Background: Previous research on physical education (PE) teaching practice indicates that an exercise physiology discourse has assumed a dominant position within the field. Research shows that PE teachers are likely to emphasise physical fitness training in their teaching, and PE teachers seem to appreciate pupils who show high levels of physical exertion. Purposes: Our aim is to examine how vigorous activity/exercise is represented in the practice of PE teaching. We will also examine teaching as a discursive practice, and thereby contribute to a critical perspective on PE pedagogy. Research design: This study was conducted in four upper secondary schools in Oslo, Norway. Data material was produced through fieldwork, during which we observed 92 PE lessons. Additionally, we conducted qualitative interviews with the eight teachers who participated in the study. Our methodological framework was discourse analysis. Findings: Our material shows that vigorous activity plays a complex role in PE class: it can be beneficial, but it can also be punitive. The PE teachers we observed drew on an exercise physiology discourse to portray vigorous activity/exercise as beneficial and valuable to the promotion of pupils' physical fitness and health. However, the teachers also drew on a military discourse when assigning vigorous activity to rebuke a disobedient pupil. The teachers also introduced vigorous activity in the form of additional exercise "punishment," which they assigned to losers in competitive activities. In these instances, the teachers drew on exercise physiology and sports discourses. Thus, we identified how vigorous activity changed value according to context, and discuss how teachers' use of vigorous activity as punishment can seem paradoxical in a PE setting. Conclusion and recommendation: Our study indicates that, rather than adhering to modern educational practices, PE is rooted in ideas and practices derived from military, sports and exercise physiology discourses. PE teachers inculcated with these discourses have limited ability to discern the paradox of assigning vigorous exercise to their pupils as both a high-value activity and a punishment. PE Teacher Education should therefore problematise how teaching practice is influenced by these discourses, and facilitate discussions on how such discourses constitute PE.