AnyBody: CAE for the human body John Rasmussen Department of Mechanical Engineering, Aalborg University, Denmark

Summary This paper introduces the AnyBody Modeling System and its capability to extend the field of CAE applications 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.

ANSYS Conference & 25th CADFEM Users’ Meeting 2007 November 21-23, 2007 Congress Center Dresden, Germany

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Introduction

Technologies such as Computer-Aided Design, Finite Element Methods, and Computational Fluid Dynamics 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 mechanical 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 industry 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. On the other hand, most of us are completely encapsulated in man-made environments for the better part of our lives, and most of the products in these environments have some sort of interface with the human body: the chairs we sit in, the computer keyboards we type at, the coffee cups we drink from, the handles on the doors, the telephone, the stapler, the seat, the steering wheels and gear shifts of our cars, the joint implants we may have received to cure arthritic pain; the list goes on indefinitely. From this point-of-view it would appear that ergonomics indeed is a very important product property, and that the ability to simulate it holds a very large potential. We shall initially introduce the AnyBody Modeling System [1]. Subsequently we shall present a number of typical applications of this new technology. 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 in the human body depends on the point-of-view. The Figure 1. A complex musculoskeletraditional anatomical classification of muscles is not tal model comprising more than 500 musadequate mechanically because many of the anatomical cles. 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 muscles 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 introduces a contact problem into the mechanical modeling, and it causes a significant complication of the mathematics and the numerical procedures.

ANSYS Conference & 25th CADFEM Users’ Meeting 2007 November 21-23, 2007 Congress Center Dresden, Germany

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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 problem with 1000 variables that must be solved for each time step of the analysis. 3.

Figure 2. Statical indeterminacy. This model accomplishes flexion of the elbow by synergistic actions of a large number of muscles.

The AnyBody Modeling System

The AnyBody Modeling System is computer software developed to handle the complications mentioned above and to fill the need for CAE-type musculoskeletal analysis or computer-aided ergonomics. 3.1

The AnyBody Model Repository

AnyBody is a modeling system because its purpose is to allow the user to model the wide variety of mechanical boundary conditions we subject the body to and the variety of body proportions and capabilities of human beings. Indeed, the system is not limited to humans but allows for modeling of any living species constructed with muscles attached to a skeleton.

Figure 3. Elements of an AnyBody Model.

ANSYS Conference & 25th CADFEM Users’ Meeting 2007 November 21-23, 2007 Congress Center Dresden, Germany

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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.

Figure 4. Scaling of body models from 5th percentile female to 95th percentile male. 3.2

The AnyScript language

The availability of the model repository leaves the user with the task of developing the environment and its connection to the human body model. Modeling in AnyBody is accomplished by means of a scripting language called AnyScript. The language is object-oriented and declarative, where the latter means that the language does not invite to writing conditional statements, for-to loops and other usual programmatic constructs. Instead, the purpose of the language is merely to allow the user to declare the presence of the different elements such as segments, joints, muscles, movement, etc. Subsequently the system automatically works out the dependency between the elements, forms a mechanical system and works out the necessary mathematics. This makes the language very powerful allowing for definition of very complex models in few statements. Figure 5 shows a typical piece of AnyScript code and it appears that the syntax is very similar to C++, Java or JavaScript. AnyScript shares the feature with the latter that it does not allow the user to define new classes. The user can merely define instances of existing classes.

ANSYS Conference & 25th CADFEM Users’ Meeting 2007 November 21-23, 2007 Congress Center Dresden, Germany

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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 exchange 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 hidden 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 scrutinize models and contribute to the improvement of their fidelity. 4.

Figure 5. Typical section of an AnyScript model.

Typical applications

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 connected mechanisms. In orthopedics and rehabilitation the applications are in any sort of medical device and rehabilitation technology: joint prosthetics development, gait analysis, development of orthoses, wheelchair configuration, investigation of cruciate ligament ruptures and range-of-motion investigations. 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.

ANSYS Conference & 25th CADFEM Users’ Meeting 2007 November 21-23, 2007 Congress Center Dresden, Germany

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Figure 7. Applications in orthopedics and rehabilitation.

Figure 6. Applications in product design.

Figure 8. Applications in occupational health and workplace design. Figure 9. Applications in sports.

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Conclusions

Musculoskeletal simulation is a new and promising field of Computer-Aided Engineering and it represents 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.

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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.

ANSYS Conference & 25th CADFEM Users’ Meeting 2007 November 21-23, 2007 Congress Center Dresden, Germany

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