This trial aims at understanding the mechanisms of optimized transcranial direct current stimulation (tDCS) (16 tDCS sessions combined with exercise)] on pain control. Optimized tDCS can lead to stronger engagement of the endogenous pain regulatory system that will ultimately lead to increased pain relief in patients with fibromyalgia (FM). Therefore, the investigators designed a 2x2 factorial mechanistic trial [tDCS (active and sham) and aerobic exercise (AE) (active and control)] to evaluate the effects of 4 weeks of tDCS coupled with exercise on the endogenous pain regulatory system assessed by conditioned pain modulation (CPM) and central sensitization as assessed by temporal slow pain summation (TSPS), and compared to either intervention alone and to no intervention.