Creep deformation of viscoelastic lumbar tissue and its implication in biomechanical modeling of the lumbar spine

Creep in the viscoelastic tissues of the lumbar spine reduces the force-producing capability of these tissues. This study aimed to explore the impact of passive tissue creep on lumbar biomechanics. Sixteen participants performed controlled sagittally symmetric trunk flexion motions after a 30-minute protocol consisting of 12 min of full trunk flexion and 18 min of upright standing. Trunk kinematics and EMG activities of trunk muscles were captured as input variables in three biomechanical models: a) EMG-assisted model with no passive tissue (Active), b) EMG-assisted model with time-invariant passive tissue (No-Creep), and c) EMG-assisted model with time-variant passive tissue components (Creep). The mean absolute error (MAE) between the external moment and the estimated internal moment was calculated as a function of model type and trunk flexion. Results revealed no significant difference in MAE between the three models at 0-30 degrees trunk flexion but as the angle exceeded 30 degrees, the MAE of the No-Creep and Creep models were significantly smaller than that of the Active model. Beyond the trunk flexion angle of flexion-relaxation of erector spinae muscles, the MAE of the Creep model was significantly smaller than that of the No-Creep model (21.8 Nm vs. 40.3 Nm), leading to reduced compression and shear forces of the L4/L5 disc by 784.7 N (31.7 %) and 280.6 N (21.6 %) at full flexion. These results indicate the modulation of the time-dependent stiffness of passive tissues led to a more accurate prediction of the net internal moment at near full flexion postures, preventing overestimation of spinal loads.