Highly skilled swimming is characterized by an extraordinarily high frequency of events. Within a second in time the swimmers must solve the difficult task of coordinating the motions of their body segments, for example, when swimming the crawl: (a) the pull-push phase of the right arm with the pull-push phase of the left arm; (b) the rolling motions pivoting on the longitudinal axis with the pull-push phase, the recovery phase, and breathing; (c) arm strokes with leg kicks; and (d) head movements when the swimmer inhales and exhales. Moreover, during different movements (e.g. during a pull-push phase), the geometric and dynamic movement pattern changes continually. The high frequency of events and the dynamic and geometric variations make subjective evaluation difficult even for the most observing and experienced coach, unless he or she has a method for obtaining objective data to support subjective conclusions. These data are achievable by quantitative measuring and scanning consistent with a theory-based conceptional model (Barthels, 1979; Brown & Counsilman, 1970; Schleihauf, 1977, 1979). In this way, the motions can be evaluated both qualitatively and quantitatively. Being norm-oriented, the evaluation of the movements involves a comparison between the factual values, or actual motions, and the "statistical" or "ideal" norms (Ballreich, 1976; Letzelter, 1985). The diagnosis of the performance levels must consider two elements: the ability of the swimmer and his or her developed skills. Although an evaluation of abilities can furnish useful knowledge for orientation and training, this study was limited to the evaluation of skills. The evaluation of swimming motions is based on a number of criteria, videorecordings (slow-motion and frame advance), and quantitative biomechanical parameters. Velocity-time and pressure-time are recorded, graphed, and measured via biomechanical methods that will be described later. The horizontal intracyclic velocities are gauged by means of an impulse light method and electrical resistance variation method; the pressure-time graphs are recorded by the difference-pressure method. Using the difference-pressure method, it was possible to measure the amount of time-dependent resultant of lift and drag. The different biomechanical methods are synchronized by a sensor-video personal computer (PC) unit in order to evaluate swimming skills qualitatively and quantitatively. This evaluation system offers the advantage of delivering objective and subjective knowledge of results simultaneously to the trainer and the swimmer.
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