The aim is to analyse the reasons for practising extracurricular physical activity in secondary school students, in order to obtain useful information for creating more motivating and attractive programmes for them, favouring their adherence to physical activity and the maintenance of healthy habits. For this purpose, the Motivation Self-Report for the Practice of Physical Exercise, Spanish version adapted by Capdevila, Niñerola and Pintanel (2004), with a sample of 97 subjects, was used as an instrument. After analysis, the results show that the factors of prevention and positive health, well-being and fun, and muscular strength and endurance are the most valued, regardless of gender, course or whether they are active or not, while social recognition and health emergencies are the least valued factors in all groups. As far as comparisons are concerned, there are differences according to gender in the competition factor, while according to the course the students of 1º E.S.O. show a significantly greater interest than the rest in the variable of weight and body image. As for the practice of physical activity, those who carry it out value more the factors of fun and well-being, competition, muscular strength and endurance and challenge.