Spine stability: the six blind men and the elephant.

Stability is one of the most fundamental concepts to characterize and evaluate any system. This term is often ambiguously used in spinal biomechanics. Confusion arises when the static analyses of stability are used to study dynamic systems such as the spine. Therefore, the purpose of this paper is to establish a common ground of understanding, using standard, well-defined terms to frame future discussions regarding spine dynamics, stability, and injury. A qualitative definition of stability, applicable to dynamic systems, is presented. Additional terms, such as robustness (which is often confused with stability) and performance are also defined. The importance of feedback control in maintaining stability is discussed. Finally, these concepts are applied to understand low back pain and risk of injury.