Objective: To investigate whether early changes in fear of movement (kinesiophobia), self-efficacy and catastrophizing were mediators of the relationship between allocation to the pre-habilitation intervention and later changes in health outcomes. Methods: The original pre-habilitation trial (PREPARE, ISRCTN17115599) recruited 118 participants awaiting lumbar fusion surgery, half of whom received a prehabilitation intervention designed based on the modified fear-avoidance model and half of whom received usual care. Mediation analysis was performed to test each mediator separately. Analysis was performed on each outcome of interest separately (Oswestry disability index, patient-specific function, EQ general health and moderate/vigorous physical activity). Mediation analysis was carried out using PROCESS. Beta coefficients and bootstrapped 95% CIs were used to interpret the results. Results: None of the potential mediators was found to mediate the relationship between allocation to the intervention and 3-month scores on any of the health outcomes tested. Conclusions: Screening patients for higher levels of catastrophizing and fear avoidance and lower levels of self-efficacy could help ensure only the patients who are most likely to benefit from the intervention are included. Significance: Prehabilitation interventions for spinal fusion surgery have been found to improve health outcomes for patients. Theory-based interventions that target key mechanisms are more effective at improving outcomes than non-theory-based interventions. While no mediating effects were found for this particular intervention, the analysis suggests that the underlying theoretical model and treatment targets are appropriate and could drive improvement if more strongly targeted.