I find PEM hard to trigger and below is a discussion of that. My CFS is considered mild and 60 (or a bit less) on this scale: http://drmyhill.co.uk/wiki/CFS_Ability_Scale_-_a_rough_measure_of_how_disabled_you_are The only repeatable PEM I have had is when feeling particularly good, going for a 20 min run. I attempted this twice. Both times my heart rate got to ~180. i felt great during and after. The next day was definitely a PEM crash and lasted for a few days and didn't really return to baseline. I've done this twice and both times I've had the same result. The rest of my PEM instances I've had aren't repeatable. When I go on my exercise bike I get my heart to ~130bpm average (~160 max). This is usually ok, although may occasionally may trigger PEM. Thoughts: 130bpm isn't normally doable. I need ritalin else I can only manage ~115bpm due to chronotropic imcompetence. Ritalin cures the CI as far as I can tell. So my hypothesis is that while PEM usually isn't attainable, ritalin fixes the CI problem and allows my heart to beat fast enough to trigger PEM, which in my case seems to need quite a high bpm (~180) Do other milds find a problem with triggering PEM consistently? Do you find that heart rate doesn't seem to have much predictive value in regards to PEM?