Mohammed Khudair, Anna Marcuzzi, Kwok Ng, Gavin Daniel Tempest, František Bartoš, Ratko Peric, Maximilian Maier, Flavia Beccia, Stefania Boccia, Mirko Brandes, Greet Cardon, Angela Carlin, Carolina Castagna, Helmi Chaabene, Anna Chalkley, Simone Ciaccioni, Joanna Cieślińska-Świder, Vilma Čingienė, Cristina Cortis, Chiara Corvino, Eco J. C. de Geus, Angela Di Baldassarre, Andrea Di Credico, Patrik Drid, Rosa Ma Fernández Tarazaga, Francesca Gallè, Esther Garcia Sánchez, Mekdes Gebremariam, Barbara Ghinassi, Marios Goudas, Grainne Hayes, Samuel Honorio, Pascal Izzicupo, Henriette Jahre, Judith Jelsma, Petra Juric, Athanasios Kolovelonis, Atle Kongsvold, Evangelia Kouidi, Fiona Mansergh, Bojan Masanovic, Teferi Mekonnen, Paul Jarle Mork, Marie Murphy, Kelly O'Hara, Ayse Ozbil Torun, Federico Palumbo, Stevo Popovic, Olaf Prieske, Zrinka Puharic, José Carlos Ribeiro, Penny Louise Sheena Rumbold, Petru Sandu, Maroje Soric, Mette Stavnsbo, Ioannis Syrmpas, Hidde P. van der Ploeg, Aurélie Van Hoye, Sofia Vilela, Catherine Woods, Kathrin Wunsch, Laura Caprinica, Ciaran MacDonncha, Fiona Chun Man Ling

• Introduction Physical activity among children and adolescents remains insufficient, despite the substantial efforts made by researchers and policymakers. Identifying and furthering our understanding of potential modifiable determinants of physical activity behaviour (PAB) and sedentary behaviour (SB) is crucial for the development of interventions that promote a shift from SB to PAB. The current protocol details the process through which a series of systematic literature reviews and meta-analyses (MAs) will be conducted to produce a best-evidence statement (BESt) and inform policymakers. The overall aim is to identify modifiable determinants that are associated with changes in PAB and SB in children and adolescents (aged 5-19 years) and to quantify their effect on, or association with, PAB/SB. Methods and analysis A search will be performed in MEDLINE, SportDiscus, Web of Science, PsychINFO and Cochrane Central Register of Controlled Trials. Randomised controlled trials (RCTs) and controlled trials (CTs) that investigate the effect ofIntroduction Physical activity among children and adolescents remains insufficient, despite the substantial efforts made by researchers and policymakers. Identifying and furthering our understanding of potential modifiable determinants of physical activity behaviour (PAB) and sedentary behaviour (SB) is crucial for the development of interventions that promote a shift from SB to PAB. The current protocol details the process through which a series of systematic literature reviews and meta-analyses (MAs) will be conducted to produce a best-evidence statement (BESt) and inform policymakers. The overall aim is to identify modifiable determinants that are associated with changes in PAB and SB in children and adolescents (aged 5-19 years) and to quantify their effect on, or association with, PAB/SB. Methods and analysis A search will be performed in MEDLINE, SportDiscus, Web of Science, PsychINFO and Cochrane Central Register of Controlled Trials. Randomised controlled trials (RCTs) and controlled trials (CTs) that investigate the effect of interventions on PAB/SB and longitudinal studies that investigate the associations between modifiable determinants and PAB/SB at multiple time points will be sought. Risk of bias assessments will be performed using adapted versions of Cochrane's RoB V.2.0 and ROBINS-I tools for RCTs and CTs, respectively, and an adapted version of the National Institute of Health's tool for longitudinal studies. Data will be synthesised narratively and, where possible, MAs will be performed using frequentist and Bayesian statistics. Modifiable determinants will be discussed considering the settings in which they were investigated and the PAB/SB measurement methods used. Ethics and dissemination No ethical approval is needed as no primary data will be collected. The findings will be disseminated in peer-reviewed publications and academic conferences where possible. The BESt will also be shared with policy makers within the DE-PASS consortium in the first instance. Systematic review registration CRD42021282874.…