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View Full Version : Human Body Models Including Tissue Analogs (BIONET Topic 3)



aleardini56
01-22-2002, 06:55 PM
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)
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