Framework Theory of Conceptual Change (FTCC) is the prevailing theoretical approach guiding current thought and research into the contextualized development of students' mental models. In FTTC Vosniadou (1994) theorized the role academic beliefs and social and contextual variables play in model development. Physical education scholars have not yet applied FTCC to an examination of students' fitness conceptions and little is known about the role academic beliefs play in knowledge development. The purpose of this dissertation was to apply FTCC to an examination of students' mental models of fitness concepts. I conducted a descriptive study using an ethnographic research design to examine the contextualized development of students' mental models. Participants included one class of sixth-grade students and their teachers at two middle schools. Student data (n=18) were collected using written questionnaires and interviews. Additionally, I collected contextual data through document collection, physical education (n=2) and science teacher (n=1) interviews, and field observations of the physical education lessons conducted at the respective schools. In the first analysis, I identified five generic mental models based upon diverse configurations in students' naive theories to explain exercise induced physical changes. Findings suggested students' diverse explanations reflected the inherent complexity of the concept. The emerging coherence of students' perspectives towards scientific views is gradual. Developing sophisticated conceptions entails developmental, applicational, and integrated processes that evolve into complex relational conceptions. In the second analysis, I identified three mental models students used to explain the concept of intensity and it's relation to other elements in the FITT principle. In contrast to previous research, all 18 students within this study were familiar with the concept of intensity and the FITT principle. Students' explanations were diverse and reflected variations in their conceptual transitions from a holistic elementary school level conception of FITT and intensity. The diverse models reflected students' purposeful and creative attempts to seek coherence and make interdisciplinary and multi-sensory connections. A myriad of variables appeared to interact to facilitate and sometimes limit students' mental models, including school support, language and tool support, and teachers' values and beliefs about teaching, fitness, and student learning.