A test of cognitive mediation in a 12-month physical activity workplace intervention: does it explain behaviour change in women?

BACKGROUND: Attempts to demonstrate the efficacy of interventions aimed at increasing physical activity (PA) have been mixed. Further, studies are seldom designed in a manner that facilitates the understanding of how or why a treatment is effective or ineffective and PA intervention designs should be guided by a heavier reliance upon behavioral theory. The use of a mediating variable framework offers a systematic methodological approach to testing the role of theory, and could also identify the effectiveness of specific intervention components. The primary purpose of this paper was to test the mediating role that cognitive constructs may have played in regards to the positive effect that a workplace behavioral intervention had on leisure-time PA for women. A subsidiary purpose was to examine the cross-sectional relationships of these cognitive constructs with PA behavior. METHODS: The Physical Activity Workplace Study was a randomized controlled trial which compared the effects of stage-matched and standard print materials upon self-reported leisure-time PA, within a workplace sample at 6 and 12-months. In this secondary analysis we examined the mediation effects of 14 psychosocial constructs across 3 major social-cognitive theories which were operationalized for the intervention materials and measured at baseline, 6 and 12-months. We examined change in PA and change in the psychological constructs employing a mediation strategy proposed by Baron and Kenny for: (1) the first 6-months (i.e., initial change), (2) the second 6-months (i.e., delayed change), and (3) the entire 12-months (overall change) of the study on 323 women (n = 213 control/standard materials group; n = 110 stage-matched materials group). RESULTS: Of the 14 constructs and 42 tests (including initial, delayed and overall change) two positive results were identified (i.e., overall change in pros, initial change in experiential powerful intervention approaches processes), with very small effect sizes. However, these mediating results were eliminated after adjusting for the multiple statistical tests. CONCLUSIONS: The intervention did not change these mediators in any substantive way, and show a similar pattern to prior research where interventions generally do not result in a change in mediation of behavior change. It is important to report mediation results in randomized controlled trials whether the findings are null or positive. Future studies may wish to focus on more detailed dose-response issues between mediators and behavior, the inclusion of moderators that could affect individual change, or different mediator constructs at higher levels of measurement specificity. Continued work on innovative and more powerful PA intervention approaches are needed.