Podcast No. 29: Heart Rate Variability (HRV), with Kristen Holmes and Emily Capodilupo

In this episode we tell you everything you want to know about HRV--heart rate variability.

WHOOP Vice President of Performance Kristen Holmes and Director of Analytics Emily Capodilupo are back to give you a full break down of HRV. Kristen and Emily explain what HRV is, how it’s measured, and why it’s an indicator of your overall fitness. They also discuss how WHOOP uses HRV to calculate your daily recovery and optimize your training, including factors that affect it and behaviors and lifestyle choices that can help you improve it. Heart rate variability is one of the core metrics behind a lot of what we do here at WHOOP, so I hope you’ll find this podcast to be a must-listen!

Heart Rate Variability (HRV) Podcast Show Notes:

3:39 - Definition of Heart Rate Variability. “It’s literally the variability in the timing between the beats of your heart,” Emily says. If your heart beats 60 times in a minute, it’s not once every second--sometimes it could be 1.2 seconds, sometimes 0.8, etc. “That variability comes from competing inputs from your nervous system.” 4:22 - Sympathetic and Parasympathetic, the two branches of the autonomic nervous system. “The do stuff, activating part [sympathetic], and the rest/digest, slow down, recover [parasympathetic]. When both are giving instructions to the heart it causes variability,” which is a good thing, Emily explains. “When those systems are well balanced you see a lot of variability because they are both getting their way. … HRV is a signal of your nervous system being balanced.” 7:06 - Better Metric than Just Heart Rate. Why is HRV an important metric and relevant beyond your resting heart rate? “Because it’s your nervous system manifesting in your heart,” Emily says. 8:05 - Parasympathetic Saturation. “If you only had heart rate, you would completely miss a valuable signal,” Emily says, when HRV and resting heart rate both trend downwards. Parasympathetic saturation “is a pretty rare state you see for the most part in highly trained endurance athletes,” and is a sign they are ready to peak. 10:11 - Resting Heart Rate & HRV as Fitness Indicators. “When your heart rate goes down, it means that each heart beat is more effective.” Emily explains. “When your HRV is higher there is more capacity to allocate resources towards exercise.” HRV naturally goes down while you’re exercising hard. “The more you’re not sympathetically dominating the more room there is for sympathetic activities to come in and dominate.” 12:59 - What Things Affect HRV? Activity level, stress, fatigue, illness, hydration, alcohol, and more. “It’s one of the most sensitive metrics that there is,” Emily states. Which makes it powerful, but also tricky. “HRV changes a whole bunch day-to-day, but also within the day.” 16:23 - HRV Variance. “You want to send the right signal to your body at the right time based on what it is you need to do,” Kristen says. “Your goal is not to have maximum HRV at every second of every day,” Emily adds. “Resources need to go to stuff. But if you start to understand how different things affect your HRV, you can start to manipulate the timing of these things relative to moments you need to peak.” 17:27 - We Choose Not to Show HRV 24/7. “It’s a complicated and noisy metric during the day,” Kristen says. “While we do take it 24/7 throughout the day, we chose to measure it during sleep, when we can mitigate the noise of all the inputs that go into HRV.” Emily adds, “Looking at average HRV throughout the day becomes not that meaningful. … We want to avoid creating more data for our users that is not actionable and doesn’t add anything.” 19:40 - WHOOP Tracks HRV During Sleep. Previously, the norm for top researchers (Emily mentions Daniel Plews and Martin Buchheit) was to take HRV first thing in the morning, but they admittedly did that for convenience, not because it was functional. “Our whole sleep platform was inspired by the need to get the most perfect HRV reading for the day that we possibly could.” 21:09 - And Why Slow-Wave Sleep? “Your HRV changes a ton during sleep. … During slow-wave sleep you’re the most dead to the world,” Emily says, so it’s the best time to compare HRV on a night-by-night basis. Changes is respiratory rate during other stages can affect it, but during SWS we “maximize the likelihood of getting a clean signal.” 23:04 - HRV & Recovery. “The [WHOOP] recovery score is primarily based on HRV,” Emily says. We also use resting heart rate and sleep to some extent, but she explains why sleep isn’t weighed that heavily into the equation. “When it’s misleading, it’s so misleading,” like if you’re sick or hungover. “We let HRV and resting heart rate tell that story.” If you don’t get quality sleep your HRV and RHR will be adversely affected anyway, so sleep can be redundant. Conversely, if you do get a lot of good sleep, we’ll see the positive effects in HRV and RHR. 26:42 - Day-to-Day Fluctuation. “Variability in heart rate variability is a good thing,” Emily states. Being “in the red” can be beneficial sometimes. “Functional overreaching followed by constructively resting.” 30:07 - What’s a Good HRV for Me? “I hate that question,” Emily says. “We do see HRV declines dramatically with age, if you’re older, you’re HRV is probably lower. We see slightly lower HRV in females than in age-matched males. … And we do see higher HRV in endurance athletes than strength-based athletes.” We discourage athletes from comparing HRVs with each other. “It’s only meaningful when you’re looking at your own data, day after day after day.” 32:11 - HRV Trends Over Time. “It’s me vs me, and what is my HRV today relative to my baseline,” Kristen says about how best to analyze it. Emily adds “That’s why we grey out recovery for the first 4 days on WHOOP, it doesn’t mean anything until you’ve established a trend.” 33:00 - Lifestyle Choices to Improve HRV. “After drinking alcohol it took four days for one collegiate team to get their HRV back to baseline,” Emily says about a study we did in 2016. Kristen adds “Once you understand the factors that influence your performance, performance is no longer a guessing game, it becomes a choice.” 36:42 - Hydration. “Hydration determines your blood volume, the more liquid in the system, the less hard your heart has to beat to circulate oxygen and nutrients,” Emily explains. “Generally speaking, almost an ounce per pound” per day, Kristen recommends. 39:19 - Nutrition. “I think it goes back to sending the right signals to your body relative to what you’re going to be asking of it,” Kristen says. The timing of nutrition affects HRV, and eliminating something your body has an aversion to (gluten, for example) can also improve HRV. “It’s highly personalized,” Emily adds. 42:30 - Sleep. “Meeting your sleep need in obviously huge.” Kristen says. “Anything that’s going to make it hard to sleep is likely going to harm HRV,” Emily adds. 43:40 - Other Factors. Psychological stress, even “being physically comfortable” Emily says, like not being too hot or too cold. “It’s really, really hard to sleep when you’re hot.”   Learn More: The Ultimate Guide to Heart Rate Variability