Cognitive behavior therapy (CBT) has made important contributions to chronic pain management, but the process by which it is effective is not clear. Recently, strong arguments have been raised concerning the need for theory driven research to e.g. identify mechanisms of change in CBT and enhance the effectiveness of this type of treatment. However, the number of studies addressing these issues is still relatively scarce. Furthermore, the arrival of varieties of CBT with seemingly different process targets increases the need for such information. The present study explored the processes of change in a previously reported successful randomized controlled trial evaluating the effectiveness of an exposure-based form of behavioral and cognitive therapy, Acceptance and Commitment Therapy (ACT), on improvement in pain-related disability and life satisfaction for patients suffering from whiplash-associated disorder (WAD). Several process variables relevant to theories underlying traditional CBT were included: pain, distress, kinesiophobia, self-efficacy, and the process primarily targeted by ACT: psychological inflexibility. Mediation analyses were performed using a non-parametric cross-product of the coefficients approach. Results illustrated that pain intensity, anxiety, depression, kinesiophobia, and self-efficacy did not have significant mediating effects on the dependent variables. In contrast, significant indirect effects were seen for psychological inflexibility on pain-related disability (pre- to post-change scores) and life satisfaction (pre- to post; pre- to 4-month follow-up change scores). Although tentative, these results support the mediating role of psychological inflexibility in ACT-oriented interventions aimed at improving functioning and life satisfaction in people with chronic pain.
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