For people living with energy-limiting illness, too much exertion can cause a delayed worsening of symptoms. This is frequently referred to as Post Exertional Malaise (PEM).

Heart rate monitored pacing is a management strategy that can help reduce PEM. It uses heart rate data to help you find the optimal balance of activity and rest, and therefore reduce the chances of symptom flare-ups or 'crashes'.

Published research shows that for people with PEM, their bodies' energy systems don't function as it typically should.

The body ordinarily uses two basic energy systems for physical exertion: short-term and long-term. The short-term energy system lasts less than two minutes does not use oxygen (anaerobic). The long-term energy system uses oxygen for activities lasting more than two minutes (aerobic). These energy systems work together because the long-term energy system also "pays back" the short-term energy system when it becomes depleted.

The physiological threshold at which the body changes from using long-term to short-term energy systems is referred to as the anaerobic threshold (AT).

In people with PEM the long-term energy system doesn't work well, so crossing the anaerobic threshold happens much faster and the “payback” takes much longer. This means that even light activity can cause a reaction, similar to a severely overtrained athlete, and a worsening of symptoms with a longer recovery time.

Alerts act as your personal guide, signaling when you're crossing your anaerobic threshold and potentially over-exerting. Real-time notifications can help you to consider slowing down or taking a break to reduce the chances of a crash.

These alerts can be helpful for two reasons:

The key aim of heart rate monitored pacing is to help you balance your rest and activity so that you can do more, with less.