Large compressive preloads decrease lumbar motion segment flexibility

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Abstract
The bending, shear, and torsion flexibilities of 13 intact adult lumbar motion segments (from 11 men, two women, 48–83 years of age) were compared under three different compressive preloads, 0, 2,200, and 4,400 N. Test forces and moments up to 160 N and 16 Nm were applied at the center of the upper end plate of the intact disc. A compressive preload of 2,200 N resulted in a significant decrease in motion segment flexibilities in all seven test directions (p < 0.06) when compared with results obtained with no preload; the preload decreased flexibility 2.6, 4.5, and 6.1 times in bending, axial torsion, and shear, respectively. These results suggest that studies of internal trunk load-sharing between active and passive tissues during strenuous tasks, which engender large spine compressive loads, should take these changes in spine passive resistance into consideration.