Music teachers produce carry out pedagogical content knowledge (PCK) in the field they are experts. So do Design teachers. Music and Design education are singular art disciplines. The knowledge, contents and pedagogy produced by Music and Design teachers mostly come from music and design professional experts or practitioners (Terrien, 2015; Moineau, 2015). Despite their expertise, teachers often experience difficulties when teaching these Music and Design subjects. Reasons have little to do with problems of resources, competence or expertise, but rather with the representations teachers have of the knowledge they need to work; and these prevent them from engaging in effective training activity. This research focuses on how, when and why Music and Design teachers’ representations of teaching knowledge and of pedagogical situation may be impediments to learn. How does knowledge migrate from music and design work to Music and Design pedagogical situations? How are skills and capabilities transferred from professional practice to learning situations? What is the influence of specific values and beliefs music teachers apply to professional practice? What is the influence of scientific and specific knowledge of design professional activity on design teachers? The assumption is that teachers’ practical epistemology prevents them from teaching efficiently and hinders the students’ actions. In other words, teachers’ beliefs on knowledge, on pedagogical situations and devices, block student action. This presentation tries to highlight how teachers in arts education generally project and practice their teaching despite practical epistemology (Östman & Wickman, 2014) and, somehow, epistemological obstacles (Bachelard, 2002). Practical epistemology implies that teachers develop get values and beliefs that can blur the knowledge discourse (Gusewell, Joliat, & Terrien, 2017). Values may be the significance of arts in a social and democratic education: teachers believe artistic disciplines are an effective way to learn the citizen issues. Beliefs can be as well the necessity of art practice as a way to eliminate epistemological obstacles. On the one hand, teachers think there is a strong link between art practice and soft skills in order to develop social capabilities among students. On the other hand, they do not identify scientific knowledge that underlies the knowledge to be taught. Teachers’ values and beliefs powerfully influence the creation of pedagogical situations and the appropriation of experiences. Values and beliefs also initiate PCK as well as scientific knowledge. They transform both training activity and creative gestures teachers do within pedagogical sequences. The current theoretical framework underlines didactics and cognitive psychology to highlight teacher activity (Terrien, 2015; Bonnardel & Didier, 2016). PCK literature emphasises the possible absence of resources (lack of time, equipment and staff), competence and experience (Hultén & Björkholm, 2016). This literature deals with arts and technology teaching and learning. It merely tries to focus on Music and Design learning and teaching despite the lack of epistemological research in this field (Terrien, 2015). Teachers mobilise a kind of epistemology that may be observed and analysed through the ways teachers act or experience pedagogical situations (Dewey, 1934; Ingold, 2011). The plan of this presentation is as follows. First, this study identifies the main theoretical framework: didactics, PCK, practical epistemology and activity theory, especially within artistic education. In Music and Design education situations, knowledge and pedagogical content flow between teachers and learners. Hence, the research carries out a clinical activity methodology from ergonomics and work psychology (Clot, 2009). Some displayed analyses enhance main outcomes: teachers’ beliefs prevent students’ action within training situations. A discussion questions the differences between technical and practical epistemology: how the teaching- learning situations modify the pedagogical content knowledge of both teachers and learners (de Oliveira, 2017; Simons, 2017). Methodology, Methods, Research Instruments or Sources Used This research is an adapted clinical activity methodology from a field of ergonomics and occupational psychology (Engeström, 2018). This so-called “self-confrontation” method (Kloetzer, 2018) allows making up a cognitive and semiotic analysis. Videos help to analyse teachers’ activity and students’ actions with both intrinsic and extrinsic analyses through recorded learning and training situations and interviews as dialogues. The focus is on the teacher’s gesture. First, it involves video recording of pedagogical situations (teacher and students in a classroom during an hour-long music or design lesson with a stable camera). Second, teachers watch their videos alone in order to shed a light on specific gestures. Researchers’ requirements also refer to specific moments teachers want to extract and to discuss because they are interested in them. Therefore, they choose two or three excerpts (with recording times of two to three minutes each). Teachers watch and comment on the videos for researchers to highlight their choices and acts. The videotaped self-confrontations provide new-collected data: teachers’ discourses on their own activity highlight this activity. Based on a written transcription of all dialogues, qualitative and quantitative analysed data provide some cross results with a grid for both data collection and data processing. The researchers previously analyse selected videos to identify and outline some significant outcomes and dialogues with four teachers: two Design preservice teachers in applied arts disciplines matter (vocational high school) and two Music expert teachers (elementary school orchestra). In the case of the A Design teacher, a photographic point of view is used to explain how to draw a kitchen plan. Design teacher B uses an approximate definition of the parallels in order to explain the vanishing lines on a conical perspective: students have to find the definition on their own and use their knowledge as students. In the design teaching cases, analyses of collected drawings made by students (Brösamle & Hölscher, 2018) reveal the initial design decisions of students. This method can also link them to priorities and the resulting design solutions. Music teacher A takes the place of a student and puts her fingers on the flute because she cannot find a way to show him otherwise. Music teacher B acknowledges that he does not know the influence of the pedagogical device on learning. Conclusions, Expected Outcomes or Findings This study tries to find indicators that allow understanding the foundations of representations on teaching knowledge. Depending to the complexity of the teaching situation and on these representations, teachers and students have trouble making connections with a body of knowledge contained in the activity at hand. Three levels of the influence of such representations on teaching appear quite clearly like a “palimpsest” (Terrien, 2017) in which the relationship to knowledge evolves. The first level corresponds to the absence of distance taking. Design preservice teachers get a bad representation of its geometrical knowledge and its scientific rules: there is no distance from the taught subject. Therefore, they are “tinkering”, using bad tools, increasing bad training. This is some kind of epistemological obstacle. At the second level, the teacher takes the place of the students. One of the Music teachers does not let them learn by themselves, taking hold of the instruments and making the gestures herself. The third level is the one in which the teacher has a misrepresentation of the influence of the intended pedagogical device. The teacher does not see interactions between learners even though the pedagogical situations he produced helps to encourage them. This study aims to show how practical epistemology blocks real work on technical epistemology. The assumption is that understanding such an impediment fosters efficiency in pedagogical situations. Practical epistemology leads teachers to be directive preventing students’ actions and, in return, leading teachers to use representations on didactical framework. Besides, this paper allows thinking the link between both technical epistemology and practical epistemology. Because technical epistemology shapes the pedagogical situation, it builds knowledge, i.e. the teacher’s epistemology. The perspective of this research is to observe other teaching situations in order to better understand and characterise the phenomena involved in technical epistemology.