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STUDY DESIGN.: The authors conducted an in vitro biomechanical flexibility study of T2S1 specimens in flexionextension under compressive follower preloads of physiological magnitudes. OBJECTIVES.: The objectives of this study were to test the hypotheses that 1) the thoracolumbar spine will support compressive preloads of in vivo magnitudes and 2) allow physiological mobility under flexionextension moments if the preload is applied along an optimized follower load path that approximates the kypholordotic curve of the thoracolumbar spine. SUMMARY OF BACKGROUND DATA.: In the absence of muscle forces, the ligamentous thoracolumbar spine specimens cannot support the compressive loads expected in vivo. As a result, the flexibility of the thoracolumbar spine in flexionextension has not been studied in vitro under physiological compressive preloads. METHODS.: Seven human thoracolumbar spines (T2sacrum) were subjected to flexion and extension moments (up to 8 and 6 Nm, respectively) under compressive preloads from 0 to 800 N applied along an optimized follower preload path. The experimental technique applied the compressive preload such that: 1) it minimized the internal shear forces and bending moments resulting from the preload application, 2) made the internal force resultant compressive, and 3) caused the preload path to approximate the tangent to the curve of the thoracolumbar spine. The range of motion was measured in the T2sacrum, T2T11, T11L1, and L1sacrum regions. RESULTS.: All thoracolumbar specimens supported the compressive follower preload up to 800 N without damage or instability. At 800 N preload, the total flexionextension range of motion of the T2sacrum region decreased by 22%, from a mean of 73 to 57 (P < 0.05). The range of motion of the T2T11 and L1sacrum regions decreased from the baseline value by 23% and 30%, respectively, at a preload of 800 N. The sagittal mobility of the thoracolumbar junction (T11L1) was not affected by the preload. The follower preload did not significantly affect the proportion of the total T2sacrum flexionextension range of motion contributed by the T2T11 and L1sacrum regions of the thoracolumbar spine. CONCLUSIONS.: The optimized follower preload vector minimizes the effects of artifact moment and shear force on the range of motion of the thoracolumbar spine in flexionextension. This model allows the entire thoracolumbar spine to be investigated under physiological loading for different clinical applications. [ABSTRACT FROM AUTHOR]