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We investigated the mechanical load of the lumbar spine caused by changes of posture and carried load.To clarify the relationship between the actual and theoretical load of the lumbar spine with and without application of external weights.There is sparse knowledge regarding the relationship between the intradiscal pressure and trunk muscle activities during forward bending motion.The measurements of intradiscal pressure, electromyogram activities of the trunk muscles, and motion analysis of the trunk in 3 young healthy men were performed simultaneously with and without lifting a weight of 10 kg. The recorded values were compared with theoretical values based on the principle of levers.The load of the lumbar spine and electromyogram activities of the back muscles increased in proportion to the tilting angle with and without external weights. However, the external weights induced no significant activity in the abdominal muscles. The actual measurement values were larger than the theoretical values at all tilting angles of the trunk.We found that the increase of the actual spinal load during trunk flexion in healthy individuals consisted of the theoretical spinal load and load generated by the back muscle activities.