Engineering definition
For engineers, a stable system is often defined as a system that will attract toward a fixed position when perturbed a small amount. It is a mathematical concept and a definite one. A system may be characterized as either stable, neutrally stable, or unstable. An example of a stable system is a marble in a bowl. Any small perturbation will be move the marble away from its stability point. In this perturbed position, the slope of the bowl and the acceleration of gravity will result in a net force that will pull the marble back toward the stability point in the center of the bowl. Notice that the rate in which the marble returns to the stability point or how far the marble moved away from the stability point is not a factor in determining if the system is stable from an engineering perspective. This is equivalent to stating that the slope of the bowl does not matter as long as it is sloping inward and not outward (unstable) or simply flat (neutrally stable). Based on these definitions, if a system is determined to be stable, it is senseless to state that the system is more or less stable—a system is either stable or it is not.
Medical definition (from an orthopedic perspective)
For clinicians, the term “stability” is often used to describe a single joint such as the knee or a series of joints such as the lower back (lumbar spine). A joint is considered stable if a person is able to use it without collapsing. The term “stability” does not have a formal definition as in engineering. Other clinicians may determine joint stability by the amount of movement that the joint will undergo during manual manipulation by the clinician. Some clinicians refer to this as “joint laxity” rather than “stability.” Clinicians are generally interested in the function outcome of stability and they use joint stability as a relative term. As an example, a person who is able to walk over level ground but sometimes collapses when climbing stairs may be said to have a more stable knee than a person who immediately collapses when any weight is put on the leg. Putting a knee brace on the leg may increase the stability as defined by less uncontrollable joint movement and/or higher passive stiffness.
Confusion
Conflict arises because of differences in this definition. The first is because engineers consider stability as a “yes” or “no” question, and clinicians consider it as quantifiable parameter such as “low”, “medium” or “high”. This often leads to engineers feeling that the clinician does not understand the workings of a mechanical system, whereas the engineer may tend to dismiss the clinician’s knowledge, understanding, and experience working with patients. Clinicians tend to feel that engineers are being picky about the definition of the word, getting lost in the details, and do not understand what is important to the patient.
