Rasmussen - 2007 - AnyBody CAE For The Human Body
Rasmussen - 2007 - AnyBody CAE For The Human Body
Rasmussen - 2007 - AnyBody CAE For The Human Body
John Rasmussen
Summary
This paper introduces the AnyBody Modeling System and its capability to extend the field of CAE ap-
plications to ergonomic product design. The paper introduces the general field of musculoskeletal
modeling and proceeds to present the AnyBody Modeling system, the AnyScript modeling language
and the AnyScript Model Repository. Finally the some of the applications fields of the technology are
presented.
Keywords
biomechanics, musculoskeletal simulation, ergonomics, orthopedics.
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1. Introduction
Technologies such as Computer-Aided Design, Finite Element Methods, and Computational Fluid Dy-
namics have had a very significant impact on modern product design. Hardly any advanced product
today is designed without the use of some sort of computer simulation, and virtually any technical
property of products can be analyzed, including strength, vibration, heat conduction, magnetism, flow,
acoustics, and light reflection just to mention a few.
However, one prominent property has been missing from the range of analysis facilities: The me-
chanical influence of the product on the human body. This property – also often called ergonomics
may not seem like a very important addition at first glance because the tradition in many fields of in-
dustry has been to regard ergonomics as something that is handled outside the technical realm by
specially trained professionals. The human body was simply not regarded as something that could be
subjected to CAE analysis.
2. Musculoskeletal modeling
Ergonomic simulation deals with the mechanics of the
human body, which, as mechanical systems go, is very
complicated indeed. The body has more than 200 bones
connected by different and in some cases very complex
joints. The bones are articulated by several hundred
muscles. An account of the precise number of muscles
Figure 1. A complex musculoskele- in the human body depends on the point-of-view. The
tal model comprising more than 500 mus- traditional anatomical classification of muscles is not
cles. adequate mechanically because many of the anatomical
muscles span large areas, have fibers going in different
directions, and can be activated separately in individual
motor units. An estimation of the necessary number of muscle units for a reasonable mechanical
modeling of the human body is around 1000, and the most comprehensive models today number
roughly half of that (Figure 1). A muscle’s effect depends among other things on its moment arm about
the joint(s) it actuates, and correct modeling requires correct moment arms. Many of the body’s mus-
cles and in fact most of the shoulder muscles wrap around bones and other tissues on their way from
origin to insertion, and this has a profound and complex influence of the moment arms because the
contact between muscle and bone changes when the body moves. Muscles can come into contact
with bones or release existing contacts, and they may slide along bones as the body moves. This in-
troduces a contact problem into the mechanical modeling, and it causes a significant complication of
the mathematics and the numerical procedures.
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Another major complication of
musculoskeletal analysis is
that the system is inherently
statically indeterminate
because there are usually
many more muscles in the
system than degrees of
freedom. This means that
each degree of freedom is
carried by several muscles,
and there are not enough
equilibrium equations
available to uniquely distribute
the load between the
muscles. The usual solution
to this problem is to presume
that the body in some sense
works optimally and let the
recruitment of individual
muscles depend on an
optimality criterion. With 1000
muscles in the system, this
creates an optimization Figure 2. Statical indeterminacy. This model accomplishes
problem with 1000 variables flexion of the elbow by synergistic actions of a large number of mus-
that must be solved for each cles.
time step of the analysis.
3
A typical AnyBody model can be divided into three parts as illustrated in Figure 3:
1. The model of the human body
2. The model of the environment, such as a chair, a bicycle or a piece of sports equipment
3. The connection between the model and the environment.
Of these three parts the human body is by far the more complicated, and having to develop human
body models from scratch would be prohibitive for the use of the technology by mainstream users.
Therefore, models of the human body are being developed in a coordinated, international, scientific
effort and placed in a public domain library from where single body parts, popular collections of body
parts or entire bodies can be imported into user models.
Models in the model repository are scalable to different body proportions as shown in Figure 4. The
scaling is based on input of length and mass of each segment individually. This enables the models to
scale to proportions of individuals such as a particular athlete as well as cross-sectional proportions
over a population.
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AnyScript derives a number of advantages from the
fact that it is stored in clear text format:
1. Models are very compact and easy to ex-
change between users.
2. “The script is the model”, meaning that
there is nothing in an AnyBody model that
cannot be seen in the script. Unlike many
other software systems, AnyBody does not
maintain an underlying data structure hid-
den from the user. If there is a problem with
a model it can be found in the AnyScript
code.
3. The fact that the AnyScript model is the
complete and unambiguous description of a
model allows scientists and users to scruti-
nize models and contribute to the im-
provement of their fidelity.
Figure 5. Typical section of an AnyScript
4. Typical applications model.
Applications of this type of musculoskeletal modeling typically fall into the categories represented by
figures 4 through 7.
Applications in product design are in fields where the comfort or operability of a product depends on its
ergonomic qualities. Typical examples are seating, accessibility, pedals and levers and their con-
nected mechanisms. In orthopedics and rehabilitation the applications are in any sort of medical de-
vice and rehabilitation technology: joint prosthetics development, gait analysis, development of or-
thoses, wheelchair configuration, investigation of cruciate ligament ruptures and range-of-motion in-
vestigations. Occupational health applications are found in any situation of manual materials handling,
pushing, pulling, lifting and workplace configuration for repetitive work processes. Sport applications
are investigation and development of techniques for specific sports performances and in strength
training for performance enhancement.
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Figure 7. Applications in orthopedics and re-
Figure 6. Applications in product design. habilitation.
5. Conclusions
Musculoskeletal simulation is a new and promising field of Computer-Aided Engineering and it repre-
sents an important opportunity to expand virtual prototyping into new fields of applications. It has the
potential to revolutionize the design of products in contact with the human body in addition to many
medical treatments and rehabilitation.
6. References
[1] Michael Damsgaard, John Rasmussen, Søren Tørholm Christensen, Egidijus Surma, and Mark
de Zee: „Analysis of musculoskeletal systems in the AnyBody Modeling System“. Simulation
Modelling Practice and Theory. 14(8), 1100-1111, 2006.