One important mechanical function of the lumbar spine is to support the upper body by transmitting compressive and shearing forces to the lower body during the performance of everyday activities. To enable the successful transmission of these forces, mechanical stability of the spinal system must be assured. The purpose of this study was to develop a method and to quantify the mechanical stability of the lumbar spine in vivo during various three-dimensional dynamic tasks. A lumbar spine model, one that is sensitive to the various ways that individuals utilize their muscles and ligaments, was used to estimate the lumbar spine stability index three times per second throughout the duration of each trial. Anatomically, this model included a rigid pelvis, ribcage, five vertebrae, 90 muscle fascicles and lumped parameter discs, ligaments and facets. The method consisted of three sub-models: a cross-bridge bond distribution-moment muscle model for estimating muscle force and stiffness from the electromyogram, a rigid link segment body model for estimating external forces and moments acting on the lumbar vertebrae, and an 18 degrees of freedom lumbar spine model for estimating moments produced by 90 muscle fascicles and lumped passive tissues. Individual muscle forces and their associated stiffness estimated from the EMG-assisted optimization algorithm, along with external forces were used for calculating the relative stability index of the lumbar spine for three subjects. It appears that there is an ample stability safety margin during tasks that demand a high muscular effort. However, lighter tasks present a potential hazard of spine buckling, especially if some reduction in passive joint stiffness is present. Several hypotheses on the mechanism of injury associated with low loads and aetiology of chronic back pain are presented in the context of lumbar spine stability.