Biomechanical testing of cadaveric spinal segments forms the basis for our current understanding of healthy, pathological, and surgically treated spinal function. Over the past 40 years there has been a substantial amount of data published based on a spinal biomechanical testing regimen known as the flexibility method. This data has provided valuable clinical insights that have shaped our understanding of low back pain and its treatments. Virtually all previous lumbar spinal flexibility testing has been performed at room temperature, under very low motion rates, without the presence of a compressive follower-load to simulate upper body weight and the action of the musculature. These limitations of previous work hamper the applicability of published spinal biomechanics data, especially as researchers investigate novel ways of treating low back pain that are intended to restore the spine to a healthy biomechanical state. Thus, the purpose of this thesis work was to accurately characterize the rate-dependent flexibility of the lumbar spine at body temperature while in the presence of a compressive follower-load. A custom spine simulator with an integrated environmental chamber was developed and built as part of this thesis work. Cadaveric spinal motion segments were tested at 12 different rates of loading spanning the range of voluntary motion rates. The testing methodology allowed for comparison of spinal flexibility at room and body temperatures in the three primary modes of spinal motion, both with and without a compressive follower-load. Additionally, the work developed a stochastic model for rate-dependent spinal flexibility that allows for accurate prediction of spinal flexibility at any rate within the range of voluntary motion, based on a single flexibility test. In conclusion, the biomechanical response was significantly altered due to testing temperature, loading-rate, and application of a compressive follower-load. The author emphasizes the necessity to simulate the physiological environment during ex vivo biomechanical analysis of the lumbar spine in order to obtain a physiological response. Simplified testing procedures may be implemented only after the particular effect is known.