Voluntary oscillations of the human torso are analysed using engineering beam theory to estimate the flexural and torsional stiffness of the back. Antero-posterior and lateral flexural stiffness are measured as well as the torsional stiffness for both relaxed conditions and at maximum voluntary exertion. The ratio of flexural stiffness in two mutually perpendicular planes is a guide to the torsional stability of a beam in bending. These criteria are used to assess the torsional stability of the human torso when subjected to large bending moments.
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