Calf lumbar spines have been widely used for the biomechanical tests of various spinal fixation devices as a substitute for human cadaveric lumber spines. While similar size, loss cost, and less inter-specimen variations are positive attributes, little effort has been made to determine if the use of calf spines is biomechanically justifiable. Wilke et al. investigated the range of motion, neutral zone, and stiffness of thoracolumbar calf spines. It was determined that these values were similar to those previously reported for the human spine. In this study, however, the test was limited to the intact calf spine, and no statistical analysis was made since the results were compared to the results of various studies reported in the literature. Direct comparison of biomechanical testing results of calf and human models has not been presented yet. The purpose of this study was to perform flexibility tests on calf and human lumbar spines using identical methods in order to determine if the anatomical and mechanical differences in the two models affect the results of flexibility tests.