Electromyography (EMG)-assisted biomechanical models of the lumbar spine have been developed to estimate spinal loading, but these models often have limited representation of passive tissue contributions to the trunk extension moment. Recent evidence suggests that sustained near full trunk flexion can lead to increased contribution of the passive tissues to resist the external moment due to increased lumbar flexion as the extensor muscle fatigue. This leads to our hypothesis that spinal loading might be increased due to load transfers between active and passive tissues. Sixteen participants maintained a trunk flexion posture that was ten degrees less than the trunk flexion angle inducing flexion-relaxation of erector spinae muscles for 12 min with breaks every three minutes. Trunk kinematic and EMG measures were collected. A muscle fatigue-modified EMG-assisted model with passive tissue components was employed to estimate the time-dependent force and moment profiles at the L4/L5 level. Results revealed that these postures led to a time-dependent increase in the proportion of passive tissues to resist the external moment (39.9 % to 49.5 %) during each 3-minute time block, thereby resulting in the time-dependent increase in the compression and anterior-posterior shear forces of the L4/L5 disc by 181.7 N and 125.2 N, respectively (all p-value <​ 0.001). These results indicate that the load transfer from active to passive tissues can lead to increased compression and anterior-posterior shear forces of the L4/L5 disc at a constant external moment. This study suggests that a time-dependent approach to an EMG-assisted model with passive tissue components can provide more accurate estimates of tissue stresses.