Heart rate variability is now a standard feature on running watches, rings and chest straps, and many runners use it to decide whether to train hard or take it easy. A guidelines paper published this month argues that its popularity has got ahead of the science. The paper, Guidelines for rigor and reproducibility of heart rate variability within human cardiovascular research, was written by a panel of cardiovascular and exercise physiologists. It sets out how HRV should be measured and what it can and cannot tell you.
The panel's main conclusion is that HRV has some value as a general indicator of cardiovascular risk. It specifically rejects the idea that HRV measures cardiac sympathetic activity or the balance between the sympathetic and parasympathetic nervous systems. The authors also say that describing it as a measure of 'vagal tone' overstates what it shows, because common time-domain and spectral measures largely reflect how breathing affects cardiac vagal activity.
For anyone using a wearable, the most useful section covers the things that change the reading. Age, sex, race and ethnicity all shift HRV, and so do fitness and exercise, so population averages say little about any one runner. How the reading is taken matters too. A wrist or finger sensor using photoplethysmography (PPG) is different from an ECG chest strap, and the length of the recording, whether it is taken in a lab or at home, and how fast and deeply you breathe all change the numbers. The authors say the growth of wearable HRV makes these limits more important, not less.
The paper doesn't mean HRV is useless for runners. This week we covered a 24-week trial in which HRV-guided intensity beat a fixed training plan on the same training volume. Coaches who work with the data give the same advice as the guidelines: measure with the same device, at the same time and in the same position every morning, and compare each reading with your own recent baseline rather than with published norms or other people's scores. An earlier single-athlete study of a professional cyclist found that day-to-day HRV variation was two to three times higher during a Grand Tour than in normal training. That suggests a single reading can become too noisy to use once you are racing or accumulating fatigue over several days.
The paper is a narrative consensus document, not a new experiment, so it doesn't settle every argument about HRV. For runners, the practical point is to treat a low HRV reading as a signal to check how you feel, how you slept and how your legs are, not as an instruction to skip a session. The trend over several days tells you more than any single morning's number.