In a previous study by Demetropoules et al., (1998), it was shown that both cadaveric and Hybrid III lumbar spines exhibit loading rate dependency when loaded in a quasi-static mode up to a velocity of 100 mm/s. In these tests, the Hybrid III lumbar spines were generally found to have higher stiffnesses than the human lumbar spines, except in compression. This is probably due to the fact that muscle loading was not simulated when testing the human spines. Additionally, the speed previously used to test the spines was less than that typically seen in automotive crash environment. The purpose of this study was to use a high-rate testing machine to establish the flexion and extension stiffnesses of the human lumbar spine with simulated extensor muscle tone.Two Hybrid III lumbar spines were used to develop the test methodology and to obtain the response of the Hybrid III lumbar spines. A low-mass, high-frequency response, five-axis load cell was used to measure forces and moments at the inferior end of the spine, and an angular velocity sensor was attached at the superior end of the spine in order to measure angular displacement. When comparing the high rate (4 m/s) and low rate (100 mm/s) loading characteristics of the Hybrid III lumbar spine in flexion, the initial high rate response resembles that at low rate. However, at about six degrees of flexion, the high rate curve takes a sharp rise, having a notable effect on maximum load. Similar results were found for extension tests. Human lumbar spines were tested by adopting the same methodology developed for the Hybrid III lumbar spines. A cable pulley system was designed to maintain a constant compressive load on the spine during testing, in order to simulate the effect of erector muscle loading in flexion. Results demonstrated that the Hybrid III lumbar spine is stiffer in flexion than in extension. While cadaver tests demonstrated that the cadaveric lumbar spine is stiffer in extension than in flexion when erector muscle tone is not considered, it is less stiff when erector muscle tone is accounted for.