Heart monitoring? NO!!!!! Your body will tell you when you've gone too far (your brain might not so you need a kitchen timer for computer work). Heart monitoring is for those who don't realise they feel sick, ill, weak, yuk, tired. Your heart can work well while you feel like death. (I had 24 hour monitoring).
Heart rate monitoring is a non-literal life-saver for many ME patients. While many of us do often start to feel quite bad during or immediately after exertion, sometimes we don't, and sometimes we don't start to feel bad until we've gone to far and crash badly the following day. The immediate symptoms may be more indicative of Orthostatic Intolerance than PEM, and don't seem well-enough connected to triggering a crash to rely on those symptoms to avoid crashes.
I use a heart rate monitor as a pacing tool. In addition to symptoms and previous experience, it's a way to indicate that I've gone too far. I thought I was pacing as well as I could, but a heart rate monitor showed that my heart rate was getting too high doing basic tasks: walking slowly up 1 flight of stairs, bathing/showering, cooking, gardening, vacuuming, etc. I was able to use that information to restructure some of those activities, and pretty much had to abandon some others due to the lack of having a less strenuous way to do them. Now that I know how I react to certain activities, I can pace them better, such as doing 5 minutes of cooking activities every 30-60 minutes in my slow cooker. So I just use the monitor now for new or especially intensive activities, like traveling, shopping (in a wheelchair/scooter), or bathing.
For example, I felt awful after going upstairs, so would lie down until I felt a bit better before starting a shower. Similar with resting after a shower. But with a heart rate monitor, I realized my heart rate was getting up to 140-150 very frequently just washing myself, raising my arms over my head to wash my hair, drying myself, etc, even while sitting down and resting if I felt tired. And I usually would start to feel a little better after resting before my heart rate had properly recovered, so would resume activities too quickly. But without a heart rate monitor, I was pretty much inevitably crashing after each shower.
So now I rest every time my heart rate hits 100, so it usually stops rising before 110, and then start scrubbing again when I'm back down to 90 or under. In this way I can (slowly) shower and not crash afterward. Though from the "experience/planning" aspect, it's still going to be my only "big" activity for the day.
Basically heart rate monitoring has helped me avoid crashes almost completely, and to be a lot more consistent regarding my functionality. Which is nice, because then it's a lot easier to make plans (such as cooking dinner versus ordering delivery), and to now accomplish some other pleasant activities like cooking coffee for myself and my fiance, and usually cleaning the machine in the afternoon as well.
I almost never crash unexpectedly now. Which means I almost never crash, except in the very rare situation where I've over-committed to something. Even if my tolerance for activity changes from day to day, my heart rate monitor lets me see that pretty well. So if my heart rate is staying at 100 just from sitting up with my feet up, then no shower. And some days it'll get high too easily even if my resting heart rate is decent, so the shower might be a lot slower and less thorough than on other days. But the heart rate monitor is a key player in getting my through those activities without triggering a crash.
Therapists and doctors who profess belief in a bio-psycho-social framework for disease, yet curiously only treat psychological aspects. Basically the "polite" way to explain supposed psychosomatic disorders in a manner which confuses instead of offends unaware patients.