Psychosocial mediators of physical activity and fitness changes in the activity counseling trial.

Background: Improved understanding of the mediators of physical activity (PA) interventions could lead to improvements in theory and programs.

Purpose: To examine the 24-month mediating effects of psychosocial variables on PA and cardiorespiratory fitness (CRF) outcomes in 878 initially sedentary adults aged 35-75 participating in the Activity Counseling Trial.

Methods: Participants were assigned to one of three intervention arms: physician advice, assistance, or counseling. MacKinnon's product of coefficients was used to test for longitudinal and contemporaneous mediation.

Results: Changes in behavioral processes of change from baseline to 24 months significantly mediated the relationship between the active intervention arms and improvements in PA and CRF from baseline to 24 months in both men and women. None of the other psychosocial variables tested met criteria for mediation.

Conclusions: Results indicate that behavioral interventions should incorporate methods to encourage participants to use these behavioral strategies as they attempt to become more active.