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Free body diagrams and biomechanical foot models

Discussion in 'Biomechanics, Sports and Foot orthoses' started by Petcu Daniel, Jun 9, 2016.

  1. Petcu Daniel

    Petcu Daniel Active Member

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    As many of you I believe the free body diagrams and biomechanical foot models are the main "instruments" to be used to understand and practice in the frame of tissue stress theory. To me some articles are hard or even impossible to be understood but I think it could be somehow useful ! So, I'm thinking to put in this thread what I found on this subject hoping there will be other more useful contributions in this sense !
    To me the main resources are :

    -Eric Fuller and Kevin Kirby, Subtalar Joint Equilibrium and Tissue Stress Approach to Biomechanical Therapy of the Foot and Lower Extremity, in Lower Extremity Biomechanics: Theory and Practice, Volume 1, edited by Stephen F. Albert and Sarah A. Curran, Bipedmed, LLC, Denver, Colorado, 2013, p.260

    -Fuller, E A. ?The Windlass Mechanism of the Foot. A Mechanical Model to Explain Pathology.? Journal of the American Podiatric Medical Association 90, no. 1 (January 2000): 35?46. http://www.ncbi.nlm.nih.gov/pubmed/10659531

    -Kevin Kirby's "- Foot and Lower Extremity Biomechanics I-IV: Precision Intricast Newsletters

    Other examples:
    -Yarnitzky G, Yizhar Z, Gefen A. Real-time subject-specific monitoring of internal deformations and stresses in the soft tissues of the foot: a new approach in gait analysis, J Biomech. 2006;39(14):2673-89 http://www.ncbi.nlm.nih.gov/pubmed/16212969
    -Heung-Youl Kim, Shinji Sakurai, and Jae-Han Ahn. ?Errors in the Measurement of Center of Pressure (CoP) Computed with Force Plate Affect on 3D Lower Limb Joint Moment During Gait.? Int. J. Sport Health Sci. 5 (2007): 71?82. https://www.jstage.jst.go.jp/article/ijshs/5/0/5_0_71/_article

    Hope these helps !
  2. efuller

    efuller MVP

    This article looked at more and less complex models. Depending on the accuracy that you need, simple models can suffice.

    Theoretical considerations and practical results on the influence of the representation of the foot for the estimation of internal forces with models.
    Morlock M, Nigg BM.
    Clin Biomech (Bristol, Avon). 1991 Feb;6(1):3-13. doi:
  3. Petcu Daniel

    Petcu Daniel Active Member

    An excellent explanation of the free-body diagrams comes from Sheri D. Sheppard, Benson H. Tongue, "Statics: Analysis and Design of Systems in Equilibrium", Wiley 2007 ( http://eu.wiley.com/WileyCDA/WileyTitle/productCd-0471947210.html )

    Two quotes from Chapter 6:
    "The free-body diagram is the most important tool in this book. It is a drawing of a system and the loads acting on it. Creating a free-body diagram involves mentally separating the system (the portion of the world you?re interested in) from its surroundings (the rest of the world), and then drawing a simplified representation of the system. Next you identify all the loads (forces and moments) acting on the system and add them to the drawing."(page 215)

    "...the general rule that describes a boundary?s restriction of motion can be used to identify the loads at the support. This rule states:If a support prevents the translation of the system in a given direction, then a force acts on the system at the location of the support in the opposite direction. Furthermore, if rotation is prevented, a moment opposite the rotation acts on the system at the location of the support." (page 267: steps to create a free-body diagram)

    More useful information in chapter 6: "Drawing a free-body diagram" which is free to download from: http://www.wiley.com/college/sc/sheppard/free.html

    Also, you can find some "free-body diagrams self tests" at: https://www.physics.uoguelph.ca/tutorials/fbd/Qmenu.htm

  4. Petcu Daniel

    Petcu Daniel Active Member

    Thank you! Excellent article !

    Quote from conclusions: "...However, forces in internal structures of the foot are of interest to all researchers who investigate the influence of therapeutic measures (such as insoles or special shoes) on internal loading, the influence of different shoes or floor surfaces on internal loading, or injury mechanisms in general. At this point in time, models comprise the only possibility to estimate these forces. It has been shown in this study that the absolute magnitude of internal force estimates depends on the representation of the foot (the model) and that a comparative approach can eliminate systematic errors, such as systematic overestimation of forces, and, therefore, yields similar results with different (but still appropriate) models...."

  5. Petcu Daniel

    Petcu Daniel Active Member

    Even if not directly related with foot biomechanics the next one is in the same line with the article indicated by Eric. The approach is similar with Tissue stress theory (but with an emphasis on the use of free-body diagrams) as it can be seen from this quote:

    "...the professionals involved should ideally provide answers to one or more of the following questions:
    1.-Which structures in the low back are supposed to be overloaded (intervertebral disc, vertebra, muscle, or ligament) and under which types of load, e.g. compression force, shear force, or axial torque (i.e.the load criteria)?
    2.-What is the actual load value for each load criterion in a working situation?
    3.-What is the maximum acceptable load value for each load criterion (i.e. the norm)?" (ZOOS ?)

    Clin Biomech (Bristol, Avon). 1992 Aug;7(3):138-48. doi: 10.1016/0268-0033(92)90028-3.

    Value of biomechanical macromodels as suitable tools for the prevention of work-related low back problems.

    Delleman NJ, Drost MR, Huson A.


    Biomechanical macromodels are evaluated with respect to their possible usefulness for health professionals and ergonomists, as well as for applied research on the prevention of low back problems. It is concluded that in the context stated geometrically simple models, in particular the model by Schultz and co-workers, are to be favoured over more complex models. However, load predictions in extreme trunk postures should be dealt with carefully. It is recommended that the model load predictions should be used only in the comparison of work situations and not for an assessment of the absolute acceptability of a work situation. Low back problems are related to mechanical (over)load at work. This study shows the pros and cons of various biomechanical macromodels as tools for health professionals and ergonomists, as well as for applied research on the prevention of work-related low back problems.


  6. Petcu Daniel

    Petcu Daniel Active Member

    Prosthet Orthot Int. 1977 Dec;1(3):161-72.

    Graphic analysis of forces acting upon a simplified model of the foot.

    Veres G.


    Application of a graphical technique to analyse internal forces on a simplified model of the foot in various external loading patterns. The method is applied when the external load is acting purely upon the forefoot, the hindfoot and on both locations. The pes planus situation and the effect of the "rocker" and inlay sole are studied.
  7. Petcu Daniel

    Petcu Daniel Active Member


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