Data from two randomized clinical trials (RCT) were used to examine the extent to which a health promotion intervention affected changes in growth trajectories of psychosocial constructs and if so, whether these constructs in turn explained changes in physical activity (PA). PA and psychosocial measures on 842 overweight adults in the United States were collected in two RCTs evaluating Internet-based behavior change interventions with assessments at baseline, 6 and 12 months. A physical activity latent variable at 12 months was created using indicators of self-reported walking and leisure time activities. Intervention-mediated effects on PA at 12 months were found via latent growth curves representing self-efficacy and behavioral strategies, where increasing growth curves across time were associated with higher PA values at 12 months. These findings provide some evidence that web-based self-help intervention programs worked through targeted behavior change constructs to influence physical activity levels in overweight adults.
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