What is heart rate variability and does it matter?
Marco Altini highlights the importance of an overlooked marker of fatigue
As a cyclist experiences stress – physical or psychological – their body responds via changes in autonomic nervous system activity. This innervates the heart, affecting heart rhythm. This is why measuring heart rate and heart rate variability (HRV) gives us insights into the stress response.
HRV refers to the variability between heartbeats – because even at a steady heart rate, heartbeats are not uniform or entirely regular. Measured in milliseconds, HRV is the variation in timing between consecutive beats. As such, HRV captures information not present in heart rate alone, and is a more sensitive marker of stress. As a non-invasive proxy of our body’s response to stress, HRV allows us to better understand how we are responding to training and other stressors, enabling us to make meaningful adjustments. Aside from training, stressors that may have an effect on resting physiology include psychological stress, sickness, alcohol intake, travel, and general life demands.
Very strong stressors usually have a clear visible effect on our resting heart rate. However, more subtle stressors such as small changes to training are better captured by HRV. Everyone experiences stress, so HRV is a useful tool regardless of the level at which you’re training and competing. In fact, in many cases HRV is more useful to recreational athletes who need to balance training with work and other commitments, potentially dedicating less time to proper recovery.
It’s important to understand that HRV is not a replacement for a training plan, nor does it trump subjective feelings. HRV is only meaningful in the context of a solid training plan and in combination with how you feel. Starting with a plan, we can use HRV to make adjustments while paying attention to how the data corresponds to feelings of fatigue and freshness.
When choosing an HRV app or sensor, look for verification that the data it captures is equivalent to an ECG. If not, it may not be sufficiently accurate. HRV should be measured as soon as you wake up, while sitting relaxed. Wearables that measure HRV
“We make changes according to how data corresponds with fatigue and freshness”
automatically may fall short in this regard, especially those that provide only sporadic data points. Validated options include the app that I co-founded, Hrv4training, the Oura ring, the Polar Vantage, and Polar chest-straps.
Variable rate
Now to interpreting the data. HRV of between 50 and 100 milliseconds is typical for a healthy young adult, but the principle “higher is better” is too simplistic. Instead, aim to keep HRV in the normal range specific to you – which your software of choice needs to provide. An unusually high HRV (for you), together with a suppressed heart rate, might denote a state of fatigue, not a state of high readiness. This is why accurate interpretation really matters. A good HRV is not necessarily a high HRV but a stable one. Comparing your normal range with your daily score is the only meaningful way to detect a change in physiology on any given day.
A proxy for stress
Heart rhythm is influenced by a series of processes triggered by stress. HRV is a useful proxy for stress. However, to make use of HRV data, we need to collect accurate data, at a meaningful time, and interpret it with respect to an individual’s normal range. By doing so, we can obtain objective feedback on our response to training and other stressors.
This feedback can then be used to assess whether everything is going according to plan. Using a well-designed training plan and making Hrv-guided adjustments as necessary can lead to improved performance, as shown in recent research.