BACKGROUND People with low back pain (LBP) alter their motion patterns during level walking and stair climbing due to pain or fear. However, the alternations of load sharing during the two activities are largely unknown. The objective of this study was to investigate the effect of LBP caused by lumbar disc herniation (LDH) on the muscle activities of 17 main trunk muscle groups and the intradiscal forces acting on the five lumbar discs. MATERIAL AND METHODS Twenty-six healthy adults and seven LDH patients were recruited to perform level walking and stair climbing in the Gait Analysis Laboratory. Eight optical markers were placed on the bony landmarks of the spinous process and pelvis, and the coordinates of these markers were captured during the two activities using motion capture system. The coordinates of the captured markers were applied to developed musculoskeletal model to calculate the kinetic variables. RESULTS LDH patients demonstrated higher muscle activities in most trunk muscle groups during both level walking and stair climbing. There were decreases in anteroposterior shear forces on the discs in the pathological region and increases in the compressive forces on all the lumbar discs during level walking. The symmetry of mediolateral shear forces was worse in LDH patients than healthy adults during stair climbing. CONCLUSIONS LDH patients exhibited different kinetic alternations during level walking and stair climbing. However, both adaptive strategies added extra burdens to the trunk system and further increased the risk for development of LDH.