The aim of this study is to examine the effects of a physical activity intervention plan developed based on the theory of planned behavior on the physical activity levels of adolescent girls, their sedentary time and their knowledge about physical activity.

Within the scope of this research, answers to the following research questions will be sought;

1. Is there a difference in physical activity level between participants in the physical activity intervention plan groups and the control group?
2. Is there a difference in terms of daily sedentary time between participants in the physical activity intervention plan groups and the control group?
3. Is there a difference in beliefs about physical activity between participants in the physical activity intervention plan groups and the control group?

In this context, researchers will compare 3 groups within the scope of the data provided by two preliminary studies conducted previously. These groups; (1) intervention group based on the theory of planned behavior, (2) intervention group based on the theory of planned behavior with family participation, (3) control group.

While the physical activity participation of adolescents in Turkey and around the world is quite low, this is more pronounced for girls. The low physical activity participation rates seen in this group shed light on the negative situations to be seen in the future in terms of public health and the economy. The fact that health-related behavior patterns acquired during adolescence form a highly effective basis for the rest of life makes it necessary to develop effective, low-cost physical activity intervention plans for adolescents that take into account individual needs. Research on physical activity intervention, while generally targeting groups with low physical activity levels, is applied in different contexts (school, family, school and family, living environment) where these groups spend their time. In this respect, school-based interventions, where adolescents spend most of their time, come to the fore. For plans to be effective, they should be based on a research process and their theoretical foundations should be inclusive. While the theory of planned behavior, which has an important place in shaping health behaviors and explains the formation and continuity of behavior by addressing the attitude, norms created by the social environment, and the control perceived by the person towards the behavior, constitutes the theoretical basis of this research, the aim of the research is to create and test a school-based physical activity intervention plan based on the theory of planned behavior, targeting adolescent girls.

Qualitative and quantitative research methods were applied through a multi-stage design process, and the findings revealed the need to create an effective intervention plan guide for adolescent girls in Turkish society. These phases included (I) a study to explain which factors are important for physical activity behavior in adolescent girls, including open-ended questions and semi-structured interviews; (II) the process of developing a measurement tool in the light of the data obtained with these qualitative methods; and (III) survey research that adopted the general survey model and reached a large population. As a result of this process, which has been going on for about 48 months, the physical activity intervention plan to be implemented has been shaped. With this intervention, it is aimed to create significant effects among adolescent girls in terms of physical activity and sedentary time and to positively affect their beliefs about physical activity.

The physical activity intervention plan to be developed according to the planned behavior theory will be based on the information obtained in pilot studies that have already been completed. The plan will be implemented in selected high schools affiliated with the Turkish Ministry of National Education. The developed protocol is planned to be implemented over two different time periods. The reason for this is that seasonal factors are emphasized as an important factor in carrying the gains provided in physical activity intervention plans into daily life (Rowland et al., 2006; Tucker & Gililand, 2007; Belanger et al., 2009; Kornides et al., 2018). The contents of the intervention plan will be delivered to students for 2 hours in selected physical education and sports classes. It is planned to implement this plan in physical education classes for one semester. Considering that the implementation will take place in one academic year, data will be collected at the beginning and end of the first semester and at the end of the second semester. Data collection processes are planned to take approximately 1-1.5 months. Thus, it will be ensured that this application will be carried out for 3-3.5 months in a semester. While planning this period, Owen et al. (2017) reviews and meta-analyses were taken into consideration.

Structurally, the physical activity intervention plan is planned to include four dimensions based on the theory of planned behavior. The first two of these are: (I) transferring information on health-related physical fitness and physical activity; (II) taking individual responsibility for physical activity follow-up; and (II) simple individual planning, which will affect the level of physical activity at the individual level. Another dimension is the inclusion in the program of the types of activities that the female adolescents (III) chose and declared that they would enjoy participating in. The last dimension, as emphasized in the theory of planned behavior, is the invitation of significant others, which are the family, teachers, and other students, including male students, with whom the target group is in constant social interaction, to this plan, and all of the practices are open to them. However, data will not collected from them.

Inclusion Criteria:

• Bein high school student
• Do not have any health problems that would prevent from participating in exercise,
• Not participating any another exercise/training program during the research process,
• Voluntarily participation -Being suitable for exercise participation according to Physical Activity -- Readiness Questionnaire (PARQ) test results

Exclusion Criteria:

• Presence of any significant health conditions that may pose a risk during exercise (e.g., heart conditions, severe respiratory disorders, uncontrolled hypertension).
• A history of musculoskeletal injuries or conditions that would limit participation in physical fitness activities.
• Current or recent participation in another exercise or fitness program.
• Inability to provide informed consent or unwillingness to participate voluntarily.
• Inability to provide parental consent.
• Any medical condition that contraindicates exercise or physical activity as determined by a healthcare professional.
• Age restrictions (younger than 15 and older than 18).
• Cognitive impairments that would hinder the ability to understand and follow exercise instructions.