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One of the worst things about COVID-19 is the uncertainty—even experts don’t know who’s more likely to get it, why, when, how, and why some people have severe symptoms and some none. The human immune system in general is outrageously complex, influenced by a staggering number of variables that fluctuate by the moment. How an athlete reacts to training is also incredibly complex, with lots of variables, lots of uncertainty.
Can one metric, like Heart Rate Variability (or HRV), crunch all those variables and all those unknowns into a meaningful message?
“Would I use HRV to gauge my susceptibility to COVID? Oh God no.” That was Shawn Arent’s oversimplified answer to an oversimplified question. That doesn’t mean Arent, chair of the Department of Exercise Science at University of South Carolina, has no use for HRV. His short answer demonstrates a point about the metric—if you oversimplify a complex subject like HRV, to say nothing of the universes of immunology and virology, you get only part of the story, you lose context and meaning.
Let’s back up. HRV, the new darling of competitive athletes, measures the variation in distance between peaks in the heart’s wave pattern over a period of a minute or two or five. Heart rhythms should, in rested and healthy condition, vary significantly in response to what’s going on around you. High variability is usually associated with an autonomic nervous system in its rest-and-digest mode, meaning your body has recovered from a previous workout. Low variability is associated with the sympathetic system—a body in fight-or-flight mode—so, not recovered. TL;DR: high HRV = good; low HRV = bad.
“HRV has been touted as useful because it’s very sensitive to change in diet, sleep, stress, a huge number of variables. And therein lies the downside—it’s highly susceptible to noise, so interpretation isn’t so simple,” Arent said. “Let’s say an athlete’s HRV is low but she actually feels good. Maybe the low score is because she ate dinner late the previous night. Should that guide what she does that day? We don’t know really what’s causing that low number. Using HRV by itself can give a misconception. It’s a useful tool but not as a stand-alone.”
Here’s how Arent recommends using HRV for training purposes. First, always measure at the same time of day, right before bed or, better yet, immediately on getting up. Standardize the conditions as best you can. Look at trends over time, an entire training cycle for example, lining up HRV with what you’re doing in that same time period.
“I use HRV along with other monitoring tools,” Arent said. “Heart rate is still a very useful training tool, particularly within zones matched to your paces or to gauge energy expenditure. I like biomarkers—blood-based measurements like testosterone, hemoglobin, hematocrit, iron, cortisol, and others—to see how the system is repairing itself. You can also measure changes in fitness variables, like VO2 max or ventilatory threshold. And how about this—simply ask athletes how they feel and look at their performance! HRV is more confirmatory than predictive the way it’s typically used. Don’t get caught up in its daily fluctuations.”
When you add the complexity of the immune system to noise tendencies of HRV, it’s not a definitive measure of immune strength. For example Arent pointed out, inflammation may trigger a low HRV, but not all inflammation is bad. Any high-intensity workout will trigger a low HRV for a period of time, but that by itself doesn’t necessarily mean your immune system is compromised because there are myriad other factors involved.
“The pillars of good training at any time—sufficient calories, enough protein that you’re rebuilding, carbs, hydration, adequate sleep— these are still the things that ensure a strong immune system,” Arent said.
And as for using HRV as an indicator of coronavirus vulnerability “Does a low HRV mean that you have COVID or that your system is vulnerable or that you just had an argument with your spouse? We don’t know,” Arent said. “This might not be the time to crush training. And as far as coronavirus goes, nothing beats washing your hands and staying distant from others. Now is the time for healthy habits and overall wellness.”