A three-dimensional nonlinear optimization based model was formulated to predict forces in muscles of the back and the disc at the L3-4 level during dynamic and static lifting modes. The data needed for the formulation (lines of action of muscles and the corresponding x-sectional areas; points of application, external loads, inertia forces, etc.) were obtained from the CT scans, the kinematic data obtained using the Selspot II system, and the force-plate. Muscular activities during lifting were recorded using the surface electrodes. Most of the muscles were active during lifting and forces in the left and right side muscles were different. A linear relationship between the predicted forces and the corresponding EMG values was observed for the erectore spinae muscle group. The model predicted disc compression was 30 to 40% higher for the dynamic lifting case as compared to the static lifting mode. A comprehensive, realistic three-dimensional model, for the first time has been developed to predict forces in the muscles during lifting. The predicted results support earlier findings that dynamic lifting activities are more stressful as compared to the static lifts.