Announcement

Collapse
No announcement yet.

Human Body Models Including Tissue Analogs (BIONET Topic 3)

Collapse
This topic is closed.
X
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Human Body Models Including Tissue Analogs (BIONET Topic 3)

    Dear Colleagues,

    After a good start with the discussion of TOPICS 1 and 2 of the current
    discussion series on contemporary issues in biomechanics, we continue with
    TOPIC 3 of this series. A brief summary of comments received to TOPIC 2
    (THE FUNDAMENTAL PROBLEM OF MYOSKELETAL INVERSE DYNAMICS AND ITS
    IMPLICATIONS FOR HUMAN MOTION ANALYSIS) was given by H. Hatze on Monday, 14
    January.

    The undersigned are indebted to the moderators of BIOMCH-L for their
    efficient and polite handling of the discussion moderation and also thank
    all discussants who spent time and effort by contributing their opinions to
    the lively debates. Please keep it up.

    Herbert Hatze,
    and
    Alberto Leardini on behalf of the BIONET Consortium


    Now follows the introduction of TOPIC 3:

    HUMAN BODY MODELS INCLUDING TISSUE ANALOGS (Topic Proponent: H. Hatze)

    First, it will be necessary to DEFINE A FEW TERMS that will be used in the
    sequel. There exists a large number of definitions of the terms "model" and
    "biomechanics", and their combination. For the purpose of the present
    discussion, I shall take the liberty to propose my own definitions which
    have appeared in the literature and have also been adopted and cited by
    numerous colleagues:

    A DESCRIPTIVE MODEL, henceforth called model, will be defined as the
    "ABSTRACT REPRESENTATION OF SELECTED ATTRIBUTES OF A REAL OBJECT, EVENT, OR
    PROCESS "
    (H. Hatze, 1981, and 2000: Progression of Musculoskeletal Models Toward
    Large-Scale Cybernetic Myoskeletal Models, Chapter 33 of Winters, J. M.,
    Crago, P. E. (eds), Biomechanics and Neural Control of Posture and
    Movement, Springer, New York, 425-437),
    and

    BIOMECHANICS as "THE SCIENTIFIC DISCIPLINE THAT INVESTIGATES THE STRUCTURE
    AND FUNCTION OF BIOLOGICAL SYSTEMS BY MEANS OF THE METHODS OF MECHANICS"
    (translated from German from H. Hatze, 1971, Was ist Biomechanik, J.
    Leibesueb.-Leibeserz. 71/2, 33-34).

    A (DESCRIPTIVE) BIOMECHANICAL MODEL may therefore be defined as "THE
    ABSTRACT MECHANICAL OR MECHANOMATHEMATICAL ANALOG OF SELECTED ATTRIBUTES OF
    A REAL BIOLOGICAL OBJECT, EVENT, OR PROCESS".

    This definition is sufficiently general to cover practically all
    biomechanical models currently in use. It is clear that any model can only
    be an incomplete and simplified analog of the real system or object it is
    supposed to represent, no matter how complex the model is.

    If one accepts the above definition of a biomechanical model then a
    (descriptive) BIOMECHANICAL HUMAN BODY MODEL is "the abstract mechanical or
    mechanomathematical analog of selected attributes of the structure and
    functional behavior of the human body".

    In biomechanics we have to deal with both, models of the TOTAL HUMAN BODY
    and with those of its SUBSYSTEMS (COMPONENTS) such as joints, bones,
    organs, special tissues, etc.

    Having clarified the terms used in the discussion of the present topic, I
    shall now turn to the (deliberately provocatively formulated) PROBLEM
    STATEMENTS. The second problem statement concerning total human body models
    is identical with the one already formulated in the problem description of
    TOPIC 2, but will be repeated here for the convenience of discussants
    wanting to join the discussions now:

    1. A (functional) MODEL IS AS GOOD AS ITS VERIFIABLE PREDICTIONS. Many
    human body models currently in use fail in this respect. The APPROPRIATE
    DEGREE OF COMPLEXITY OF BIOMECHANICAL HUMAN BODY MODELS and, indeed, of all
    biomechanical models, as well as MODEL VALIDATION PROCEDURES have been
    hotly debated issues for some time now. It is a maxim in modeling that
    deduction should always be carried as far as possible. This ensures the
    incorporation into the model structure of as many as possible of the known,
    and with respect to the selected attributes relevant, properties of the
    real biosystem. (A detailed discussion of this maxim and the problem of
    model complexity selection is contained in the reference H. Hatze (2000)
    already cited above.)

    This maxim determines the degree of model complexity. In fact, it
    determines the MAXIMUM DEGREE OF COMPLEXITY because we could not create a
    better model at that specific point in time. As more details become known
    about the structure and (or) functional behavior of the biosystem in
    question, the maximum degree of complexity increases and the model should
    be modified accordingly. In this sense, appropriately designed human body
    (or subsystem) models of greater complexity are more adequate, that is,
    more powerful and reliable in producing biologically realistic results.

    However, the practical implementation of this maxim is frequently thwarted
    by limitations such as unrealistically long periods required for model
    development, oversized computer programs, excessively long execution times
    of the computerized model version, etc. Thus, a compromise has to be struck
    between the theoretically possible maximally complex human body or
    subsystem model and its less complex but economically feasible realisation.
    The resulting compromise model may, however, turn out to be inadequate. For
    functional models, this is the case when their simulation responses deviate
    from the corresponding responses of the real biosystem by more than a
    prescribed (acceptably small) margin, for all conceivable modes of
    operation. These MODEL VALIDATION TESTS are rarely performed. If they were,
    many of the currently used human body or subsystem models would have to be
    classified inadequate.

    2. TOTAL HUMAN BODY MODELS currently used in biomechanics are unrealistic
    and, for most investigations, inadequate. The human limb system can not in
    general be considered as, or adequately approximated by, an assemblage of
    interlinked rigid body segments. Rather, it is a RIGIDO-VISCOELASTIC HYBRID
    composed of numerous (nearly) rigid bony segments, interconnected by
    (visco)elastic tissue structures (ligaments, cartilage, joint capsules,
    etc.), and having attached to them (visco)elastic active (musculotendinous
    units) and passive (e.g. inner organs, blood vessels, etc.) components,
    which, in turn, are interconnected among themselves. In reality, there
    exist no limb segment boundaries: joints are spanned by elastic structures
    (mainly muscles) that belong simultaneously to two or more limbs. The
    motions of the elastic body structures relative to the skeleton can be
    highly significant, especially in motions with pronounced accelerative
    phases, such as impacts (e. g. heel strike in gait). The consistency of
    some of these elastic structures (muscles) changes with their degree of
    contraction. The detailed three-dimensional structures (bones, ligaments,
    muscles, tendons, soft tissue structures, etc.) of terminal segments
    (hands, feet) have to be fully accounted for, which is hardly the case with
    present-day models. Joints generally have six degrees of freedom (with the
    possible exception of the hip joints) and are exceedingly complex
    structures whose detailed functional behavior still awaits elucidation.

    3. BIOMECHANICAL MODELS OF HUMAN BODY SUBSYSTEMS cover an extremely
    diversified range of the structures and the functional behavior of body
    components, including haemodynamics, skeletal muscle architecture and
    function, joint kinematics, bone behavior under varying loading conditions,
    etc. Although the state of development and the rate of its progress is
    vastly different in the various areas, it is certainly true that a
    considerable number of human body subsystem models must be regarded
    inadequate. Discussion contributions on this subtopic and on the
    identification of problem areas concerning subsystem models are especially
    welcome.

    4. Present-day PARAMETERIZATION OF BIOMECHANICAL HUMAN BODY AND BODY
    SUBSYSTEM MODELS is inappropriate. Any proper biomechanical model should
    contain a sufficient (minimal) number of parameters that allow the model
    to be individualized, that is, to be adapted to a specific subject or
    patient. It is of the utmost importance that the subject-specific values of
    these parameters are determined by appropriate (non-invasive) experimental
    IN-VIVO methods, especially in the clinical realm, in sports, and in
    ergonomics. To take population averages for these parameters is
    inappropriate and leads to erroneous results. Furthermore, because all
    living systems are subject to continuous adaptation and change, it should
    be realized that the values of these subject-specific parameters are VALID
    FOR A RESTRICTED PERIOD OF TIME ONLY. This fact is ignored in most of the
    present-day biomechanical modeling attempts, as has recently been pointed
    out by Dr. Tashman in a TOPIC-2 discussion contribution containing an
    excellent account of the current unsatisfactory situation in clinical gait
    analysis. (A special topic on the identification of subject-specific
    parameters will soon appear on this discussion forum).

    Herbert Hatze

    ************************************************** ******
    Prof. Dr. Herbert Hatze
    Head, Department and Laboratory of Biomechanics, ISW,
    University of Vienna

    Auf der Schmelz 6 Tel: + 43 1 4277 48880
    A-1150 WIEN Fax: + 43 1 4277 48889
    AUSTRIA e-mail: herbert.hatze@univie.ac.at
    ************************************************** ******

    ************************************************** ************************
    Alberto Leardini, DPhil
    Movement Analysis Laboratory
    Centro di Ricerca Codivilla-Putti
    Istituti Ortopedici Rizzoli
    Via di Barbiano 1/10, 40136 Bologna ITALY
    tel: +39 051 6366522
    fax: +39 051 6366561
    email: leardini@ior.it
    http://www.ior.it/movlab/

    "Where is the Life we have lost in living,
    Where is the wisdom we have lost in knowledge,
    Where is the knowledge we have lost in information."
    Thomas Stearns Eliot, Choruses from ''The Rock'' (1934)
    ************************************************** ************************

    ---------------------------------------------------------------
    To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl
    For information and archives: http://isb.ri.ccf.org/biomch-l
    ---------------------------------------------------------------
Working...
X