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  • joint forces

    Dear Biomch-l readers,

    I have been reading with interest the debate concerning the terminology
    of joint forces.
    I think the term "joint force" or "resultant joint force" is adequate
    to describe the sum of the various forces acting on the joint (F1).
    However I prefer to refer (especially during teaching) to the joint
    moment or resultant joint moment, because drawing a circular arrow to
    represent the joint moment is clearer than drawing a vector anywhere on
    the joint, pointing to an arbitrary direction.
    The term "joint muscular force" is more appropriate for the componenet of
    F1 generated by muscles, instaed of "active force" etc.
    If a collective term for F2 is necessary, then I prefer the term "joint
    contact force". That is the resultant joint force is the sum of muscular
    and contact forces. One could argue that there is no actual contact between
    segments in a joint but in that case one shouldn't use terms such as action
    and reaction.
    However, I think that including the names of the bones is more appropriate.
    Using the example of knee extension by R. Hinrichs, I prefer the term
    tibio-femoral (contact) force with two components:
    1. Tibio-femoral compressive force (absorbed mainly by the menisci)
    2. Tibio-femoral shear force (absorbed mainly by ACL)
    Similar terms should be used in other joints. I believe this terminology
    is more accurate than confusing or misleading derived collective terms.

    On the subject of terminology, I would like to make a comment on the
    continued use of the term "contraction"=shortening as "muscular
    contraction", "isometric contraction !" etc. Accepting the argument
    of "intended" contraction of proteins at sarcomere level even in isometric
    or eccentric conditions, I think that "contraction" is totally inappropriate
    to describe whole muscle movement, especially under isometric or eccentric
    Peter Cavanagh proposed the term "muscular action" (Cavanagh, P. (1988) On
    "muscle action" versus "muscle contraction". Journal of Biomechanics, 21:69).
    I think the term "muscular activation" is more appropriate than "contraction"
    or "action", because it can be used to describe whole muscle movement
    (eccentric or concentric) or no movement (isometric).
    The appropriate terms therefore are eccentric, concentric, isometric,
    isokinetic activation, maximum voluntary activation (MVA vs MVC) etc.


    Vasilios Baltzopoulos

    PS I hope the weather is better in your part of the world.

    V Baltzopoulos, PhD
    Dept. of Movement Science,
    Faculty of Medicine,
    University of Liverpool,
    PO Box 147, Liverpool L69 3BX, U.K.

    tel +44 51 7943223
    fax +44 51 7943229