How the inherent stiffness of the in vivo human trunk varies with changing magnitudes of muscular activation

BackgroundThe abdominal muscles provide stiffness to the torso in a manner that is not well understood. Their unique anatomical arrangement may modify their stiffening ability with respect to the more commonly studied long strap-like muscles of the limbs. The purpose of this study was to examine stiffness inherent to the trunk, as modified by different torso, and in particular, abdominal muscle activation levels.MethodsNine healthy male participants were secured in a “frictionless” apparatus and subjected to applied bending moments about either the flexion/extension or lateral bend axes. Abdominal muscle activation levels were modified through biofeedback from the right external oblique muscle. Moment–angle curves were generated and characterized by an exponential function for each of flexion, extension, and right-side lateral bend, at each of four abdominal muscle activation target level conditions.FindingsStiffness measured in extension increased in a linear fashion throughout the range of motion and increased with each successive rise in abdominal activation. Stiffness in flexion and lateral bend increased in an exponential fashion over the range of motion. In flexion and lateral bend, stiffness increased with each successive rise in abdominal activation from zero to approximately 40% and 60% of the range of motion, respectively. After these points, stiffness at the highest levels of activation displayed a “yielding” phenomenon whereby the torso stiffness dropped below that characterized at lower levels of activation.InterpretationIncreasing torso muscle co-activation leads to a rise in trunk stiffness over postures most commonly adopted by individuals through daily activities (neutral to approximately 40% of maximum range of motion). However, towards the end range of motion in both flexion and lateral bend, individuals became less stiff at the maximum abdominal muscle co-activation levels. The source and mechanism of this apparent yielding are not fully understood; future work will be directed toward elucidating the cause.