The biomechanical "stability" of the whole human lumbar spine was investigated experimentally and theoretically.

The three-dimensional angular load-displacem ent behavior of the whole lumbar spine subjected to pure moments at L1, with the sacrum fixed, was determined. The load-displacement characteristics of the main motion, those in the same direction of loading, demonstrated nonlinear behavior. Distinct coupling patterns were observed. Theory for describing three dimensional vertebral displacements relative to a moving and directly unobservable coordinate system, fixed to a vertebra, was developed.

The lumbar spine was approximated as an Euler column. The experimental and theoretical limits of stability were determined. In vitro testing demonstrated that the ligamentous spine was laterally unstable, in the sense of Euler, at a load of 67 N, decreasing as the severity of injury to the specimens increased. Theoretically, the Euler phenomena of the ligamentous lumbar spine was modeled as a discrete elastic column.

The lateral, Euler stability of the muscular spine was also theoretically examined. Stiffness of the stretch reflex was assumed to be the regulated variable in the control of posture. The stabilizing efficiency of various muscle architectures was investigated. Coactivation of back muscles was predicted to maintain upright posture for any load that increased the compressive load on the spine.

A new definition of biomechanical stability was proposed. It was hypothesized that the biomechanical characteristics of the intervertebral articulations and whole spine are such that they minimize the expenditure of muscular energy in attaining biomechanical stability.