Elsevier

Clinical Biomechanics

Volume 22, Issue 3, March 2007, Pages 266-274
Clinical Biomechanics

Review
Spine stability: The six blind men and the elephant

https://doi.org/10.1016/j.clinbiomech.2006.11.011Get rights and content

Abstract

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.

Section snippets

The blind men and the elephant

John Godfrey Saxe’s famous poem about six blind men and the elephant tells how different experiences lead to different perceptions. For the blind men, depending on what part was touched, they perceived the elephant to be a wall (side), spear (tusk), snake (trunk), tree (leg), fan (ear), or rope (tail). As the poem goes, these six blind men “disputed loud and long, each in his own opinion – exceeding stiff and strong, though each was partly in the right – and all were in the wrong!”

So how is

Why is stability important in spine biomechanics?

Stability is one of the most fundamental concepts to characterize and evaluate any system. For a system to carry out its goals or functions requires the system to be stable. In terms of the spine, stable behavior is critical for the spine to bear loads, allow movement, and at the same time avoid injury and pain.

Bergmark was the first to apply stability analysis to the spine by evaluating the potential energy of the system (Bergmark, 1987). (There are a number of simple explanations of static

Stability, robustness, and performance

Although the focus of the paper is on stability, there are other system characteristics, such as robustness and performance, which are also important when describing a system’s behavior. However, before assessing the robustness and performance of a system, the system must be stable. Consequently, we will start with the concept of stability.

Feedback control and stability

Given that the spine has similar characteristics to an inverted pendulum, it can be shown to be unstable and hence we assume that it will not behave “well”. Therefore, we want to control it in some fashion (1) to ensure that it is stable, and (2) to improve its robustness and performance. The principal approach for control is feedback. The information concerning the output of the system is feedback and used to modify the input (Fig. 4). In control parlance, the isolated system is called the

Plant versus system characteristics

In Fig. 3a and b, a perturbation produces a deviation away from the original position or trajectory. A stiffer system will be displaced less than a compliant system and will respond faster, suggesting that it performs better. So it would appear that increasing spine stiffness through trunk muscle co-activation would (1) ensure that the spine is stable, (2) reduce the amount of displacement, and (3) minimize the risk of injury. However, one has to remember that stability is context dependent

Feedback control model of LBP

Now that the framework for discussing the spine system has been presented, we can apply it to better understand biomechanical aspects of LBP. Clearly, there are many pathways for the development of LBP. In this section, we will discuss a number of possible mechanisms by which impairment in feedback control can lead to pain and injury. This is not intended to be an exhaustive list, but rather a few examples of how the presented framework encompasses various scenarios. To start, we will review

Conclusions

Stability is an elusive term that could lead to confusion. To help facilitate future discussions, a formal definition and a framework was presented which encompasses both static and dynamics spine systems. When considering the spine as a dynamic system, a static concept of spine stability is not adequate for discussing the involved issues. The stability of the system is maintained with feedback control, which also affects the system’s robustness and performance.

Stability is context dependent;

Acknowledgement

This publication was made possible by the NIH Grant Number R01 AR051497 from the National Institute of Arthritis and Musculoskeletal and Skin Diseases.

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