To reveal deformation behaviour of normal lumbar discs (L((3/4)), L((4/5)) andL(5)/S(1), the lumbar spines of eight asymptomatic volunteers were examined cineradiographically during flexion and extension of the trunk. Disc deformation could be evaluated by displacement of the superior corners of the disc, which were measured with respect to the upper surface of the adjacent lower vertebra. Furthermore the in-vivo strain distribution of each lumbar disc was analysed by the finite element method and in-vivo measurement of the disc deformation. During flexion, deformation of the lumbar disc increased rapidly after a certain delay from the start of trunk motion and reached maximum value before the finish of trunk motion. It was also confirmed that time lags were present between the onsets of disc deformation. Namely, each disc deformed not simultaneously but stepwise from the upper to the lower level with time lags during flexion. When the lower disc deformation started, the strain at the adjacent upper discs had already reached more than half of the value at full flexion. RELEVANCE: The present study revealed one of the phenomena of lumbar spinal kinematics, which will provide helpful information to clinical problems such as evaluation of spinal instability.