Subjectively it feels that a simple measure like calories per unit of time does not correspond to what ever culminates in triggering PEM. Other things interact like the novelty of the activity, additional sensory stimuli or orthostatic issues. For example walking x metres on level ground alone, might not trigger PEM but walking x metres over uneven ground whilst talking to someone might. A subjective, in a non scientific sense, feeling of effort seems to relate to triggering PEM over and above what theoretically could be measured in joules.
Also units of whatever cumulates to trigger PEM, again subjectively, does not seem to have a simple relationship with time; for example walking a hundred metres in one go might trigger PEM, but walking ten metres ten times over a period of time might not. However doing different things, each alone does not trigger PEM, over a period of time may cumulatively trigger PEM. It is not obvious whether the relevant time period is hours or days, whether interspersing rest impacts the consequences of activity, what the impact of switching between types of activity (physical, cognitive, etc) is.
Also in relation to PEM, either pre triggering or when in ‘recovery’, what are the vital components of rest. Some years ago I experimented with the idea that periods of lying flat on my back in a darkened room with out any distraction, sound, etc and trying to empty my mind of any thoughts might enable me to increase productive activity in any time period. Although this may be a necessary response to PEM once triggered, I found trying to do this preemptively was anything but restful and messed up my psychological state, I found such preemptive rest required some form of distraction to be achievable, that is som low level cognitive activity made the rest more restful.
