Obsessive compulsive disorder (OCD) is a severe and debilitating anxiety disorder afflicting 2% of the population. Cognitive behavioural therapy (CBT) is considered first line psychological treatment for OCD, but there are a large number of treatment non-responders, and the majority of responders have residual symptoms. Aerobic exercise has shown potential benefit for general mood and anxiety disorders, but has not been widely tested in OCD. This study will examine the additive benefits of a standard 12-week aerobic exercise program to a standard 12-week CBT protocol in the treatment of OCD. The study will test if Exercise+CBT results in significantly better clinical outcomes compared to either treatment alone or no treatment at all. Treatment outcomes will be assessed in relation to symptom and cognitive measures of clinical improvement.

This study aims to determine if a standard, evidence-based 12-week aerobic exercise program results in significant reduction of obsessive-compulsive symptom severity and associated cognitive dysfunction as a stand-alone intervention and when combined with the first-line psychological treatment, cognitive behavioural therapy (CBT). There are 3 broad aims to this study: 1) to compare the relative efficacy of Exercise, CBT, and their combination (CBT+Ex) versus a non-treatment waitlist control (WL), 2) to examine the extent to which neuropsychological features of OCD improve following treatments, and 3) to determine the extent to which the BDNF gene and protein are a) associated with learning and cognitive factors, and b) moderate symptom and cognitive change across treatments.

The study design allows for a novel, well-powered and potentially landmark study on the impact of exercise on obsessive-compulsive symptom severity and cognitive functioning in OCD, both as a stand-alone treatment and when augmenting CBT. If this study can demonstrate that a short aerobic exercise program of 12 weeks duration can confer significant clinical gains for those suffering with OCD, then it could easily be translated into highly accessible, routine clinical care. Further, demonstration of improvement in OCD-associated cognitive dysfunction, given the refractory nature of the illness, would provide another avenue into the long-term enhancement of outcomes for this chronically affected population.

Inclusion Criteria:

• Presenting with DSM-5 diagnosed OCD
• Score of >16 on the Yale-Brown Obsessive Compulsive Scale (YBOCS)
• A Physical Activity Readiness Questionnaire (PAR-Q) score of ≤1
• If on medications for OCD, must be stabilized, i.e. are currently receiving an adequate dose (equivalent of 40mg/day of fluoxetine) for an adequate duration (at least 12 weeks of treatment) prior to the initiation of the study
• Has physician letter of approval for safe participation in fitness assessment and exercise protocol

Exclusion Criteria:

• Previous course of CBT treatment (≥ 8 sessions) in past two years
• Engaging in an active exercise regimen (>2 days/wk of moderate-to-vigorous exercise training) at the time of recruitment
• Concurrent diagnosis of a severe mood disorder, schizophrenia or other psychotic disorders, or substance abuse/dependence
• Suspected organic pathology
• Active comorbid medical condition that may require urgent intervention during the treatment
• Incapable of providing informed consent