This research focuses on the evolution of primary school teachers' practices engaged in a training and research process in mathematics, lesson study. The study deals with the following question: what practices will change and what resistance in practices will be observed? The practices are analysed in a double didactical and ergonomical approach theoretical framework. In lesson study process, a group of teachers and facilitators prepare a lesson and then one of the teachers teaches it in his-her class. By appropriating it, he-she creates modifications between the collective preparation (prescribed task) and the lesson (conducted task). The activity of the teacher is analysed as a process of modifications of the prescribed task. This local analysis is complemented by a categorisation of i-genre and a component analysis of practices. This research highlights changes in practices before, during the class time or more indirectly as a formative practitioner, but also some resistances.