What factors cause fatigue when working below anaerobic threshold?

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We all probably know about pacing by heart rate. Theoretically, if you keep your exertion light enough that you never cross over into anaerobic respiration, you should be able to continue going at that pace indefinitely (assuming that you aren't doing significant cognitive work - brain exertion doesn't seem to affect HR much). Of course, that isn't how it works in reality. We still need to take breaks from exertion while pacing. What other factors contribute to fatigue?
 
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Getting overly emotional causes me a lot of fatigue, even laughing too much (though I suppose that raises the heart rate). Worry, fear, knowing things are left undone like unanswered emails or an uncleaned house all cause fatigue for me. Anticipating going leaving the house for appointments too. I could go on about the details of everything that causes me fatigue.
 

nerd

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I think it's similar with cognitive and physical exertion. I can do low level of both without necessarily triggering PEM. But if I reach a certain threshold, like attending a gathering with many people, like trying to solve a difficult challenge, pushing myself to get a result, giving 100% and not just a relaxed 50%, same for physical activities, then it's almost granted I will feel worse in the coming days. I never try to get things done because this will subconsciously put me in a situation when I try to give 100%, like what was normal in the old days. If I do something now, then only so much that I have something to do, that I don't get bored, but not to get things done.
 

Wishful

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I didn't have an anaerobic threshold (that I could find). As far as I could determine, my physically-induced PEM was triggered by cell damage (which triggers an immune response to clean up the damage) by straining muscles beyond their typical usage. So, a 6-hr bike ride (with some steep hills) did not trigger PEM, yet a few seconds of climbing a ladder (strains those muscles differently) did. Overhead arm work was an easy trigger for me, since I don't regularly do that. I could dig soil for hours in one movement pattern without triggering PEM, yet a few shovels dug using different movements did.

Your question is about fatigue, but I think pacing is meant to avoid PEM. ME has a fatigue-like symptom (worse during PEM), but it may be central fatigue, rather than physical fatigue, so heart rate may not be a factor.
 
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We all probably know about pacing by heart rate. Theoretically, if you keep your exertion light enough that you never cross over into anaerobic respiration, you should be able to continue going at that pace indefinitely (assuming that you aren't doing significant cognitive work - brain exertion doesn't seem to affect HR much). Of course, that isn't how it works in reality. We still need to take breaks from exertion while pacing. What other factors contribute to fatigue?
What do you mean with cognitive and physical?
When you are very severe it doesn't make a difference if you do physical or cognitive exertion... If you only mean that you think of sth it depends if it's stressful or not... But things that are commonly not related to physical exertion and are more concerned cognitive like looking at a picture or screen...Both increase the HR... That's why I don't believe in the hypoxia theory of PEM... How could looking at a screen lead to hypoxia? And it does increase my HR. I'm wearing a HR monitor all the time and when I look at a screen for too long it warns me bc my HR is getting above 95Bpm