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.