The importance of tracking (stability and prediction) in longitunal designs: an applied study to the epidemiology of physical activity and sports performance This study aims at the presentation of the tracking phenomena in epidemiological and motor performance research. The problem of tracking, its essence and importance were presented based on two sets of data. The first one was related to physical activity (i.e., sports practise) of 588 youngsters followed longitudinally for 6 years, starting at the age of 12.7 years and ending at 17.7 years. A quasi-simplex model was used, and showed the high stability of sports practise, as well as the moderate to high tracking. In the second study, information came from the best performance of 12 long jumpers in six consecutive years. This data was analysed according to the concept of canalisation, using the procedures suggested by Cohen, and Foulkes and Davies. Results showed a low stability of performance over time, as well as its non-linear behaviour. In conclusion, three aspects deserved main attention: the first one related to the use of novel approaches to the study of tracking; the second one showed the high stability of sports practise, as well as the moderate to high tracking; the third one evidenced the low canalisation of the performance of the long jumpers due to its low stability.