STRESS STUDIES IN TENNIS – DIFFERENCES BETWEEN TENNIS RACKETS OF DIFFERENT LENGTHS

INTRODUCTION: Recently several manufacturers of tennis rackets have been offering elongated rackets. The extension of these rackets (27 to 32 inches) ranges between 1 and 5 inches. Considering the biomechanical characteristics of handling and efficiency of shot of these "Long-Bodies" leads to the following presuppositions: 1. With equal body-segment-velocities the longer lever results in an increased velocity of the racket head. 2. For the realization of both equal bodysegment -velocity and an increased velocity of the racket head, the player requires more power within one shot cycle than is the case with standard racket lengths. 3. In case the muscle power per time unit which has to be used with standard racket lengths cannot be further increased, the timing must consequently be adapted so that the same body-segment-velocities can be realized. It can be assumed that not just positive results will appear when using "Long- Bodies". Regarding these presuppositions, in this study the following parameters in the use of tennis racket with different lengths will be investigated: first impact points on the tennis rackets, second impact points in the field, muscle actions during the strokes, accelerations of the tennis racket handles during the impact, velocities of the balls. METHODS: A better quality player had to play 30 shots with each racket length. The first impact points were recorded with a newly developed measuring device, the Treffpunkt-Analyzer. The EMG recordings were realized with a 25 Channel EMG recorder. To record the accelerations of the racket handles we used a threedimensional accelerometer. The determination of ball velocities was realized using a kinematographic procedure. RESULTS AND CONCLUSIONS: The investigation shows different results within the use of different racket lengths. The acceleration of the racket handle is more dependent on the stiffness of the racket than on the length. We measured 70g maximum acceleration along the vertical axis using the 32" racket and 102g by using the 27" racket. A clear increase in accelerations dependent on racket-length could not be found. The velocities of the balls are significantly different (Sig. 2- tailed: 101.6 km/h, 29" -> 107.6 km/h, 32" -> 107 km/h). The first impact points are significantly different (Sig. 2-tailed: .005). When using the 27" racket the first impact points are located closer (ca. 10 mm) to the racket head. Differences in the precision regarding the second impact points in the field could not be found. The EMG evaluation shows that players using longer rackets do not need more muscle activity (integral of the EMG signals) than when using the 27" racket. When using 27" rackets, significantly higher muscle activity could be found for several muscles

FORWARD DYNAMICS FOR THE EVALUATION OF PRACTICAL PROBLEMS IN SPORTS

INTRODUCTION: In athletic movements there are often situations where one cannot rate varying executions, because the effects of single actions are unknown. At a tennis stroke for example, the movement of the ball after hitting is well visible as an effect of the action. However, the conditions at hitting the ball and the actions that lead to the torque of hitting are not reliably visible. Their interpretation is only subjective. Nevertheless, the trainer and the player have to give statements of the muscular activity like "hold the racket loosely or firmly "or"relax or stiffen your wrist." This paper focuses on a controversial problem: the use of the wrist in tennis. Some favor a firm wrist, others an actively moving wrist. The group which favors the active wrist based their idea on the higher velocities of the racket head. For this idea biomechanical considerations are only based on kinematic data and on analysis in muscular physiology (see KLEINÖDER 1997, ELLIOT 1991, HUIJING 1994, KOMI 1994) and not on kinetic analysis. With this work we try to fill these gaps with computer simulation. In a similar way we worked on a problem in gymnastics: the increase of swings on the horizontal bar, which is necessary for all swing elements. Little work has been done in this area (see BAUER 1976, BÖHM1997 and WIEMANN 1993). Nevertheless, the research that allows a development of a general theory of the swing increase is lacking (except for the efforts of WIEMANN). The goal of this paper is to show that computer simulation can be a first step towards the development of such a theory

A functional approach to movement analysis and error identification in sports and physical education

In a hypothesis-and-theory paper, a functional approach to movement analysis in sports is introduced. In this approach, contrary to classical concepts, it is not anymore the ideal movement of elite athletes that is taken as a template for the movements produced by learners. Instead, movements are understood as the means to solve given tasks that in turn, are defined by to-be-achieved task goals. A functional analysis comprises the steps of (1) recognising constraints that define the functional structure, (2) identifying sub-actions that subserve the achievement of structure-dependent goals, (3) explicating modalities as specifics of the movement execution, and (4) assigning functions to actions, sub-actions and modalities. Regarding motor-control theory, a functional approach can be linked to a dynamical-system framework of behavioural shaping, to cognitive models of modular effect-related motor control as well as to explicit concepts of goal setting and goal achievement. Finally, it is shown that a functional approach is of particular help for sports practice in the context of structuring part practice, recognising functionally equivalent task solutions, finding innovative technique alternatives, distinguishing errors from style, and identifying root causes of movement errors