Both patients with peripheral structural pathologies, like rheumatoid arthritis (RA)-patients, or patients with central sensitivity syndromes (CSS) suffer chronic pain. CSS are characterized by an increased responsiveness of central pain neurons. An impaired endogenous pain inhibition is already demonstrated in CSS. In the present study the investigators want to evaluate the efficacy of pain inhibition in response to physical stressors and whether the efficacy is opioid-mediated in two chronic pain populations (RA & CCS) compared to controls.

Therefore a triple-blinded randomized controlled trial (RCT) with cross-over design will be performed. The efficacy of wind-up of pain and spatial summation of pain is evaluated before and after a submaximal exercise, while the experimental group receives a selective serotonin reuptake inhibitor. Participants are 20 RA-patients and 20 CSS-patients, more specific patients with fibromyalgia and chronic fatigue syndrome, and 30 healthy controls. This way, the investigators analyze how pain inhibition reacts on different types of physical stressors in different pain patients and if pain inhibition is opioid-mediated.

Inclusion Criteria:

• Each study participant should be aged between 18 and 65 years. All three groups will be comparable for age, gender, education level and socioeconomic status; both patient groups will be comparable for illness duration.
• The Central Sensitivity Syndrome (CSS) group will comply with the diagnostic criteria for Fibromyalgia (FM) as defined by the American College of Rheumatology and the Centre of Disease Control criteria for Chronic Fatigue Syndrome (CFS).
• At the time of study participation, healthy control subjects cannot suffer any pain complaints. Sedentary is defined as a sedentary job and < 3 h moderate physical activity/week. Moderate physical activity is defined as activity demanding at least the threefold of the energy spent passively.
• Patients have to be able to cycle on a bicycle ergometer.

Exclusion Criteria:

• In order to preclude confounding factors, participants cannot be pregnant (pregnancy test) or until 1 year postnatal and will be asked to stop medication two weeks prior to study participation, not to undertake physical exertion, and to refrain from consuming caffeine, alcohol or nicotine on the day of the experiment. For ethical reasons, Rheumatoid arthritis(RA)-patients are able to take non-opioid pain medication as described in the 1st step of the World Health Organization analgesic ladder (Non-Steroidal Anti-Inflammatory Drugs and paracetamol).