Heart rate variability (HRV) is becoming more and more popular as a training tool for competitive athletes. The basic idea is that when HRV is high, an athlete is primed for optimal performance. HRV is one of the metrics used to determine WHOOP Recovery each day. However, when calculating Recovery, WHOOP takes the application of HRV to the next level.

From a new WHOOP white paper:

“The WHOOP Data Science team has expanded on the last decade of medical and academic HRV research to build a physiological monitoring platform capable of delivering athletic-performance optimizing analytics.”

Included in the paper is an analysis of WHOOP data in relation to a pair of studies run by Antii M. Kiviniemi in 2007 and 2010. Kiviniemi compared recreational athletes on a pre-defined workout schedule to a group prescribed varying degrees of activity depending on HRV levels (they were given more stressful workouts when HRV tested higher). Generally speaking, the Kiviniemi research suggested that training based on daily HRV measurements improves cardiovascular fitness at a greater rate than a traditional training method with a set schedule of workouts.

With data from athletes using WHOOP, our scientists demonstrated that the system’s Recovery calculations are largely consistent with Kiviniemi’s method. But unlike the Kiviniemi studies, there is more to WHOOP Recovery than just HRV.

WHOOP also takes into account resting heart rate (RHR) and Sleep Performance. Why is this important?

A training model using strictly HRV works for most people nearly all of the time. Normally, when HRV increases there is a corresponding decrease in RHR. But, every once in awhile that’s not the case, and WHOOP is able to make that distinction.

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As mentioned above, the Kiviniemi studies involved recreational athletes. However, there is a phenomenon that can take place with elite athletes, known as parasympathetic saturation, which the Kiviniemi research didn’t account for. Parasympathetic saturation occurs when one of the two branches of the autonomic nervous system, the parasympathetic, dominates the other, the sympathetic. When the human body is in this state, low HRV coincides with a low RHR. This also happens to be the time when the body is peaking physically.

With only HRV as an indicator, an athlete who attains parasympathetic saturation might see a low heart rate variability level and determine that it’s not a good day to train hard–a great opportunity would be lost.

Imagine a scenario where the coach of track and field team, for example, monitored the HRV of his runners and used it to help determine who raced on any given day. He’d misinterpret the signs if one of his athletes was in a state of parasympathetic saturation, and likely withhold him from competition. In reality, that runner was the perfect candidate to succeed. WHOOP Recovery would’ve shown this.

In addition to HRV and RHR, Sleep plays a big part in WHOOP Recovery. Energy levels reported by athletes using WHOOP correlate with better Sleep performances, but not with greater HRV. By combining HRV, RHR and Sleep Performance, WHOOP Recovery takes previous science to a new level.

The scope of what WHOOP is capable of to further advance our understanding of Recovery is fascinating. Kiviniemi’s studies used only a few dozen people. With every new athlete on the WHOOP system, the database grows, as does the possibility to expand our knowledge.