Evaluation of the threshold of stability for the human spine
Journal of Biomechanics 42(8), 1017-1022. doi:10.1016/j.jbiomech.2009.02.008
Martin L. Tanaka, Maury A. Nussbaum, Shane D. Ross
Orthopaedic Surgery, Wake Forest University
Virginia Tech - Wake Forest University School of Biomedical Engineering and Sciences
Industrial and System Engineering, Virginia Tech
Engineering Science and Mechanics, Virginia Tech
The threshold of stability (ToS) is introduced as a new tool for evaluating spinal stability.
Current methods have evaluated the magnitude of spinal kinematic variability, stability diffusion,
or Lyapunov exponents. This present method differs by modifying task difficulty to obtain a
critical value delineating regions of stability and instability. Conceptually, as task difficulty
increases, kinematic variability of the system increases and the basin of stability decreases.
When kinematic variability exceeds the basin of stability, stability cannot be maintained over
time. This concept is first illustrated using a mathematical model, from which an effective
potential function was calculated to show the relationships among kinematic variability, the
basin of stability, and task difficulty. In addition, an experiment was performed to evaluate the
sensitivity of the ToS to changes in postural control using visual feedback as a control variable.
The ToS was found to be more sensitive than the Lyapunov exponent to removal of visual
feedback, suggesting it may have use as a diagnostic indicator (e.g. for low back pain).
Furthermore, this new method has an additional advantage in that minimal instrumentation is
needed. Its simplicity, sensitivity, and low cost suggest that the ToS has potential as a diagnostic
or prognostic tool in a clinical setting.
Keywords: threshold of stability, Lyapunov, spinal stability, kinematic variability, low back pain
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