Browsing by Author "Toosizadeh, Nima"

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    Load-Relaxation Properties of the Human Trunk in Response to Prolonged Flexion: Measuring and Modeling the Effect of Flexion Angle
    Toosizadeh, Nima; Nussbaum, Maury A.; Bazrgari, Babak; Madigan, Michael L. (PLOS, 2012-11-05)
    Experimental studies suggest that prolonged trunk flexion reduces passive support of the spine. To understand alterations of the synergy between active and passive tissues following such loadings, several studies have assessed the time-dependent behavior of passive tissues including those within spinal motion segments and muscles. Yet, there remain limitations regarding load-relaxation of the lumbar spine in response to flexion exposures and the influence of different flexion angles. Ten healthy participants were exposed for 16 min to each of five magnitudes of lumbar flexion specified relative to individual flexion-relaxation angles (i.e., 30, 40, 60, 80, and 100%), during which lumbar flexion angle and trunk moment were recorded. Outcome measures were initial trunk moment, moment drop, parameters of four viscoelastic models (i.e., Standard Linear Solid model, the Prony Series, Schapery's Theory, and the Modified Superposition Method), and changes in neutral zone and viscoelastic state following exposure. There were significant effects of flexion angle on initial moment, moment drop, changes in normalized neutral zone, and some parameters of the Standard Linear Solid model. Initial moment, moment drop, and changes in normalized neutral zone increased exponentially with flexion angle. Kelvin-solid models produced better predictions of temporal behaviors. Observed responses to trunk flexion suggest nonlinearity in viscoelastic properties, and which likely reflected viscoelastic behaviors of spinal (lumbar) motion segments. Flexion-induced changes in viscous properties and neutral zone imply an increase in internal loads and perhaps increased risk of low back disorders. Kelvin-solid models, especially the Prony Series model appeared to be more effective at modeling load-relaxation of the trunk.
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    Time-dependent assessment of the human lumbar spine in response to flexion exposures: in vivo measurement and modeling
    Toosizadeh, Nima (Virginia Tech, 2013-03-01)
    Among several work-related injuries, low back disorders (LBDs) are the leading cause of lost workdays, and with annual treatment costs in excess of $10 billion in the US. Epidemiological evidence has indicated that prolonged and/or repetitive non-neutral postures, such as trunk flexion, are commonly associated with an increased risk of LBDs. Trunk flexion can result in viscoelastic deformations of soft tissues and subsequent mechanical and neuromuscular alterations of the trunk, and may thereby increase LBD risk. While viscoelastic behaviors of isolated spinal motion segments and muscles have been extensively investigated, in vivo viscoelastic responses of the trunk have not, particularly in response to flexion exposures. Further, most biomechanical efforts at understanding occupational LBDS have not considered the influence of flexion exposures on spine loads. Four studies were completed to characterize viscoelastic deformation of the trunk in response several flexion exposures and to develop and evaluate a computational model of the human trunk that accounts for time-dependent characteristics of soft tissues. Participants were exposed to prolonged flexion at different trunk angles and external moments, and repetitive trunk flexion with different external moments and flexion rates. Viscoelastic properties were quantified using laboratory experiments and viscoelastic models. A multi-segment model of the upper body was developed and evaluated, and then used to estimate muscle forces and spine loads during simulated lifting tasks before and after prolonged trunk flexion at a constant angle and constant external moment. Material properties from the earlier experiments were used to evaluate/calibrate the model. Experimental results indicated important effects of flexion angle, external moment, and flexion rate on trunk viscoelastic behaviors. Material properties from fitted Kelvin-solid models differed with flexion angle and external moment. Nonlinear viscoelastic behavior of the trunk tissues was evident, and predictive performance was enhanced using Kelvin-solid models with ≥2 iii retardation/relaxation time constants. Predictions using the multi-segment model suggested increases in spine loads following prolonged flexion exposures, primarily as a consequence of additional muscle activity. As a whole, these results help to characterize the effects of trunk flexion exposures on trunk biomechanics, contribute to more effective estimates of load distribution among passive and active components, enhance our understanding of LBD etiology, and may facilitate future controls/interventions.