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Analyzing power files doesn’t make you faster. Training Stress Score (TSS) and Training Stress Balance (TSB) charts cannot say for certain whether you should go hard tomorrow or not. And a simple heart rate monitor won’t tell you when you are sick until it’s too late. But heart rate variability, or HRV for short — that’s a metric that could well be useful.
Devices such as the Wahoo TICKR, Oura Ring, and Polar H10 can now provide insights into your underlying cardiovascular system that, just a few years ago, were limited to only the high-cost, high-tech world of experimental medical science. But what is HRV, and why is it so valuable to us athletes, as well as the general population?
What is heart rate variability (HRV)?
HRV is the variation in time intervals between each heartbeat, measured in milliseconds. In other words, it is the regularity (or lack thereof) of each consecutive heartbeat.
Even at rest, an individual’s HRV can be quite substantial. In general, a higher HRV indicates a well-functioning heart, and a body that is adapting well to physiological stress. This is important to note, as HRV differs from resting heart rate (RHR). A high RHR is indicative of high stress and low cardiovascular fitness, while a lower RHR is indicative of a strong heart, and a healthy, fit individual.
HRV (along with many other physiological metrics such as RHR) is affected by much more than the individual’s training load. Alcohol and drug use, poor diet, and high levels of stress can all have profound effects on HRV, sleep, and overall recovery. Thus, HRV is an individual metric, meaning that you should never compare your numbers to someone else’s. Age, genetics, time of day, and current health status are just a few of the factors that affect an individual’s HRV.
It is most important to establish your own individual baseline, gathering HRV data from a number of days – preferably weeks – before looking for possible insights into training, recovery status, and signs of illness or fatigue. For example, an HRV of 75 ms means nothing without context. But for an individual whose baseline is 55 ms, a reading of 75 ms is likely a sign of good recovery and positive physiological adaptations. On the other hand, for an individual whose baseline is 90 ms, a 75 ms reading could be a sign of poor adaptation, illness, or fatigue.
As with almost all physiological data, the numbers are not a be-all-end-all; you can still perform at or near your best on a day when you have low HRV.
The Benefits of Using HRV & How to Use It to Optimize Your Training
Devices such as the WHOOP strap and Oura ring, which are affordable and effective for both athletes and the general population, can be used to glean important insights into our cardiovascular health, aerobic fitness, and responsiveness to training. Anyone with a smartphone can use HRV — all you need to do is strap on the Wahoo TICKR or slide on the Oura ring, connect the device to your smartphone, open up the app and poof … there’s your HRV!
Reigning XC Mountain Bike World Champion Kate Courtney has been using WHOOP to track her HRV for the last two years. In a recent conversation, she told me, “the WHOOP software helps make HRV easy to track over time, and integrates it in a way that makes big data usable in training decisions.”
HRV can be a useful early indicator of illness, overreaching, and adaptations to varying training loads. A low HRV may be a warning sign for injury, illness, or poor adaptation to training or stress. One study showed that athletes experience a drop in HRV 24 hours post-workout, after completing a high-intensity interval session. Other studies suggest that HRV decreases during a period of high training load, and rebounds following a period of rest.
HRV-guided training is perhaps the most important takeaway from this new technology boom. As opposed to pre-planned training (i.e. the traditional way of prescribing training, such as two days on, one day off), HRV-guided training involves prescribing intense interval sessions when the athlete’s HRV is above normal (high), and rest or recovery days when the athlete’s HRV is below normal (low).
Studies have shown that athletes using HRV-guided training achieved greater improvements in performance and higher VO2max values than those using pre-planned training. These data are especially interesting because both groups completed the same amount of training in terms of overall load and intensity. This means that for the same amount of exercise, the HRV-guided group got stronger, fitter, and faster.
Limitations to Using HRV
HRV alone is not always the best indicator of overall stress, physiological adaptations, illness, or fatigue. But when combined with RHR and sleep, HRV can be used to make “physiologically nuanced and better-informed” training recommendations. Kate Courtney, a big data nerd who loves tracking her training on the bike and in the gym, says that it’s just as important to keep an eye on how your body is adapting.
While HRV is a great way to keep track of recovery, she says, you “really have to pay attention to how you are feeling! No one data point tells the whole story … I only change my training if I see a trend in the data, and if the way I am feeling matches up.”
Based on the newness of HRV technology and its recent introduction to the general population, studies have been somewhat limited in their scope and subject pool. Most studies have been performed on generally healthy, fit individuals, but more data on the physiologically elite is still forthcoming.
Traditional training models rely on athletes and coaches to do all the guesswork. It is hard for an elite athlete to know when they are tired, and even harder for them to tell their coach the truth. The best athletes in the world are incredibly strong, resilient, and determined to succeed – to admit to tiredness and fatigue can weigh on the mind more than it does on the body.
HRV puts a number on it, quantifying the athlete’s stress in a way that they could never describe themselves, and providing both athlete and coach with unique physiological insights that can be used to prescribe the most efficient and effective workouts that the world of sports science has ever seen.
About the author
Zach Nehr is a Level 3 USA Cycling coach, a Cat 1 cyclist and a graduate from Marian University where he received a Bachelor’s Degree in Exercise Science and Psychology. He is currently undertaking a Masters degree in Physiology.