Intersegmental Forces and Moments

[Tim Derrick]

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

https://doi.org/10.1016/j.jbiomech.2019.109533

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

[Ian Stokes]

Re: Intersegmental Forces and Moments

This paper “ISB recommendations on the reporting of intersegmental forces and moments during human motion analysis” [https://doi.org/10.1016/j.jbiomech.2019.109533] 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 http://www.uvm.edu/~istokes/pdfs/motseg.pdf

[Vasilios Baltzopoulos]

A bit late to this discussion but after the recent publication of my theoretical perspectives paper on Inverse Dynamics in the Sports Biomechanics journal, I hope to resurrect this thread for those interested.

The paper is entitled ‘Inverse dynamics, joint reaction forces and loading in the musculoskeletal system: guidelines for correct mechanical terms and recommendations for accurate reporting of results’ and can be accessed here: https://www.tandfonline.com/doi/full...1.2020.1841826

The main points related to the topic of this thread:

I disagree with using the terms intersegmental forces, resultant or net forces to differentiate between joint reaction forces calculated from different inverse dynamics approaches. All joint reaction forces from inverse dynamics are intersegmental, resultant or net forces, so using these terms to differentiate between joint reaction forces calculated from different approaches is inappropriate and misleading.

The so-called ‘bone on bone’ force is not different to the joint reaction force as usually presented but it is, in fact, a correctly calculated joint reaction force.

There is another misconception that some joint reaction forces are calculated through inverse dynamics whereas joint reaction forces that include the contribution of muscle forces are calculated through ‘musculoskeletal modelling’. However, this is a serious misunderstanding because musculoskeletal modelling is required for the construction of the free body diagram in every inverse dynamics procedure, irrespective of the approach used to model the muscle and other forces acting on the free body diagram. Musculoskeletal modelling is not a mechanical methodology that is different from inverse dynamics. All joint reaction forces are calculated through inverse dynamics and musculoskeletal modelling is the process involved in constructing the free body diagram of the segment that includes the forces acting on it and this process is required in all inverse dynamics procedures.

I hope that this paper clarifies these misconceptions related to inverse dynamics in biomechanics and the suggestions help with using appropriate and relevant mechanical terms.

Bill Baltzopoulos