Do you know what fitness people like? Expensive gadgets that tell us about our fitness! Fitbits, Garmins, Apple watches and the like are our jam! So when a new data collection point shows up overnight we want to know about it, all about it. Apple watch at least got HRV data in this fall’s update but what the hell is it? Also is mine good? How good? Am I super-fit, kinda fit, overtraining or dying? I found at least that a simple google search wasn’t going to cut it on this one. But googling does tell you it is super important, maybe. So I decided to do a deep dive into the literature again and find out for you. Read on a by the end you’ll know what it is, what it measures, whether yours is good or whether you’re dying.
Definition: HRV or heart rate variability is a measurement of the differences in times between heartbeats. It’s calculated by taking the standard deviation (or other variation calculation) of time between successive heart rates. Basically what that means is your device is measuring the time between your heart beats and records that. Then it uses a formula to quantify the amount of change in the amount of time between hear beats. That means, are they all over the place or mostly the same with a few outliers. So basically if most of your heart beats are the same distance apart then your HRV is low and if the time between your heart beats is very different your HRV is high. PS it’s usually measured in ms, as is the variation if you are a unit geek like me.
How it’s measured: In a clinical setting it’s usually measured via ECG where the shape of the heartbeat is visible and this is the gold standard. The same points on successive waves are measured and compared to each other giving the most trust worthy results. Since it’s used in cardiology heartbeats not originating from the sinusoidal node can be excluded. But it can also be monitored by blood pressure or as your device does by detecting blood flow. That is to say if you want yours checked out further see your doctor for ‘better’ monitoring before you freak out based on something on your wrist.
Why do we measure it: HRV data can be clinically significant when added to pulse data alone. HRV data gives an idea of nervous system control of the heart. A heart with good nervous system control is able to respond and detect to small changes in demand for blood flow quickly and it does an effective job resulting in higher HRVs. A heart with poor nervous control either can’t detect these changes on demand or can’t respond to them effectively resulting in lower HRV numbers. So HRV data gives us an idea of how healthy your heart is and how healthy your nervous system is. That’s pretty cool!
What is the clinical use: There are a few actually but it’s a pretty new measurement so even within the fields it’s used in the importance isn’t totally clear yet. Arguably the most important use is that it is a good predictor of mortality after a heart attack. I say arguably for two reasons one that other experts offer that simple heart rate data is as good a predictor and that this data is only relevant if you have recently suffered a heart attack. Just because yours is low doesn’t mean your headed for disaster in the form of a myocardial infraction. It’s not a predictor of heart attacks just afterwards, so breath! Looking at HRV data can give ‘extra’ information in patients with congestive heart failure, diabetic neuropathy, depression, after a heart transplant. It can be a predictor of poor outcomes in premature babies or be predictive in cases of SIDS. That being said no data exists that says if your HRV is below value X make a will. It’s a new thing that we don’t fully understand yet and other measures are better predictors of outcomes in almost all fields at this point.
Can other things effect your HRV data: In a word, yes. Things beyond medical issues and fitness can affect your HRV data. Since HRV activity is related by the sympathetic (fight or flight) and the parasympathetic (rest and digest) nervous systems it’s data is of interest to psychologists. Under acute stress HRV levels are lower than when a person is relaxed and people who describe themselves as worriers tend to have lower HRVs. Basically that means if you are stressed in some way either mentally or physically your HRV will go down. But it is also an indicator or cardiovascular health and nervous system health. So the healthier and fitter you are the higher it is.
How it’s used in training: If you google HRV information you’ll find more than a couple of stories from elite athletes that used their HRV data to reach for new heights. And in a case of two how they did that. Basically it comes down to avoiding overtraining injuries like you might have suspected but also scheduling your workouts for when your body is ready for them. I think we all know how terrible it is to have overtraining injuries and that watching your resting heart rate is about the only way to see one coming objectively. Basically the wisdom goes like this. Don’t overtrain or you’ll wind up injured, so dial back if that’s happening. Well you feel really tiered, drug out, cranky and unmotivated. Isn’t that also just called marathon training? So you watch your daily resting heart rate if it’s ticking up your at risk.
Basically HRV does the same thing, but in reverse only maybe faster. HRV data is thought to respond faster than the resting pulse to overtraining in the body. So if you see your HRV is low on a given day your might be extra tiered. Conversely you might feel like skipping your speed work because it might be too much today but your high HRV for the day says you can get it done.
So how are the elites using HRV data to smash their records? Essentially as an alternative to weekly schedules. That sounds trivial, but it’s not because it’s working for them. Basically they and their coaches track the athlete’s HRV data and decide whether to push it on a given day, take it easy or dial it up or down for a while in general. And now you can do the same thing. Establish a baseline overtime ideally outside of a training cycle or look back over the last few months. Then when your training allow yourself to move your weekly workouts around in response to that day’s values. Is it high today? might be the right time for that speed workout. If it’s lower after a long run a few extra days maybe move your next hard workout to later in the week. Is it still really high after a hard workout perhaps you should push harder next time. And that’s it really but remember it’s working for the pros and it might work for you too.
So is yours high or low: That’s why you’re here right? So the extreme-ish values are 15 ms and 100 ms, however values are recoded above and below that. If your HRV is hovering above 80 most of the time you’re probably in pretty awesome shape. If your consistently around 20 ms you’re likely not in the best shape of your life and you might want to schedule a check-up. In general females tend to have lower HRV values than their male counterparts and it drifts downward as you age. For someone in their 40’s you would expect a HRV of about 60 ms and the value goes down about 6 ms for every decade you are alive. Here is a table of how you measure up by age. Remember 95% of people will fall within this range. But being consistently at the bottom of the range could mean a trip to the doctor is in order. PS does zero mean you’re already dead? chances are if you got a zero reading and your reading this now you have a problem with data collection.
* Twenty-Four Hour Time Domain Heart Rate Variability and Heart Rate: Relations to Age and Gender Over Nine Decades, 1998, Journal of the American College of Cardiology, Unetami et al.
So that is that. HRV and what it means. Keep in mind the way your fitness tracker measures it is viewed as the least accurate but there is a lot of data there. The clinical standard is 24 hours of observation.