Heart Rate Variability (HRV) is a measure of the variance between heartbeats. Unlike Resting Heart Rate (RHR), which measures the heartbeats, HRV is tracking the time when the heart is not beating. For example, if your heart rate is 60 BPM, it does not beat once every second, but perhaps 0.8 seconds between two beats and 1.1 seconds between the next two beats.
HRV is modulated by the autonomic nervous system (ANS) and its two branches: the parasympathetic nervous system (PNS, or “rest and digest”) and the sympathetic nervous system (SNS, or “fight or flight”). A high HRV indicates that the ANS is well-balanced between the two systems, enabling the body to respond to physical and emotional stressors quickly and efficiently. When HRV is low, it suggests that the ANS is imbalanced, often with a dominant “fight or flight” response.
Let’s review the same two weeks of data, this time with HRV, to further understand. After the 1st of Sunday, there is a decrease in HRV. It’s not until the rest day on the 8th of Sunday that HRV increases again. The data mirrors RHR but with an inverse relationship—as HRV increases, RHR decreases. However, there are differences worth noting. In our experience, changes in HRV are a more acute response to training compared to RHR, which is a more long-term recovery metric. For example, on the 3rd of Tuesday, I had an active recovery day with yoga. We see a rebound in HRV to 97, but RHR still hovered at 43 BPM, indicating an acute recovery but not yet a long-term one. I responded to this on the 4th of Wednesday by doing an intense Zone 4 running session, which resulted in a substantial drop in HRV to 81.
When tracking HRV for recovery, it’s better to monitor trends over time rather than focusing on day-to-day fluctuations, as you might with RHR. Additionally, it’s important to understand that HRV is highly personalized, so it’s more beneficial to track your individual numbers than to compare them with others. Below is a WHOOP study showing average HRV by age. Athletes and performers with a statistically low HRV score need not be alarmed. We simply recommend incorporating more parasympathetic activities, such as yoga, walking, or resonance breathing.
To track HRV, it’s easiest to use a fitness device like WHOOP, Aura Ring, or Garmin, but there are also apps like Elite HRV that help you record your score. For more information on how to increase your HRV, check out this article.
