Physical (mechanical) models for sports equipment research, development and testing

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,’’ he developed a mathematical description of human muscle contraction, buildingup a well-reflected system of side-long equations, which not only described the biochemical process of bridgebuilding 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. 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 them 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. 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 present 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