HRV: The Endurance Athlete's Complete Guide

Heart Rate Variability (HRV) can help sports performance in a number of ways. It can help us understand how stressful a workout is (how much physical work the athlete does); and how strainful a workout was (how much the stress applied affected the athlete’s body).  HRV also plays a key role in telling athletes (and their coaches) how much recovery they need. AS we know, reaching peak performance is all about the “stress-recovery balance”!

HRV – what is it? 

Heart Rate Variability (HRV) is the measure of the variation in time between heartbeats. It’s a relevant marker reflecting cardiac modulation by sympathetic and vagal components of the autonomic nervous system (ANS). Unlike basic heart rate that counts the number of beats per minute, HRV looks at the exact changes in time between successive beats, and the balance between sympathetic (intense fight-or-flight) and parasympathetic (rest-and-digest, which relaxes the body and mind) tone. The ANS balances these 2 forces so you can respond appropriately to daily stressors, and helps regulate heart rate, respiration and digestion.

Although historically the clinical application of HRV is associated with the prediction of sudden cardiac death and assessing cardiovascular and metabolic illness progression, HRV is now becoming one of the most useful tools for tracking the time course of training adaptation/maladaptation of athletes and in setting the optimal training loads leading to improved performances. 

Some situations result in an increase in variation (high HRV), while others cause the intervals between beats to stay more constant (low HRV). 

Low HRV: If the intervals between your heartbeats are relatively constant, then you are in a fight-or-flight state.

High HRV: If the interval length varies, you are in a more relaxed state. A higher HRV correlates with better health, resilience, and increased fitness.

Age CERTAINLY plays a role in what constitutes a ‘good’ HRV score:

What HRV can tell athletes?

A higher HRV correlates with better health, resilience, and increased fitness. It means that your body is reacting to stimuli and constantly adapting. 

A lower HRV indicates your nervous system is fatigued and not responding as quickly to the stresses of training. 

Rather than comparing yourself to others, it’s more important to follow your own HRV trends. Pay attention to the delta between days rather than the raw number. Once one establishes a baseline HRV, one can monitor positive and negative trends in fitness, fatigue, and readiness to compete. 

HRV fluctuations are normal and, in some cases, desirable. Expect dips in HRV and a downward trend of 5-10% when deliberately overreaching, but look for a rapid rebound once the intensified period ends, and during pre-race tapering. HRV can drop substantially when you get sick and can stay low even after you feel better indicating that your body is still recovering. Differences in hormone levels, diet, outside stressors and sleep quality also affect HRV. 

HRV decreases with the total stress load placed on your body. This includes:

a) Physical stress e.g. from workouts and manual labor

b) Chemical stress from poor nutrition or toxins

c) Mental / emotional stress from work deadlines, relationships, etc. (Porges et al, 1992)

Most importantly, HRV data can be used to determine when an athlete is less likely to perform at their best or even suggest a higher risk for injury. The sympathetic (fight or flight) response to stress focuses on short-term survival. This acute response can become chronic in modern daily life with stresses related to work, relationships, financial, lifestyle, etc. Chronically accumulated stress from multiple sources can contribute to drastically reduced health and performance long term. 

We preach meditation, deep breathing exercises, yoga and even float tanks to control emotions and stress to improve HRV. Avoid road rage, do things that make you happy, follow a nourishing nutrition plan, avoid getting into heated arguments with your spouse, and foster healthy relationships - all have a positive impact on HRV. 

Therefore, HRV measures work best for evaluating trends over time and the use of rolling averages. Best not to focus on one low reading or one high reading. 

The Technology: 

The standard method to measure HRV is through an electrocardiogram (ECG) – The ‘R’ peak in the QRS complex measured by ECG marks a heartbeat. The time between two R’s is known as the RR Interval. This is measured when a stress test or a VO2Max test is performed with a 12-lead monitor.

 With advances in technology, we can now measure HRV using photoplethysmography (PPG). PPG measures inter-beat intervals or IBIs. This marks the time from the steepest increase in signal prior to the completion of the ventricular contraction (heartbeat). PPG determines the timing of cardiac cycles via continuous monitoring of changes in blood volume in a portion of the peripheral microvasculature.

Wearable sensors like Whoop and Oura, work by shining light through your skin, and measuring how much is reflected back by the blood traveling through your veins to determine how fast your heart is beating. This is how finger clips used in hospitals work, as with the pulse oximeters that many people have purchased through the COVID-19 pandemic. 

Because every optical HR sensor is slightly different and can be affected by everything from your skin tone, placement, and how tight the device is worn, there are quite a few variables at play when it comes to the accuracy of these sensors.

Optical HR sensors attached to wearables have been around for quite some time and it has been well established that they do not match the point-to-point accuracy of a chest strap to monitor interbeat variation required for HRV measures; however, they still do a fair job of obtaining your high, low and average BPM.

How should athletes measure HRV? 

Research has shown that when monitoring rested versus over-trained athletes there is little difference in overnight readings but waking up is enough stress to show differences. 

Take daily morning readings and be consistent so you can evaluate trends over time. Measure HRV every day, at the same time, in the same position (supine is best) to rule out confounding variables.  

One alternative to a wearable is to measure your HRV with EliteHRV.  You may use your compatible heart rate monitor chest strap.  As mentioned above, the chest strap can measure the interbeat variation most accurately.  Bear in mind HRV taken with a device that does not synch to Optimize, requires a manual entry. 

However, if you would prefer a wearable that is on all the time and you don’t want to think about taking an HRV measurement, the Whoop seems to be most accurate with the addition of more sensors and several peer-reviewed publications showing Acceptable agreement was found between WHOOP- and ECG-derived HR (Bellenger et al, 2021), while Oura is the most well studied with many peer-reviewed publications showing 98% accuracy for HRV measurements when compared to a medical grade ECG device (Kinnunen et al, 2020) while other studies show variation in some parameters (Cao et al, 2022). NOTE: conflicts of interest always must to be taken into consideration when reviewing these publications! 

Though there are no peer-reviewed studies in the literature comparing Whoop vs Oura, many individuals have reported on both and seem to show similar results for HRV (however differences in sleep have been significant - more on that in our next SLEEP guide!). 

I’ve also seen individual data showing better accuracy of Oura and Whoop vs chest strap on indoor rides vs outdoor rides, indicating that movement plays a role, as expected. 

The FUTURE: 

Although coaches and athletes can calculate athlete strain and recovery from heart rate data, power-based metrics can be more precise and accurate.

FasCat’s new Optimized technology leverages power meter and wearable data for a 24/7 representation of the athlete’s balance between stress and recovery.

Optimize is in development and will be available as an in app monthly subscription this Summer.  Subscribe to our email newsletter 👇 to be notified of its release!  

REFERENCES: 

Relation of high heart rate variability to healthy longevity

Usman Zulfiqar 1, Donald A Jurivich, Weihua Gao, Donald H Singer

Am J Cardiol 2010 Apr 15;105(8):1181-5.

Vagal tone: a physiologic marker of stress vulnerability

S W Porges Pediatrics 1992 Sep;90(3 Pt 2):498-504.

Wrist-Based Photoplethysmography Assessment of Heart Rate and Heart Rate Variability: Validation of WHOOP

Clint R Bellenger 1 2, Dean Miller 3, Shona L Halson 4, Greg Roach 3, Charli Sargent 3 Sensors (Basel) 2021 May 20;21(10):3571. doi: 10.3390/s21103571.

Feasible assessment of recovery and cardiovascular health: accuracy of nocturnal HR and HRV assessed via ring PPG in comparison to medical grade ECG Hannu Kinnunen 1, Aleksi Rantanen, Tuomas Kenttä, Heli Koskimäki

Physiol Meas 2020 May 7;41(4):04NT01.

Accuracy Assessment of Oura Ring Nocturnal Heart Rate and Heart Rate Variability in Comparison With Electrocardiography in Time and Frequency Domains: Comprehensive Analysis

Rui Cao 1, Iman Azimi 2, Fatemeh Sarhaddi 2, Hannakaisa Niela-Vilen 3, Anna Axelin 3, Pasi Liljeberg 2, Amir M Rahmani 1 4 5 J Med Internet Res 2022 Jan 18;24(1):e27487. doi: 10.2196/27487.

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