Abstract
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.
Highlights
Trunk flexion exposures, whether prolonged or cyclic, result in viscoelastic deformation of passive tissues in the posterior trunk and a reduction in trunk stiffness [1,2]
All three measures increased with lumbar flexion angle (Figure 4), and each of the relationships with lumbar flexion angle was well characterized by exponential functions (R2.0.93)
While RMS errors were consistent across lumbar flexion angles, R2 generally increased with angle for each model
Summary
Whether prolonged or cyclic, result in viscoelastic deformation of passive tissues in the posterior trunk and a reduction in trunk stiffness [1,2]. Several other studies have determined the viscoelastic properties of muscle using both in vitro [17,18,19,20,21,22,23] and in vivo [24,25,26,27,28,29,30] measurements. In an in vivo study by McGill and Brown [31], the wholetrunk creep was measured for prolonged flexion exposures
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