The load-displacement-time characteristics of the lumbar spine were obtained under simulated central posteroanterior (PA) mobilization. The instrumentation (the spine mobilizer) consisted of a motor-driven force applicator and a pair of displacement transducers. The PA mobility of the L3, L4 and L5 segments of 28 young normal subjects (14 males and 14 females) was examined, and this was found to be dependent on spinal level but not on gender. At each level tested (except L5), greater movement was observed in the more caudal of the two segments adjacent to the mobilized vertebra. Finally, L4 was subjected to cyclic and sustained PA loadings, and to loadings at different rates. Viscoelastic behavior such as creep and rate-dependency was observed, as was the preconditioning phenomenon. The results suggest that PA mobilization produces more mechanical effects on the lower segment, and that the preconditioning and creep effects might explain the improvement in spinal mobility after mobilization.