Hi, I want to use a heart rate monitor in order to avoid PEM, but after doing a lot of reading about the anaerobic threshold (AT) and energy pathways, I'm still a little confused as to some of the the mechanisms involved with fatigue and PEM. Here is my understanding, please help me to understand this better: PEM is triggered when the heart rate reaches near or above the AT (for a certain duration). AT is significantly lower in CFS sufferers than in others (aprox. 60% max heart rate). So here are my questions: Is the point of using a heart rate monitor to prevent anaerobic energy production...? Does keeping heart rate under AT mean that the activity is aerobic and less likely to cause PEM...? So is PEM caused by anaerobic or aerobic exercise...? How does this relate to fast resting heart rate during PEM crashes (or in general)...? I'm sure I have more questions but I will start here. Thanks for any information and clarifications!