Heart rate monitoring, pacing & ME/CFS

A good basic overview of heart rate monitoring is here:
http://www.cfidsselfhelp.org/library/pacing-numbers-using-your-heart-rate-to-stay-inside-energy-envelope

Case study from the originators of the technique:
https://www.healthrising.org/blog/2013/08/13/heart-rate-monitor-program-improves-heart-functioning-in-chronic-fatigue-syndrome-mecfs/

Research paper describing the approach (relatively technical & dense, but might be useful for sceptics!):
https://academic.oup.com/ptj/article/90/4/602/2888236

There is also a very supportive Facebook Group:
https://www.facebook.com/groups/ME.CFS.HRM/
Not quite as active as a year ago when I got involved, but still good.  There is a wealth of knowledge in the posts and the files, but it can be hard to sort through ...  If you post a specific question you are likely to get a helpful answer.

As for heart rate monitors (HRMs), you have a few options. You need something that will set off an alarm when you go over a certain heart rate (50-60% of max HR, usually about 100bpm).  It's also useful if it is waterproof so it can be worn in the shower, which can be the highest exertion a CFSer regularly does (I sit down in the shower now to keep my HR down).

* most accurate and cheapest: chest-strap sensor (e.g. Polar H10), sync'ed via Bluetooth to your phone.  Chest straps are very accurate, but some find them annoying to wear all day.  I haven't used this method so am not sure what apps are available that will sound an alarm when you go over a specified threshold.  You can also use a chest strap for advanced heart rate variability (HRV) monitoring.  HRV is used by athletes for identifying if they are over-training or not, and CFSers use it in a similar way.

* optical wrist sensors are incorporated into FitBit-type devices.  These are comfortable to wear 24/7, and also count steps and monitor sleep.  Optical sensors are not as accurate as chest straps, and can have quite a lag, which means they aren't very accurate when the HR is variable.  They also give bad readings when moving your fingers.  For this reason I wear mine on the ankle!  (Looks like a gaol bracelet, but then it kinda is ...  Also, you need slim ankles to do this.)  Having said that, I think they are accurate enough for our purposes. Can't be used for HRV though.  Polar now has a product designed to be worn on the upper arm, the OH1, but I'm not sure if it has a display or alarm (have to sync to phone).  I use a Polar a370 and am quite happy with it, though I have had to send it back twice for replacement (I think I've been unlucky).  Only the FitBit Ionic and Versa have an alarm, and FitBits have better sleep monitoring than other brands.  Also, someone has written a HR Pacing app which is available for FitBits!  Not sure if they are waterproof though.  Garmins are another option - some people use the Vivosmart 3.