Psychosocial determinants and perceived environmental barriers as mediators of the effectiveness of a web-based tailored intervention promoting physical activity in adolescents: the HELENA Activ-O-Meter

The aim was to examine if psychosocial determinants (attitudes, self-efficacy, social support from a sports partner) and perceived environmental barriers (PEB) of physical activity (PA) mediated the effect of a 3-month Internet-based intervention on PA in European adolescents. A sample of 536 adolescents (51% boys) aged 12-17 years were randomly assigned to intervention or control condition. Questionnaires were used to assess different PA behaviors, psychosocial determinants and PEB at baseline and at 3-month follow-up. Mediating effects were assessed with the bootstrapping method. PEB regarding neighborhood safety mediated the effect of the intervention on all PA indices. PEB regarding sports facilities availability at neighborhood and PEB regarding sport-related facilities availability at school mediated the effect of the intervention on moderate to vigorous PA (MVPA) and moderate PA (in leisure time and at school, respectively). Social support from a sports partner suppressed the effect of the intervention on vigorous PA and MVPA. No other factor had a mediation effect. All PEB measures appear to mediate PA behaviors of different intensities and in different contexts. Interventions promoting PA in adolescents should also focus on improving the targeted PEB as mediators of engagement in PA to bring the desired effects in actual behaviors.