RecruitingBehavioural interventionExercise and Emotional Learning in Posttraumatic Stress DisorderThe goal of this clinical trial is to test how exercise affects learning and memory processes relevant to the treatment of PTSD. Participants will complete a baseline intake followed by two experimental sessions. During the first experimental session, participants will undergo an MRI session of imaginal exposure to traumatic memory cues followed by 30-minutes of moderate intensity exercise or low intensity exercise. Participants will complete a second session of imaginal exposure with MRI 24 hours later.