International concern about "alarming" levels of childhood obesity has seen a proliferation of interventions filtering into school physical education programmes that are designed to influence children's health practices and attitudes. This article addresses one such obesity-prevention intervention, the Global Children's Challenge™, a 50-day pedometer-monitored event, aimed at children and involving their parents and teachers. Our research problematises the effects of the GCC pedometer exercise regime. We demonstrate how the pedometer measurement imperative made available in the GCC not only enables exercise to be measured for potential health benefits but also makes available tools inextricably linked with antagonistic body relations that could propel some students into a self-monitoring world dominated by numbers. We illustrate how the emphasis on measurement allows for comparisons (dividing practices), self-surveillance and surveillance of others in the formation of particular kinds of subjectivities. This study of the discursive construction of student subjectivities in the GCC took place in one strategically chosen Australian primary school. In-depth interviews were conducted with one teacher, four Year-6 students and a parent of each child in order to produce rich contextual data. Foucauldian concepts of power, knowledge and "technologies of self" underpinned the study and Gore's methodologies for analysing "techniques of power" and "regimes of truth" were used to explore the functioning of power and the formation of subjectivities in the GCC. Our analysis suggests a need to move away from the constraining construct of measurement in the primary physical education (PE) classroom and promote self-reflective mindful physical activity rather than telling students when, where and how to move their bodies.