INTRODUCTION: Training loads and methods should be adapted to swimmer`s age and performance level. International age-group records times of 50m freestyle showed a progressive improvement in performance of this event. This event could be used as an evaluating tool of the race components (RC). RC should be modified with the age, growth and physical conditionig development. Some forms of regression analysis have been applied to RC studies during international competitions (Absaliamov & Timakovoy, 1990; Nomura, 2006), but this kind of analysis has not been applied to age-group swimmers. Our research purpose was to estimate and use a sophisticated statistical model of the kinematic characteristic, stroke rate (SR), stroke length (SL) and stroke index (SI) performance evolution during the age-group development based on a 50m freestyle test. METHODS: 280 regional age group swimmers (162 males and118 females) participated in the study (age range: 9 - 22 years). Detailed swimming competition video analysis procedure was applied to obtain the RC. Regression analysis was used to discover the tendency and model of the 50m times, SR, SL and SI. Inverse function approximation of the 50 m time by age and gender was carried out. Quadratic function approximation of the SL and SI by aging was carried out. Lineal function by aging was defined for ST. RESULTS: Different equations were obtained for gender, 50m times, SR, SL and SI. Significant differences between genders in were obtained. 50 m times were different between genders. There is a tendency to improve the parameters SL and SI with age in both genders. SR don´t get a clear trend and has an irregular behaviour. DISCUSSION: Kinematics components SL and SI change with the development of many physical fitness factors. With growth, the 50m times of the swimmers studied have a tendency to improve with the development. SR, SL and SI have the same generic equation for boys and girls but there are two different models according to gender. The inverse function by age and gender was the better approximation carried out in this training test of 50m freestyle. SR don´t get a clear trend.
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