Trunk Tissue Creep Can Increase Spine Forces During a Subsequent Lifting Task

OCCUPATIONAL APPLICATIONS Results from the current study show evidence of an adverse effect of prolonged trunk flexion on spine loads during consecutive lifting tasks. The time-dependent methodology introduced here can enhance task assessment based on the duration of flexion exposures. More generally, results demonstrate the importance of considering prior trunk exposures when assessing risk factors for lifting tasks. The proposed solution incorporates “time” as an independent variable, in addition to lifting weight and posture, to better assess spinal loads and maximum lifting capacity based on prior loadings. The current results also suggest that existing ergonomic guidelines or biomechanical models that do not incorporate the viscoelasticity of soft tissues or time-dependent neuromuscular alterations may underestimate spine forces and potential injury risk in some circumstances. To account for this, especially when assessing spine forces during lifting after exposure to prolonged flexion, additional safety margins should be considered. TECHNICAL ABSTRACT Background: Prolonged trunk flexion decreases soft tissue stiffness due to viscoelastic deformations and can also lead to altered kinematics when performing a subsequent lifting task. Yet, it remains to be determined if or how these changes and alterations might increase spine forces. Purpose: A previously developed viscoelastic model was used, along with experimental data, to predict changes in peak spine forces during a lifting task performed following a prolonged flexion exposure (creep). Methods: Model inputs were obtained from an experiment using ten participants, within which lifting kinematics and muscle activity were measured both before and after creep exposure. Two sets of simulations were performed; one in which kinematics were assumed to be unchanged by creep exposure and the other incorporating measured changes in kinematics following exposure. Results: Post-exposure changes in lifting kinematics involved a reduction in the peak relative sagittal-plane flexion of superior lumbar motion segments and an increase in these flexion among inferior lumbar motion segments. Creep exposure caused increases in predicted peak spine forces during lifting at all levels of the lumbar spine (65–241 N). A substantial portion (∼51%) of this increase was estimated to be the result of muscular compensations for reduced passive tissue stiffness. Conclusions: The current study demonstrates that both changes in lifting kinematics and viscoelastic deformations resulting from creep exposures can lead to increased trunk muscle forces and spine forces during a lifting task. This evidence suggests a potential mechanical basis for previous epidemiological evidence that indicates an increased risk of low back disorders for jobs involving both trunk flexion and lifting.