Background of the study: Despite the important role of physical activity in maximizing health during the life span, impaired mobility is the main cause of diseases such as breast cancer, diabetes and a significant proportion of the burden of ischemic heart diseases. This paper will describe the protocol for a randomized clinical trial of an SDT-based (self-determination theory) physical activity motivational intervention for women aged 30-45 years. To this end, an SDT-based intervention aims at promoting physical activity, quality of life and happiness of women. Methodology: A randomized controlled trial parallel design will be used to evaluate the effectiveness of SDT-based intervention on physical activity, quality of life, and happiness. Ninety two women aged 30-45 years (non-pregnant, Body Mass Index ≤35 (kg/m2) and without a medical ban to do physical activity) will be randomly allocated into the self-determination (intervention) and control group. The primary outcome will be the physical activity and motivation for physical activity and secondary outcomes will be the quality of life and happiness. Physical activity will be evaluated using 7-day accelerometer. Moreover, participants will complete MPAM-r, BREQ-2, EQ-5D-5L, SF-36 and OHI questionnaires. The change in the mean score of the determinants at baseline, 1 and 3 months after intervention will be compared between the two groups (5% alpha error and 80% power). Discussion: This study will be conducted to assess the effectiveness of an SDT- based intervention developed for 30-45 years old women. If successful, the intervention would help women with the necessary knowledge and skills to enable them to make healthier choices for themselves. Moreover, women living in urban areas with similar economic and social conditions would be able to benefit from the current research`s experiences.
