Sudden, unexpected loading on the low back is associated with a high incidence of low back pain. Experiments in which sudden loading was applied during standing revealed increased compression forces on the spine and increased trunk angle, which may cause injury to the spine and hence explain this association. During a more dynamic daily activity, i.e. lifting, this could not be demonstrated, which may be due to experimental constraints. We therefore reinvestigated the loading of the low back when subjects were lifting an unexpectedly heavy object. Ten males lifted boxes, weighing 1.6 or 6.6kg, at a self-selected lifting velocity. In some trials the mass of these boxes was unexpectedly increased by 10kg. The ground reaction forces, body movements and trunk muscle activity were measured and from these, the L5/S1 torques and compression forces were estimated. Underestimation of the mass did not lead to an increase in low back loading. This finding was independent of the mass the subjects were expecting to lift. In conclusion, no evidence was found to support inference regarding causality of the association between sudden loading and low back pain during whole body lifting movements. Copyright © 2001 Elsevier Science Ltd.