BIOMECHANICS OF SPORTS - SELECTED EXAMPLES OF SUCCESSFUL APPLICATIONS AND FUTURE PERSPECTIVES

The performance criteria of physical activities, especially those of sport disciplines, can usually be defined in biomechanico-mathematical terms. This implies that sufficiently complex models of the human neuromusculoskeletal system can be used for the simulation and analysis of sports motions and, at least in principle, for the biomechanical optimization of the performance in the various sport disciplines. It is frequently forgotten that biomechanical optimization, in the widest sense, is the ultimate goal of the majority of all endeavors in sports biomechanics, even if this may not always be obvious. Examples of the successful generation of biomechanical models include adequate models of the human skeletal and muscular subsystem, and the creation of a functional racket-hand-arm system model for simulating tennis strokes. Simultaneously, anthropometrico-computational and dynamometric methods were developed for determining respectively the subject-specific segmental and myodynamic parameter sets. The models and methods just mentioned will be illustrated during the oral presentation. As regards the practical applications of the biomechanical modelling approach to sports, some selected examples also to be presented are: the complete optimization of a kicking motion; the successful computer simulation and analysis of a rock ' n roll Betterini somersault in connection with an accident requiring a biomechanical expert opinion; the development of an objective biomechanical method for testing the quality criteria of tennis rackets; the quantification of the variability of repeated sports motions; and investigations into the validity and reliability of vertical jumping performance testing methods. Needless to say that appropriate biomechanical models of the human neuromusculoskeletal system are indispensable in theoretical studies such as the demonstration of the comparatively high insensivity of skeletal motions to neural control perturbations. Considering the current state of the art it would appear that contemporary biomechanics of sports is still too pre-occupied with measurement, data collection, and the subsequent phenomenological description of an observed event instead of asking the (much more difficult) question concerning the causes and fundamental mechanisms underlying the observed phenomenon. The mere measurement and description of the ground reaction forces during the release phase of the javelin throw, for instance, without relating their significance to the musculoskeletal factors that determine the throwing distance, is meaningless and constitutes a futile exercise. As a future trend in sport biomechanics, the utilization of models for performance optimization may be expected to gain increasing importance

Structural Determinants of Muscle Gearing During Dynamic Contractions

In skeletal muscle, interactions between contractile and connective tissue elements at multiple scales result in emergent properties that determine mechanical performance. One of these phenomena is architectural gearing, which is quantified as the ratio of muscle velocity to muscle fiber velocity. Many pennate muscles operate with a gear ratio greater than one because muscles shorten through a combination of muscle fiber shortening and fiber rotation. Within a muscle, gearing is variable across contractions. During low force contractions, muscles operate at high gear while muscles operate at low gear during high force contractions. This variable gearing has a significant impact on muscle performance as muscle architectural changes favor muscle speed during fast contractions and muscle force during slow, high force contractions. We hypothesize that gearing in any given contraction is determined by the dynamic interaction of fiber-generated forces, fluid force transmission, and the elastic behavior of intramuscular connective tissues. Because muscle is isovolumetric, muscle fibers must bulge radially when they shorten. Radial bulging and fiber-generated forces off-axis from the muscle line of action exert forces that load connective tissues that ensheath fibers, fascicles, and the whole muscle. The way in which fluid pressures and fiber forces interact to load connective tissues in three-dimensions remains poorly understood because of the complex and multiscale nature of these interactions. Here we review evidence for variable gearing in pennate muscles, present a conceptual model that describes the fundamental interactions that determine gearing, and discuss where gaps remain in our understanding of the determinants and consequences of muscle shape change and variable gearing