I work out doing 15 mins aerobic on my elliptical trainer and feel ok at the end.
I don't have PEM. I arrived at 15 mins as a safe time limit.
However, simply going into the garden and weeding for a shorter time induces immediate malaise and struggle.
I wonder if its simply because I am not used to weeding, or is it an indication of something else?
I notice a similar thing. I don't tend to get much PEM or negative repercussions from physical exercise, and I can easily walk for 40 minutes at a brisk pace without problem (except that I feel slightly more tired and brain fogged for about half an hour or so after my walk, but that clears up quickly, and I don't think that is actual PEM).
However, recently I was cleaning out a large bedroom fitted wardrobe, and I was bending down a lot to get into the wardrobe, in order to clean the interior surfaces, and after doing this for around 30 minutes I suddenly felt quite whacked and decided to stop. Yet in terms of energy expenditure, that does not make sense, as I would think I exert more energy during my brisk walks.
Possibly the POTS / OI theory mentioned above could explain this: because I was doing a lot of repeated crouching down then standing up (as you would probably do during weeding also), and this might have been beyond my autonomic nervous system's ability to cope with all these sudden changes in gravity-induced blood pressure.
The other explanation that comes to mind is that during cleaning I was using muscles that I normally do not use much, and maybe this takes a higher toll than using muscles which are toned from frequent use, such as the muscles employed during walking (I go for a brisk 40 minute walk at least 3 times a week, so my walking muscles are reasonably toned).
Maybe muscles that are toned from regular use become more energy efficient, thereby having more resilience to PEM?