Background Physical activity is the most critical determinant of physical health which reduces the health risks during life. This designed study aims to evaluate the effect of educational intervention based on the theory of planned behavior on promoting physical activity among middle-aged women attending the health centers in Karaj city during 2017–2018. Methods This research was a quasi-experimental controlled trial, and the study incorporated 140 Middle-Aged Women Referring to Karaj Health Centers assigned randomly to a control and intervention group (70 each group). Participants enrolling in the survey were asked to complete the International Physical Activity Questionnaire and also the theory of planned behavior measures at the baseline and the end of the study. The SPSS software were used, to analyze the data. The study applied Nonparametric Wilcoxon and Mann-Whitney tests and ANCOVA to interpret the results. Results The results revealed participants' age range between 30 to 50 years with the mean and standard deviation scores 40.07 (8.750) in training and 45.81 (8.513) at the control group. The intervention improved the theory of planned behavior measures and the physical activities of participants. The results showed a significant difference in the attitude score, the perceived behavioral control score, and the level of subjective norms between two training and non-training groups (P < 0.001). The followed-up physical activity score also showed an improvement in the IPAQ score of participants in the training group (p < 0.001). Conclusions The results of this specific study support the effectiveness of the theory of planned behavior for promoting the physical activity of women.
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