The human lumbar spine has been shown to support compressive loads of 1000 N in standing and walking, and up to many thousands of Newtons in strenuous activities such as lifting. The literature presents a number of biomechanical models that seek to replicate the load-carrying capacity of the spine while adhering to physiological constraints. While many of these models provide invaluable insights into the mechanisms governing spinal stability, there is a nearly universal disregard for the magnitude of the muscle forces required in the neutral standing posture. In compliance with constraints on metabolic cost and muscle fatigue, muscle activations in excess of 5% maximum voluntary contraction (MVC) in the standing posture are physiologically infeasible. The purpose of this thesis was to investigate the hypothesis that the passive structures of the lumbar spine are sufficient to produce static equilibrium under the body weight load in the neutral standing posture. A novel method of applying physiologic loads to the lumbar spine in vitro to determine its passive stability was developed. Five cadaver specimens were tested and a passive equilibrium posture was discovered for each. Further, the parameters defining the equilibrium posture correlate well with the standing posture as reported in the literature. This is an indication that the lumbar spine is inherently capable of remaining erect in the neutral posture with muscle activations below 5% MVC. It is postulated that the iliolumbar ligament and the thoracolumbar fascia, passive components that are not typically incorporated into stability models of the spine, have the potential to provide added passive stabilization to the system. It is recommended that biomechanical models of the spine incorporate this 5% MVC constraint and place greater emphasis on the contributions of passive structures to overall stability.