Creep deformation of the human trunk in response to prolonged and repetitive flexion: measuring and modeling the effect of external moment and flexion rate.

While viscoelastic responses of isolated trunk soft tissues have been characterized in earlier studies, the effects of external loading and flexion rate on these responses in the intact human trunk are largely unknown. Two experiments were conducted to measure trunk viscoelastic behaviors, one involving prolonged flexion with several extra loads (attached to the wrists) and the other repetitive trunk flexion with different extra loads and flexion rates. Direct outcome measures included initial trunk angle, creep angle, and residual/cumulative creep. Viscoelastic behaviors in both experiments were characterized using different Kelvin-solid models. For prolonged flexion, extra load significantly affected initial angle, creep angle, and viscoelastic model parameters, while residual creep remained unchanged. For repetitive flexion, cumulative creep angle significantly increased with both extra load and flexion rate. Nonlinear viscoelastic behavior of the trunk was evident in both experiments, which also indicated better predictive performance using Kelvin-solid models with ≥2 retardation time constants. Understanding trunk viscoelastic behaviors in response to flexion exposures can help in future modeling and in assessing how such exposures alter the synergy between active and passive trunk tissues.