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Sounding out human motion

Although standing upright and moving forward may appear to be simple, they are in fact complex activities that are not yet understood in detail. Even less is known about how we hop, run and jump. Researchers at the Stuttgart-based Institute of Sport and Movement Science are looking into the ways the musculoskeletal system interacts to create movements as well as studying the effect of disorders. Computer simulation is one of the principal tools being used by the researchers to investigate human motion.

Dr. Syn Schmitt studied physics and sport science. He is now head of the Computer Simulation of Human Movement research group at the University of Stuttgart. © private

Try standing still without moving. However hard you try, you never quite manage it. Dr. Syn Schmitt and his research partners have found out that people tend to sway back and forth at a frequency of 0.4 to 1.2 Hz even though they appear to be standing completely still. Schmitt is head of the "Computer Simulation of Human Movement" research group at the Institute of Sport and Movement Science at the University of Stuttgart. Amongst other things, he is very keen to find out more about a person's ability to stand upright.

Human beings have to make slight oscillating movements in order to ensure that their musculoskeletal systems continue to function properly. "If somebody stood rigidly upright, it would create muscular strain that would quickly lead to overstraining and cramp," explains Schmitt. Scientists have known for several decades that people make small oscillating movements when they are standing upright. The prevailing school of thought in the field of biomechanics holds that humans are like inverse pendulums compared to a normal pendulum that has the mounting suspension at the top. In contrast, the point of oscillation of human beings is around the bottom of the body, i.e. the ankle. "For 15 years, it has largely been assumed that pendular movements of the human body are also regulated through other joints, principally the hip joint. We have now been able to confirm this assumption," said Schmitt highlighting that their findings have shown that human beings are "inverse double pendulums".

From individual pendulums to double pendulums – what next?

Confirming this assumption was far from simple. The researchers started off with time-consuming and complex analyses of the movement moments in the joints. “When people were standing in an upright position, we found that people’s hips swayed back and forth in a movement that was generated by the hips themselves,” said Schmitt. The single pendulum model alone did not fully explain this observation. The researchers therefore developed an equation system for double pendulums. Important work in this field was carried out by Dr. Michael Günther at the University of Jena, who is now part of the Stuttgart team. Everything then fell into place: Based on a double pendulum, the researchers were able to simulate how people stand upright. This led to further discoveries. “The simulations provided us with detailed information about how humans stand upright,” said Schmitt.

The schematic shows the attachment points of the muscles; the forces between these attachment points were then calculated. © Schmitt, Inspo University of Stuttgart

The results are not only of huge interest in terms of basic research, but they also have a practical benefit in such areas as equilibrium training. Training of this kind used to concentrate mainly on strengthening the ankle, but the new findings of the Stuttgart researchers now also enable specific training to be directed at the hip joint. The researchers also envisage that their findings will contribute to the diagnosis and treatment of movement disorders associated with certain diseases. For example, the scientists know that the upright position of Parkinson's patients is generated differently from that of healthy people. "The oscillations have a different frequency. A reason for this might be that Parkinson's patients tend to tense their ankle or hip muscles more," said Schmitt. He also has an idea for a new project in which he wants to elucidate and compare the moment fluctuations in joints. However, at present he has no money or staff for the proposed project.

Deciphering ankle function

Schmitt is also working on the ankle in another, strongly interdisciplinary project. Working with Prof. Dr. Wilfried Alt and his team at the Institute of Sports at the University of Stuttgart, Schmitt is investigating the inverse dynamics of ankles. One of his major goals is to create a basis for innovative prostheses, which will then be further developed by other cooperation partners, including Dr. Urs Schneider and his team at the Stuttgart-based Fraunhofer Institute of Manufacturing Engineering and Automation (IPA): At present, the only prostheses available are relatively rigid because little is known about how the lower ankle axis is functionally coupled with the upper axis. “We also want to understand why ankles have two axes,” said Schmitt.

Dr. Wilfried Alt has already carried out important preliminary investigations and has characterised the location of the ankle axes. “We assume that the location of ankle axes is crucial for a rolling walking action. Our group will now calculate the forces acting on the ankle axes and simulate the joint moments that arise during different movements such as running and jumping as well as the joint moments arising when people wear different types of shoes or run on different surfaces,” said Schmitt. The results of these investigations could prove to be of great interest for prostheses manufacturers as well as sports equipment manufacturers.

From virtual to artificial muscles

Computer simulation of a person in an immobile, upright position – a project carried out in cooperation with Dr. Michael Günther (University of Jena) and Biomotion Solutions GbR, Tübingen © Gernot Zech, Weil der Stadt

Schmitt's work on the development of an artificial muscle is also of great interdisciplinary importance. He is working in cooperation with Prof. Dr. Reinhard Blickhahn and Dr. Michael Günther from the Department of Movement Science at the University of Jena, with whom he has already filed a patent. "We are measuring, analysing and simulating the movement of human muscles in order to develop drive systems that function like human muscles, and use about the same amount of energy as human muscles," said Schmitt. This project might also contribute to the development of innovative prostheses.

Muscles, or more precisely the interaction of muscles, tendons, ligaments, bones and cartilage, are the subject of another cooperative project in which Schmitt is working closely with Prof. Dr. Wolfgang Ehlers and Jun.-Prof. Oliver Röhre from the Institute of Mechanics at the University of Stuttgart. The project is part of the SimTech excellence cluster. "We are working closely together in the development of a multiple body simulation of the human spinal cord. The particular challenge is to adapt our simulation methods so that they can work together," said Schmitt. Ehlers and Röhre are calculating the course of muscular forces using continuum mechanics methods while Schmitt and his team are looking at muscles as if they were threads running from one point to another, and the team is then measuring the forces acting between them. The common objective of the two different simulations is to create one and the same behaviour pattern. "We hope to obtain a detailed understanding of the conditions involved in the movement of the human spine in order to create an optimal testing environment," said Schmitt going on to point out one of the medical benefits of this project: "We hope to use these simulations to optimise spinal disc implants."


Further Information:
University of Stuttgart
Institute of Sport and Movement Science
Dr. Syn Schmitt
Allmandring 28
70569 Stuttgart
Phone: +49-0711 685-60484
Mail: schmitt[at]inspo.uni-stuttgart.de
Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/sounding-out-human-motion