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Joint axes and plane of motion

Discussion in 'General Issues and Discussion Forum' started by ACH, Jul 24, 2011.

  1. ACH

    ACH Welcome New Poster

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    Hi All,

    I am year one podiatry and last week I had my first biomechanics lecture on planes and axis. What at first I thought was very basic has both kept me awake at night and occupied my thoughts all weekend. I have gone for understanding them, to over thinking them and losing understanding... and now I have made my peace with them.. but with the following conclusions.

    Planes and axis give a much generalised picture of observed motion from an external perspective; they do however fail to present an accurate picture of the internal motion when the surfaces of the bones articulate at the joint/s involved. Furthermore if we were to look closely into a joints motion we would always find (even if only in the smallest of margins) that when any two or more joints articulate together there is always two or three axis and planes involved. .. and why...because no joint has an absolute flat or perfectly round surface. This being said, the usefulness of biomechanics is limited to pinpointing abnormal motion on a large enough degree to be observed by the eye/motion capturing software...or pinpointing at what degree in a movement a problem such as pain is felt, thus helping give a direction for further investigations- such as x-ray.

    I would love to have some feedback on my ideas, however right or wrong they may be. All debate will provide me good service and increase my understanding of biomechanics... :)
  2. I'm guessing that so far you have been taught that axes are fixed in space and time. This is not true. I'm guessing that so far you have only discussed the idea of rotation at joint surface, without considering translation. As your knowledge grows you will understand that both rotation and translation occur simultaneously at the joint surfaces; you will learn about instant centres of rotation and axial bundles.

    For now, I'd just try to get your head around: the bigger the angle an axis makes to a plane, the movement that will proportionally occur within that plane.
  3. Jbwheele

    Jbwheele Active Member

    Ach pinpointing at what degree "Where" or when pain /problem occur in joint or areas of motion, is in the hands of the gods. there are sooooo many variables Craig Payne is really into tissue stress and the actual "Motion" as the problem not the measurable point of pain, however Palption of a painful area is a totally new subject. Just learn your anatomy well and the joints will follow.......... I mean articulations, not the kind smoked by some people.....hippies mainly.
  4. efuller

    efuller MVP

    Also, remember that a joint axis is an imaginary line that describes the motion that has occurred. Many people tend to think that joint motion must occur around that axis and this is not the case. A good example is the "published" axis of the ankle joint that allows plantar flexion dorsiflexion. However, an unloaded ankle joint can have up to 10 degrees of inversion motion which occurs about an axis other than the "published" one. The motion determines the axis, the axis does not determine the motion.

    Inman's book had some good pictures on how the joint surface helps determine the axis of motion. Also see Cahil DR. The Anatomy and Function of the Contents of the Human Tarsal Sinus and Canal. Anat. Rec. 153 p1-18. 1965 for how the joint surfaces determine the location of the "published" STJ axis.

  5. David Smith

    David Smith Well-Known Member


    Planes and axes are merely reference frames to define some motion of interest. These reference frames usually exist in the frame of a room or lab where vertical is parallel to the line of action of gravity and the other plane references are orthogonal, or at right angles to that, and the rotation references turn about those plane centres . Therefore the movement of a joint of interest can be defined in terms of those reference frames. You might see that the actual movement of the joint can be outside any or all of the references and have to be defined in terms of some summation of some or all of them. Or you can rotate the reference frame into the orientation of the joint or limb and then define the motion in those terms, which might be more convenient since the motion of interest may lie or be defined in one single plane or axis.. There then becomes the problem of defining one unique reference frame in terms of some other reference frame that is universally known.
    Therefore books and papers tend to stick to the universal reference frame to describe a joint motion but sometimes that can be an ambiguous definition especially when the description is simplified for the sake of convenience, convention and/or easier understanding of the concept. Sometimes then the convention is to use a single word to describe a translation (sliding) or rotation motion. Therefore we can talk about inversion- eversion and adduction- abduction and plantarflexion- dorsiflexion and superior- inferior and medial- lateral and posterior- anterior but the actual motion of the foot is bits of several of these.

    Does that helpor just confuse your understanding :dizzy:

    regards Dave Smith
    Last edited: Jul 25, 2011

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