It is not yet fully understood in detail how the nervous system, muscle cells and entire muscle groups interact in motion sequences. Three-dimensional simulations of the skeletal muscles of the type being researched by biomechanics at the University of Stuttgart, are set to change that and help with the development of control systems for exoskeletal neuro-prosthetics.
Reaching for a cup of coffee in the morning looks so easy; most people even manage to do it whilst still half-asleep. Yet, what has to happen in the human body just for this movement alone is by no means trivial. For the hand to reach for the cup, the brain has to initiate the sequence in the spinal nerve cells. The electrical signals then reach the so-called neuromuscular junctions via the nerve fibers, which transfer the signal to the muscle fibers within the skeletal muscles, which are responsible for voluntary movements. To this end, the electrical signal is transformed into a mechanical force. Calcium ions serve as a messenger substance, which causes certain cells to contract. The mechanical force generated at the cellular level is transferred via the muscles to the tendons resulting in the arm moving towards the cup. Then the hand has to close around it with a carefully controlled pressure before the cup is moved towards the mouth. Many muscle groups are involved in this simple sequence, controlled by countless nerve cells. Just how complex this pattern of movements actually is always becomes especially clear when it is impaired as the result of an illness or accident or when engineers attempt to train a robot to perform human-like movements.
Neuromuscular question mark
Every movement originates in physical and biochemical processes within the cells, which have not yet been entirely researched in terms of their relationship to entire musculoskeletal groups and their interactions with the central nervous system. Oliver Röhrle, Professor of Continuum Biomechanics and Mechanical Biology at the University of Stuttgart, wants to change that with and a team of experts from various disciplines. “Our interdisciplinary approach”, says Röhrle, “is based on a holistic understanding of the neuromuscular system”. Simulations are their chosen tool for this, which is why computer scientists, mathematicians and visualization specialist are on board. Sports scientists, electrical engineers, biologists and physiologists provide the bridge to application-based issues. Ultimately, Röhrle explains, their work addresses the question: “how is movement generated?”.
“The total electrical potentials of such movements can be measured using electromyography, a type of ECG, for the muscle, for example on the surface of the arm or leg”, the mathematician explains. “But this only provides us with extremely noisy signals and it is difficult for us to draw conclusions about the individual muscular processes from them”. It is precisely on the upper thigh that electromyography reaches its technical limits because it can only measure down to depths of one to two centimeters – but the musculature only starts to become really interesting below that level. “We want to use realistic simulations to get a much deeper understanding of the movements and electrical potentials to deliver results that our colleagues can then validate”, says Röhrle.
Neuronal control in the model
His research group uses three-dimensional skeletal muscle models for their calculations and bases their simulations on the principle of: “activation in, movement out”. These models are extremely detailed, thus they take account of a large number of muscle fibers and their neuronal control systems. “Among the probably 20 research groups around the world who model skeletal muscles in three dimensions, we're the only ones who approach it in this way”, the scientist explains. It is still fundamental research at this stage, but simulations used in various areas, such as the sports sciences, will benefit from it in future. But the results could also be useful for achieving as natural a connection for protheses as possible or for crash tests.
One example of a research project that is already much closer to application maturity is known as KONSENS NHE, which, in addition to Röhrle's team, involves the university hospital and the University of Tübingen as well as Reutlingen University. The objective of the project, which began in 2017 and is scheduled to go on for three years, is to produce an exoskeleton for the hand controlled via the nervous system, which is suitable for everyday use. “In terms of the development of this orthosis, we're thinking of stroke victims, who often suffer from limited limb mobility, either long-term or short-term”, Dr. Leonardo Gizzi, who is responsible for the project within Röhrle’s team. The orthosis will ensure that stroke victim are able to grip things firmly enough and move their hand without restrictions. A prototype model of a brain controlled exoskeleton for a hand developed by an international team under the auspices of the University of Tübingen served as the starting point, which successfully restored the function of the hand in paraplegics almost completely. However, this exoskeleton was not portable and it could only be used with the aid of trained personnel. That's why the objective of the current project is to produce an exoskeleton that is fit for everyday use. If a stroke patient is suffering from a unilateral paralysis, for example, he or she should still be able to put it on themselves.
Communications between the patient and the orthosis
To be able to reach for something accurately, the exoskeleton should ideally be controlled by measured brain waves combined with eye-movements and a three-dimensional object recognition ability. “We use electromyographic electrodes for patients whose hand muscles are still active, but cannot exert enough force to be able to grip things securely”, Gizzi explains. The voltage that occurs naturally within the muscle can be measured using this type of electrode. These signals are also, as it were, the direct connection to the research work being carried out in Röhrle’s team on the simulations of skeletal muscle movements. “However, the orthosis doesn't only receive control signals from the patient but also provides him or her with haptic feedback via vibra-
Hardware and the electronic control system are currently being created by the project team: Gizzi is responsible for the layout of the electromyographic electrodes on the forearm. “We're trying to find the optimum layout with as few sensors as possible”, says the scientist. This will be followed be comprehensive functional testing, initially with healthy test subjects. The real work will begin as soon as the project participants reach the stage where they can attach the orthosis to a patient for trials: “That will be a crucial phase, because, ultimately, only those affected can tell us how using the orthosis feels to them”, Gizzi explains. “How it looks, the weight, operating it – all of that will play into it and may well differ from our expectations”. After that it will no longer be about technology and functionality, as Gizzi explains citing prosthetics as an example. “Experience there has shown that, whilst older people want an artificial replacement limb that looks as natural as possible, a prosthesis can't look robot-like and technical enough for children”.