Ratings of fatigue intensity with "0" being a complete lack of fatigue and "100" being the highest intensity fatigue imaginable. Each time point represents an average of the daily scores from the previous two weeks. For CFS participants, the two week time point is from two weeks of baseline activity and the four and six week time points are from increased activity. For the control participants all time points are from baseline activity. CFS participants did not change significantly over time with increased activity. # Significant difference between CFS and control participants (P < 0.001). Values are mean ± SD.