. Short- (λmax-s) and long-term (λmax-l) maximum finite-time Lyapunov exponents were calculated from measured trunk kinematics to assess stability. PLAD use did not change λmax-s, but significantly reduced λmax-l; indicating increased local dynamic spinal stability when lifting with the device. The second study was a report on the effects of lifting two different loads (0% and 10% maximum back strength) on local dynamic spinal stability and kinematic variability, expressed as the mean standard deviation (MeanSD) across cycles. It was determined that increasing the load that was lifted significantly reduced λmax-s, but not λmax-l or MeanSD. Thus, as muscular and moment demands increased with load so did subjects’ spinal stability. The third study was designed to look at changes in local dynamic spinal stability and kinematic variability resulting from 1.5 hours of repetitive automotive manufacturing work, as well as the between-day reproducibility of the measures. Operators performed a repetitive dynamic trunk flexion task immediately pre- and post-shift, as well as at the same pre-shift time on the following day. Despite significant increases in back pain scores, operators were able to maintain their stability and variability post-shift. Moreover, λmax-s was the most reproducible measure. The final study was structured to directly compare lumbar spine rotational stiffness (quasi-static mechanical spinal stability), calculated with an EMG-driven biomechanical model, to local dynamic spine stability, during a series of dynamic lifting challenges. Results suggest that spine rotational stiffness and local dynamic stability are positively associated, as they provided similar information when lifting rate was controlled. However, both models provide unique information and future research is required to fully understand their relationship. In general, the results of these studies illustrate the potential for Lyapunov analyses of kinematic data to be used to assess local dynamic spinal stability in a variety of situations.