Supplementary Material for: Effects of a web-based behavioral activation intervention on depressive symptoms, activation, motivation, and volition: results of a randomized-controlled trial

Introduction: Behavioral activation (BA) is effective for the treatment of depression. The Health Action Process Approach (HAPA), which is derived from health psychology, can provide a motivational-volitional framework of behavioral activation. Objective: This study investigates the efficacy of a HAPA-based internet-delivered BA intervention (iBA; called InterAKTIV) in individuals with depression, also assessing HAPA-based motivational and volitional outcomes. Methods: In a two-arm randomized controlled efficacy trial with a parallel design, 128 participants with a major depressive episode were randomly allocated to the intervention group (IG; TAU + immediate access to iBA) or control group (CG; TAU + access to iBA after follow-up). The primary outcome of clinician-rated depressive symptoms and secondary outcomes were assessed at baseline (T1), 8 weeks (T2), 6-month after randomization (T3). Data were analyzed on an intention-to-treat basis. Results: Linear mixed model analyses revealed a significant group*time interaction effect on clinician-rated depressive symptoms favoring the IG (F2, 156.0 = 7.40; p < 0.001, d = 0.79 at T2, d = 0.25 at T3). The IG was also superior regarding self-rated depressive symptoms, behavioral activation, most motivational, and all volitional outcomes. Conclusion: This study shows that HAPA-based iBA can significantly improve clinician-rated depressive symptoms, as well as outcomes used in the HAPA model in people with depression. Building on these efficacy results, in the next step, the relationship between BA interventions and activity levels should be investigated, taking into account motivation and volition as potential mediators.