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Intersegmental Forces and Moments

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  • Intersegmental Forces and Moments

    ISB recommendations on the reporting of intersegmental forces and moments during human motion analysis

    The International Society of Biomechanics and the authors of this paper would like to formally request comments and discussion of this paper. The complexity of a topic such as intersegmental moments and forces is such that it is impossible to cover all of the associated details within a journal article. Therefore, we would like to invite comment from the Biomch-L community concerning issues that were not covered, covered inadequately or issues that you may disagree with. We hope that this will result in a vibrant discussion that can be referred to in conjunction with the original article.

    Tim Derrick
    Ton van den Bogert
    Andrea Cereatti
    Raphael Dumas
    Silvia Fantozzi
    Alberto Leardini

  • #2
    Re: Intersegmental Forces and Moments

    This paper “ISB recommendations on the reporting of intersegmental forces and moments during human motion analysis” [] provides an excellent contribution to clarify several issues about musculoskeletal intersegmental and joint biomechanics. Notably, it addresses the often confused concepts of ‘internal’ and ‘external’ moments.
    I do have some comments about the following passage in the manuscript, in relation to spinal biomechanics:
    Joint centers
    To compute the intersegmental joint moments, a reduction point, that is the point with
    respect to which the system of forces is reduced, is required. This point is classically defined as a
    joint center. In most of the human movement analysis protocols proposed in the literature,
    adjacent bony segments are conceptually assumed to be connected by spherical pairs, and their
    relative motion is described by three joint angles about the three anatomical axes defining the
    joint coordinate system and passing through this joint center.

    My own experience of joint biomechanics relates mostly to the spine, where the articulations are flexible structures, not simple diarthroidial joints, so they transmit moments in addition to forces. Therefore, especially for the spine, “Joint Center” would benefit from further definition. The reason it is “classically defined as a joint center…. [and] adjacent bony segments are conceptually assumed to be connected by spherical pairs” is that the joint contact force (in the case of a frictionless joint) passes through the center of both spherical surfaces, i.e. the joint force is perpendicular to the surface. (This is qualified by ‘in most of the human movement analysis protocols’.) Frequently, the joint center is identified as the (kinematic) center of rotation. This is convenient, since for a frictionless, rolling and gliding joint, the center of rotation is located on a line that is perpendicular to the surface at the point of contact, i.e. on a line co-linear with the joint force.

    This concept is often (incorrectly) extended to the spine articulations. In the spinal case the instantaneous center (or axis) of rotation depends on the present or actual forces transmitted by the (flexible) articulation. There is no a priori reason why the articulation would not be transmitting moments about this instantaneous point or axis. Therefore, my colleague Mack Gardner-Morse and I have employed an ‘equivalent’ structure (at its simplest an ‘equivalent beam’) to represent the spinal articulations. The properties approximate those obtained from experimental studies. See Gardner-Morse MG, and Stokes IAF: Structural behavior of human lumbar spinal motion segments. J Biomechanics 2004, 37(2): 205-121