History,philosophy,and value of mechanical models in sports science and engineering

The role of mechanical models for knowledge gain

One of the finest scientists in the field of (sports) biomechanics, Professor Herbert Hatze who died in 2002 much too young at the of age 65, was a mathematician. In his famous manuscript “Myocybernetic Control Models of Skeletal Muscle,” ¹ he developed a mathematical description of human muscle contraction, building-up a well-reflected system of side-long equations, which not only described the biochemical process of bridge-building in the muscle cells, but also considered the specific anatomical structures of different muscle types. He was very familiar with mathematics and certainly convinced of its power to increase our knowledge and understanding of the real world. Therefore, it is hard to believe that in their research to understand the tennis stroke, he and his team developed, manufactured, and used a mechanical replicate of the human arm. This device is shown in Figure 1.

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Figure 1.

Mechanical model of the human arm and hand for testing tennis rackets.

Hatze called this artificial arm for testing tennis rackets “Manu-Simulator,” and I was lucky to talk to him in person, discussing the advantages of this device. For him, the major benefit was not only the possibility to standardize boundary conditions, but even more so, the option to systematically redefine the boundary conditions, both accurately and precisely. He dismissed counter arguments of limited external validity of his mechanical model and passionately criticized tennis racket tests, even with experienced tennis players, because of their inherent high variability and unidentified confounding variables raised by human material.

In his journey to identify the best approach, he finally used both mathematical and mechanical models, combining them with athlete experiments in the field and in the lab. As a result, he was able to derive valuable insight into the few milliseconds prior to and after ball impact, showing the relationship between grip strength, transferred vibration to the hand-arm-system, and oscillation-damping characteristics of tennis rackets.^2,3

The motives for a special section on mechanical models

Despite the power of combining mathematical and mechanical models, this special section concentrates only on mechanical (physical) models. Why? One reason is that the application of mechanical models is so widespread. They are present in sport, exercise, and training science, as well as in the daily practice of many sports. The second motivation for focusing on mechanical models is the wide variety between simplicity and amazing complexity, raising the exciting question of how much complexity is needed and where simple models meet their limits. Lastly, a little bit of a secret reason is that mechanical/physical models for the application in sport is a wonderful playground for people who call themselves engineers. Designing and realizing these models bares enough challenge and requires all the knowledge we have learned in mechanics, thermodynamics, aerodynamics, material science, and product design. We no longer need to limit our engineering skills for designing transmissions, turbines or tooling machines. Instead, we are allowed to apply these models to one of the most passionate areas – the field of sports. Thus, developing mechanical models for the application in sport could be one major reason why we call ourselves “sports engineers.”

Categories of mechanical models

The importance of mechanical models in sports, sports engineering and sports injury prevention becomes more clear, when trying to categorize them based on their purpose and application. The following open list should be seen as a proposal for such kind of categorization – other structuring however might also be appropriate:

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Figure 2.

Passive mechanical model of a skier, able to “automatically” carve down an inclined plane. This model helped to explain skiing physics and how the shaped ski initiates and controls turns.

What is the scope of this special section?

The idea behind this special section is to focus on the methodological aspects in the development and use of MechM, intended to gain knowledge in the field of sports, sports science, and engineering. This objective goes further than just describing the design or function of a certain apparatus. Instead, we want to illustrate how researchers have solved the challenge of finding the best compromise between needed complexity and necessary simplification. By this, the reader should be able to make better decisions (e.g. when having to select the appropriate material to best replicate certain human tissue behavior) or identify the advantages and disadvantages of MechM by understanding their major limitations. Finally, they should be aware of the challenges to conduct an acceptable validation process, which sometimes is impossible to achieve fully.

Two examples

Overview of papers included

We are fortunate to have a distinguished list of contributing authors, each an expert in a specific area of physical models.

Two papers address the challenging task of developing a valid representation of the human head, which is suitable for conducting realistic impact tests. These papers include Stone et al.’s Development of a representative headform for the assessment of potentially destructive head impacts and Gilchrist et al.’s Physical Properties of Biofidelic Headforms for Accurately Simulating Oblique Impacts.

Even though sport surfaces at first sight may not be considered a MechM, artificial turf definitely is. Replacing the natural grass, completely or to a certain extent, by some types of polymers while still having to provide similar mechanical properties as the original material is difficult. As players and clubs expect the perfect playing surface over longer periods of time, the contribution of Fleming et al. titled A New Model of Third Generation Artificial Turf Degradation, Maintenance Interventions and Benefits looks at the aspect of maintenance.

Coming from the playing surface to footwear, the work by Havenith et al. deals with the thermo-physics inside the shoe. Their contribution A thermal foot manikin as a tool for footwear evaluation and development will certainly be helpful to footwear developers to study and improve foot climate, and thus, reduce discomfort.

The study by Mocera et al. titled Nordic Walking Multibody analysis and experimental identification presents a methodology for the analysis of body motion during Nordic Walking by designing a numerical tool to replace human body behavior. This approach allows for studying several biomechanical aspects, including the kinematics of each body segment, while estimating the loads applied to joints for given tasks.

In the study by Love and Shannon titled A Comparison Study of Athletic Kinesio-tapes and Structural Cloth Tapes Applied to Address Biomechanical Injuries and Muscle Stabilization, the authors acknowledge the widespread use of tapes and wanted to assess distinctions between them from a material perspective. Their study focuses on measuring mechanical properties and modeling their mechanical response.

We hope this collection will motivate some of the readers to enter this exciting field of sports engineering. For those who are already working in this field, this Special Section hopefully delivers some valuable inspiration for ongoing work.

I want to thank Michael Caine for helping to guest edit this Special Section. I would also like to express my deepest thanks to all contributing authors and especially to all the involved reviewers. Special thanks to Susan Pryputniewicz and km. Ranjana for continuing to follow-up on manuscripts and James Sherwood, JSET Associate Editor, for allowing this Special Section to become a reality.