The experimentally determined torque–rotation curve of the lumbar spine is mathematically described with a proposed dual-inflection point Boltzmann equation. The result is a method for describing functional spinal unit motion data. The benefit of the model is that each of the coefficients has a specific meaning in relation to the torque–rotation curve: the points and identify the respective minimum and maximum rotations of the functional spinal unit, and indicate the inflection points of the curve where the stiffness changes markedly, and and are associated with the rates of change of the curve at and , respectively. The dual-inflection point Boltzmann captures the full quality of motion of the spinal segment and can also be used to derive relevant parameters such as range of motion, midrange stiffness, and hysteresis.
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