Discussion
Parental support is significantly correlated to children’s PA levels [According to SDT, satisfying the basic psychological needs enables the development of more autonomous forms of motivation [Previous research has shown that need-supportive behaviours from PE teachers and peers enhance need satisfaction and lower need frustration for students in the PE context [Regarding perceptions of need-supportive behaviour from parents, attitude and perceived behavioural control, no statistically relevant changes emerged. These findings are unexpected as previous research by Tilga and colleagues has demonstrated that a web-based autonomy-supportive training for teachers enhanced students’ perceptions of teachers’ autonomy-supportive behaviours [Inconsistent with our expectations, we found that intention and effort towards out-of-school PA were higher in the control group in the follow-up measuring occasion. One possible explanation could be seasonality regarding outdoor PA as both intention and effort are associated with actual PA. There are substantial seasonal differences in Estonia with rainy autumns, often snowy and windy winters, and moderately warm summers. It is also worth mentioning that from November to February daylight is scarce. Research has shown winter to be associated with lower levels of PA, especially among girls [The study has also some limitations. Firstly, we only used questionnaires for measurement, including collecting data about PA behaviour. The reason for using questionnaires solely is the aim to recruit participants as widely as possible. However, research has shown that in self-report measures participants frequently overestimate their PA levels as compared to accelerometer-measured data [The reasons for finding few intervention effects in post hoc tests can be attributed to the interaction effects being only borderline statistically significant, large standard deviations, the small number of participants in the study and uneven sample size in study groups. This does certainly reduce the statistical power of the study and the generalizability of the results. We found it difficult to get parents to participate in the training program, an obstacle also encountered by van Sluijs and colleagues [Another limitation of the study is the significant difference in the proportion of male and female students between the intervention and control groups. We cluster-randomized by school, not knowing beforehand how many students voluntarily enroll and what the proportion of male and female students would be. This does impact the generalizability of the results.A possibility for improvement is related to missing values. In the pilot study we used pen-and-paper questionnaires and had randomly missing answers to questions throughout questionnaires, reducing the reliability of the obtained findings. In further research we plan to use online questionnaires as this allows marking all answers as required. Tilga and colleagues have successfully used this approach on 11–15-year-old students previously [Some limitations also apply to the used questionnaire. We adapted perceived competence and perceived relatedness support scales from Standage and colleagues [Further research should be carried out to test the intervention in other domains, where messages related to PA are delivered, such as PE in schools. To achieve the highest impact on students’ PA levels, interventions in several domains (e.g., home and school settings) could be combined. This would mean creating a need-supportive social environment in several contexts for the children, thus leading to a comprehensive approach to promoting their PA levels. Such supportive environments enable satisfying all three basic psychological needs, making it possible to reach optimal performance and wellbeing for the children [Future research could be based on a newer classification of motivational behaviours by Ahmadi and colleagues set in the educational context [The results of the study are expected to contribute to informing the development of future interventions and educational programs for parents to promote PA in children and adolescents. The online approach makes it possible to reach large audiences and enables self-paced learning for participating parents. It is vital to find effective and cost-effective ways to support PA, that can also be adapted to various countries and age groups.