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Discussion in 'Biomechanics, Sports and Foot orthoses' started by wdd, May 31, 2012.

  1. wdd

    wdd Well-Known Member

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    Abnormal Forces.

    I am concerned with the continuing use of the term "abnormal forces" in podiatric biomechanics texts.

    My concern is that it doesn't necessarily, accurately, or comprehensively describe the situation. To put it another way the phrase 'abnormal forces' is almost invariably inaccurate and frequently wrong.

    Are the forces abnormal?

    In what way are the forces abnormal?

    If there is an abnormaility is it necessarily related to the force?

    Biomechanics is concerned with forces applied to tissues. Therefore the forces and/or the tissues may be 'abnormal'.

    I think that there is a need for a more comprehensively accurate phrase, to replace "abnormal forces", even if it is more cumbersome.

    A satisfactory phrase should incorporate these possibilities, ie the tissues and/or the force may be abnormal.

    Mechanical injury occurs when the characteristics of the applied force are beyond the capacity of the tissue to resist.

    The capacity of tissue to resist applied force without injury (tissue tolerance) is dependent upon the characteristics of the applied force and the structural and functional characteristics of the stressed tissue.

    Tissue tolerance varies from tissue to tissue and even at different locations within the same tissue. It is also history dependent.

    Any term attempting to describe the interaction between forces and tissue is likely to be more meaningful if it states something about the interaction between force and tissue.

    The application of forces stressing tissues beyond their level of tolerance results in injury.

    My attempt at an alternative phrase for "abnormal forces" is: forces stressing tissues beyond their level of tolerance.

    Should the phrase "abnormal forces" be changed?

    If so what should it be changed to?

    May the force be with you.

  2. Petcu Daniel

    Petcu Daniel Active Member

    What about "pathological forces" ?

  3. Normal forces are forces which act perpendicular to a surface, abnormal forces presumably do not ;) I too have a dislike for the term "abnormal forces".

    We talk about the zone of optimal stress (ZOOS) for a tissue because too little stress can have a negative impact too; not just too much force, but too little also. So if you like, an abnormal force could be one which results in a tissue functioning outside of its ZOOS.

    P.S. here's the content of a slide from one of my lectures on tissue stress:

    Tissue dysfunction occurs when:
    The load is excessive in relation to the biomechanical properties of the tissue
    The biomechanical properties of the tissues have decreased in relation to a “normal load”
    Both 1 and 2 occur together.
  4. efuller

    efuller MVP

    An example of an abnormal force is the car bumper hitting your tibia. It's quite unusual. An injurious force would occur if that car bumper was moving over 10 mph just before it hit the tibia.

    I agree abnormal forces is a poorly defined term. I would imagine it would be used by people who hadn't bothered to define, or examine, the forces involved. Patient: "Why does my foot hurt?" Lazy doctor: "abnormal forces." or "you have arthritis." Better doctor: " your 1st mpj hurts because of high compressive forces at the joint surface caused by high tension in the plantar fascia." (Of course this is said after physical exam leads to that conclusion.)

  5. wdd

    wdd Well-Known Member

    It seems like a move in the right direction. Although it is essentially still open to the same criticism as "abnormal forces", ie it ignores the potential tissue contribution. However pathological bacteria is a term used to describe bacteria causing disease and not diseased bacteria so there could be little argument against associating pathology with force to mean forces causing tissue injury. How about modifying it slightly to "pathogenic forces", ie forces giving rise to tissue injury?

  6. Imagine what you like Bill, but it doesn't mean what you have stated above is correct. For starters ZOOS is a state of health, not of dysfunction. Secondly, the tissue may be placed outside of it's ZOOS through a single loading cycle.
  7. Bill:

    I agree with you that the term "abnormal forces" is not a good term when discussing the etiology of various musculoskeletal injuries among health professionals or scientists. I don't use that term "abnormal forces" and don't think it is used that often that I see. However, the term "abnormal forces" would probably be sufficiently descriptive to describe how too large of a force could cause injury to the layperson.

    One of the main problems with using the term "abnormal forces" is is that the term "force" has four characteristics that directly determines its effect on the body: 1) magnitude, 2) direction, 3) point of application and 4) line of action. Therefore, to be accurate and precise, we would need to know which of these four characteristics of the force vector was "abnormal" in order to fully understand whether what specifically it was about the force that was "abnormal".

    Secondly, a force, can also describe a rotational force, or a "torque", or a "moment of force", or a "moment". Typically, in biomechanics," force" describes a linear vector quantity whereas "moment" describes a rotational force that is caused by a force acting across an axis of rotation at a given moment arm distance.

    Third, the term "force" is not specific enough to determine the known causes of injury to musculoskeletal components without including other parameters which may cause a force of a given magnitude to cause tissue damage and another force of identical magnitude to not cause tissue damage. We must also determine the following time-dependent parameters of force application to better determine its potential for deformation or damage to a musculoskeletal component. These parameters are very important since all the tissues of the body are viscoelastic in nature and, as a result, have time-dependent characteristics such as "creep response" and "stress-relaxation":

    A. Rate of force increase and force decrease (i.e. slope on the force vs time curve).
    B. Duration of force application.
    C. Repetition of force application over a given time period.

    Fourth, as Simon mentioned, the body, over time will regulate the mechanical characteristics of its structural components in response to applied forces so that these body tissues may either strengthen over time or weaken over time. Therefore, a force may produce an injury early on in training whereas the exact same force may produce no injury, and in fact may help strengthen the tissue, once the body tissue has become thicker, stronger and better able to handle the increased tissue stress.

    Fifth, the terms "tissue stress" and "tissue pressure", since they are both dependent on the magnitude of force that is acting over a given surface area or over a given cross-sectional area of tissue, are force parameters that will more likely be directly related to the likelihood of tissue injury than force, by itself. For example, to say "the posterior tibial tendon had excessive magnitudes of tensile stress causing tendon rupture" is quite specific and less ambiguous whereas to say "the posterior tibial tendon had abnormal force in it causing tendon rupture" is less specific and more ambiguous.

    Hope this helps.
  8. drsha

    drsha Banned

    I like your inclusion of structural in your drefinitions.

    What about Compensatory Threshold.

    below which forces are "normal" and aboove which, before during and after complaints, something is "abnormal"?
  9. wdd

    wdd Well-Known Member

    I think the phrase I used contained 'structural and functional'. As I see it that phrase covers everything outside the psychosomatic?

  10. wdd

    wdd Well-Known Member

    I take my previous post back. 'Structural and functional' covers everything. Psychosomatic is one aspect of the structure and function of the brain.


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