Psychological flexibility or inflexibility: examining the distinct roles of these transdiagnostic factors in mild traumatic brain injury recovery

OBJECTIVE: Psychological flexibility (PF) and psychological inflexibility (PI) are transdiagnostic processes implicated in mental health outcomes, yet their distinct roles in mild traumatic brain injury (mTBI) recovery remain unclear. This study examined PF and PI in a treatment-seeking mTBI sample and tested whether these processes operate through distinct pathways linking post-concussion symptoms (PCS), functional disability, and psychological distress. METHOD: Participants were 173 adults with mTBI recruited from outpatient concussion rehabilitation services in New Zealand. Participants completed measures of PCS (Rivermead Postconcussion Symptom Questionnaire), disability (WHODAS-12), psychological distress (DASS-21), PI (reactive avoidance; AAQ-ABI), and PF (Personalized Psychological Flexibility Index; PPFI). Structural equation modeling tested a parallel mediation model in which PCS and disability predicted distress indirectly via PI and PF. Three models were estimated, substituting PPFI facets (behavioral engagement, acceptance, harnessing). RESULTS: Across the three structural equation models, overall model fit was adequate, with the behavioral engagement model demonstrating excellent fit. Higher reactive avoidance was consistently associated with greater psychological distress, and a history of a mental health conditions, across all models. Reactive avoidance mediated the relationship between PCS and distress in the behavioral engagement and harnessing models. Behavioral engagement and acceptance were each independently associated with lower psychological distress, whereas harnessing showed a small positive association with distress. These facets did not mediate the relationship between PCS and psychological distress. No serial mediation effects between reactive avoidance and PF on psychological distress were observed. CONCLUSIONS: This study demonstrates that PF and PI represent related but distinct transdiagnostic processes shaping psychological outcomes following mTBI. Reactive avoidance emerged as a key mechanism linking PCS to psychological distress, while behavioral engagement and acceptance were independently associated with lower distress. By delineating these processes, the findings extend existing mTBI research and offer clinically relevant insights with the potential to inform transdiagnostic psychological interventions.