Motivation

Ergonomics and biomechanics are are subjects of major socio-economic importance. The cost of musculo-skeletal injuries is rapidly increasing at the same time as the fundamental understanding of the mechanical function of the body leaves many open questions. Presently, the number of injuries caused by excessive use of computer mice is exploding, and yet the actual causes of many of these injuries remain a mystery, and it is therefore difficult to issue guidelines that will reduce the problems.

One of the best opportunities to further our basic understanding of the body's mechanical function is to create and verify mechanical models and then perform detailed studies of the behavior of these models. This requires mechanical, mathematical, and numerical expertise.

An attractive quality of such models is that they are equally well-suited to further the fundamental understanding of the body's function and to solve practical ergonomic problems. Typical examples of the latter are analysis and optimization of tools and workplaces and the design of sports equipment and hand tools for maximum efficiency.

How did it all begin?

Design of bicycles has been a passion at the Institute of Mechanical Engineering since the early nineties. The interest is originated in the institute's combination of activities in design optimization and advanced materials. In 1991-1992, a group of master's students, Anette Struve Nielsen, Torben Lindby, and Claus Jørgensen, designed and built a bicycle in carbon fiber and kevlar that was well ahead of its time.

The special thing about this bike was that it was about ten times as stiff as a conventional racing bike with the same weight. This was due to the combination of materials and optimized shape.

We went on to do other projects on bicycle optimization. Lightness and stiffness are conflicting criteria. The weight can be minimized with a specified stiffness or vice versa, but one will always limit the other. We gradually realized that it was not possible to determine the best combination of the two by looking at the bicycle alone. We would have to investigate the rider and bicycle as one system.

From an efficiency point-of-view, stiffness is important because the body spends metabolic power on elastic deformations of the frame when the driving force pulsates from one pedal to the other. Knowing very little about bio but quite a lot about mechanics, we started the development of mechanical models of muscles and joints. A group of students, Peter From, Thue 

Møller Jensen, and John Flint, did the initial investigations but were unable to fully solve the problems related to redundancy in the system. Later, two Spanish guest students from the excellent technical university of Bilbao, Miguel Escartin and Leire Ibanez, used a quadratic optimality criterion to identify muscle forces in the arm while sawing. They were not toally successful in getting reasonable results either.

The mechanical problems of muscle recruitment were starting to catch the interest of a number of people in the department. Multiple informal discussions between Michael Hansen, Michael Damsgaard, and John Rasmussen lead to the conclusion that muscle recruitment could well be controlled by a min/max criterion. Michael Damsgaard and John Rasmussen developed a test algorithm for bicycling and found that not only did it produce very reasonable results, it was also extremely numerically efficient--so much that modeling, simulation and optimization of larger and more complex body assemblies would definitely be possible. A couple of conference papers were presented and received so well that we decided to go on with the work and apply for external funding. In the meantime, Michael Voigt had joined the group, providing some badly needed physiological expertise and access to the excellent lab facilities if the Center for Sensory-Motor Interaction.