Insufficient physical activity in children and adults is linked to increased risk of physical and mental health conditions. Parents have an important influence on their children's activity, and preliminary aimed at promoting physical activity suggest the family unit as a potentially efficacious means of delivery. However, there is relatively little research examining the efficacy of family interventions aimed at promoting both parents' and children's physical activity. In addition, many family-based interventions are not guided by behavioral theory known to enhance intervention efficacy and reduce variability, and do not target low-active families. This project will therefore develop and test the efficacy of a theory-based behavioral intervention to increase physical activity engagement in low-active children and their parents. The intervention will adopt a randomized waitlist-controlled design with parents and their children allocated to an intervention group that receives theory-based content targeting changes in key behavioral determinants or to a waitlist control group that receives measurement only. The intervention will be delivered to parent-child dyads in a series of four online meetings with a trained facilitator, accompanied by web-based supporting materials, and a moderated social support online chat group. The primary dependent variable will be parents' and children's leisure time physical activity, and secondary outcomes will be device measured physical activity (intervention group only) and measures of key theory-based determinants of physical activity, including autonomous motivation, attitudes, subjective norms, self-efficacy, intentions, and perceived autonomy support. These outcomes will be measured at baseline and at 3, 6, and 12 months post-baseline. Parents and children allocated to the intervention group are expected to exhibit higher levels of physical activity and behavioral determinants. Intervention effects on physical activity change are expected to be mediated by the theory-based determinants, consistent with psychological theory. The project will contribute to research by testing the efficacy of a unique intervention based on behavioral theory expected to be effective in promoting physical activity in low-active parents and their children, and providing guidelines and materials that practitioners in health promotion and public health can use in campaigns and national strategies to promote physical activity in this population.

Low levels of physical activity in adult and youth populations are associated with an increased risk of physical and mental health conditions and reduced quality of life. Conversely, regular activity participation is associated with reduced chronic disease risk and better psychological health and well-being. However, studies indicate that most young people and adults do not achieve recommended levels of physical activity. Physical activity participation also tends to decrease throughout childhood, with steep declines observed during adolescence. Consequently, effective means to promote physical activity participation in child and adult populations are needed. While family-based interventions represent a potentially useful environment for delivering behavioural interventions to promote physical activity in both parents and children, few interventions have been applied in this context. Those that have, tend not to have a strong basis in behavioural theory, often focus on short-term change, and are seldom evaluated systematically.

Within this project, we aim to develop an intervention to promote physical activity in low-active parents and their children. Based on previous research, an integrated model based on multiple theories from behavioural science, particularly social psychology, will guide the development of the intervention in the current project. Specifically, the model draws its predictions from two key behavioural theories: self-determination theory (SDT) and the theory of planned behaviour (TPB), two pre-eminent theoretical approaches that have been applied extensively to predict behaviour and guide interventions in health behaviour contexts.

Evidence suggests that a key approach to family-based behavioural interventions aimed at promoting physical activity in young people and their parents should focus on parent-child dyads, as they provide an efficient means to increase physical activity in both parties. Few studies have systematically evaluated the efficacy of behaviour change methods focused specifically on these kinds of dyad in changing physical activity in family-based interventions and further high-quality evidence is needed. Although the proposed integrated model has generally been used as a basis for interventions in individual contexts, it has not hitherto been adopted as a basis for family-based behaviour change interventions focused on dyads, yet is well placed to be adapted to inform such interventions. The model can also be modified to encompass dyad-focused versions of key determinants identified in the model. For example, the model can be modified to include dyad-based self-efficacy perceptions, a family-based revision of the self-efficacy construct in the model, which reflects perceived confidence in performing the target behaviour together with other dyad members and joint appraisal of goal progress.

Inclusion Criteria:

• Parent does not undertake at least 150 minutes of moderate-intensity (or 75 minutes of vigorous-intensity) physical activity each week.
• Child does not undertake at least 60 minutes of moderate-intensity or vigorous-intensity physical activity each day.
• Child is aged between 8 and 12 years old

Exclusion Criteria:

• Parent reports that either they or their child is currently meeting physical activity guidelines at screening.
• Parent states that they or their child has some physical condition that prohibits them from safely participating in physical activity.

All research components including protocols, materials, de-identified data, and analysis protocols and output, will be publicly shared on a permanent open-access repository (OSF.io).

Data will be uploaded following the conclusion of all data collection.

Data will be openly available.