Tuesday, February 2, 2016

An HRV Primer

This post is for those of you who may be intrigued by heart rate variability (HRV) but not quite ready to devote the time to diving head first into the fantastic but extensive three-part series on HRV that, our resident cardiologist, Dr. Larry Creswell wrote or the five-part (!) series that I did on my own blog.

I get it, you’re mildly curious about what all this HRV stuff is but not quite willing to pursue a Doctorate in the subject just yet. For you, this is a “just the facts, maam” kind of summary on what heart rate variability is and how you can apply it to improve the efficacy of your training.

What is HRV?
Heart rate variability refers to the quantifiable differences in time between heart beats. While we might assume that, if our resting heart rate is 60 beats per minute, that we have a constant 1-second spacing between beats, this is not the case. While the mean spacing (R-R interval) may be 1s, if we break it down, we’ll see sometimes the spacing might be 0.8s. On other beats, it might be 1.2s. This is especially the case if you’re feeling fresh and less the case if you’re feeling tired. This is the crux of the practical relevance of HRV.

To put it in terms that we’re all familiar with, consider your HRV file to be akin to a power file from your favorite group ride. On days that your legs feel fresh, you’ll see a lot more variability. You might go to the front more, or attack on the local hill or sprint for a city limit sign. On days that you’re fatigued, you might spend the entire ride putting out a constant wattage just hanging on to the back. Your heart (and the nervous system it represents) does the same thing. When fresh, it is very responsive to all types of stimuli. When tired it’s just doing its thing keeping you alive.

Why should we pay attention to HRV?
There are two key reasons to monitor HRV:

  1. To minimize the risk of overtraining
    Several studies have shown a link between low HRV and overtraining/under-performance symptoms (such as Pichot et al., 2000, Mourot et al., 2004, Tian et al., 2013).

    While performance modelling goes a long way towards minimizing risk by quantifying fatigue, it suffers from one key error in assumption: that fatigue comes solely from training load.

    In the real world, particularly the real world of working age-group athletes, there are many factors that come into play to determine just how tired an athlete is at any point in time, things like work stress, family stress, travel, poor nutrition, lack of sleep. None of these things are reflected in the athlete’s TSB but they are reflected in the athlete’s heart rate variability.

  2. To enable the athlete to get more performance “bang” from their training “buck”
    Related to the above, our system is not always at its best to receive a hard workout. While we may be able to “get ‘er done,” without falling into full blown over-training, if we don’t time hard workouts properly, the extended recovery time between sessions will significantly diminish the training effect of these hard workouts. In this sense, HRV can be used to identify when our body is at the crest of the wave and ready for work. In this sense, significantly improving an athlete’s training response.

    A study by Kiviniemi et al., (2007) showed the power in this approach, when over a four week training period they compared two groups -- one whose training was guided systematically by their changes in HRV each morning and another who followed a more traditional fixed training schedule. The traditional group improved their VO2max by about 2% over the four week block, while the HRV group improved their VO2max by 7%!

How do we begin to use HRV?
Okay, so I’ve piqued your interest enough to invest a little time or a little cash to begin incorporating HRV in your training. Where to from here?
A few options…

  • HRV apps
    If you’re willing to shell out a few bucks, there are a number of HRV apps that make HRV tracking easy and convenient. You’ll need a Bluetooth heart rate strap to start, such as the Polar H7

    Once you have your strap, you can collect and analyze the data via a number of apps. If you have an iPhone, I’d recommend checking out Marco Altini’s HRV4Training or Sweetbeat. On Android, check out Bioforce or Ithlete.

    These apps will all track your HRV in context of recent numbers and red flag low HRV days -- information that you (and your coach) can directly apply to your training in terms of, maybe moving a planned “hard day” if you come up against a red flag or noting multiple red flag days as a suggestion to rest, irrespective of whether a recovery week is on the plan.

    Note: Marco’s is the only app that offers straight up RMSSD. The others use proprietary scores. If you want to compare these numbers against the literature, I offer some tips here.

  • The “I have a Garmin 920XT” option
    Now that Garmin is opening up its platform for developers, I see a lot of cool stuff on the horizon for the newer units (920, 620, etc). One of these apps that falls in the “cool stuff” category is real time heart rate variability monitoring (without the need for a Bluetooth belt). If you go here, you can download the app direct to your 920 and begin tracking HRV. This method doesn’t offer the same level of contextual analysis as a lot of the apps, so I’ve provided a spreadsheet that you can use to track your numbers and identify those “red flag” days.

  • The low cost, higher time, “I’m a HRV geek” option
    Kubios is a fantastic HRV analysis program made available for free download from the University of Finland. It is set up to accept Polar and Garmin files (without the need for a Bluetooth HR strap) and will analyze and spit out a host of HRV metrics that add an extra dimension to HRV monitoring.

    While RMSSD and the associated proprietary scores of the various apps are a great starting point, there is a lot of additional info that can be had by diving a little deeper. Specifically into the frequency domain.

    Kubios (and a couple of the apps like Sweetbeat and the extensive OmegaWave system) will also provide you with a relative balance of the low frequency and high frequency components of HRV. I explore these in a bit more depth here and here.

    As a quick summary, these numbers can give you some practical guidance as to your body’s readiness for different types of training -- aerobic versus anaerobic. So, in addition to telling you if your body is ready for load today, by incorporating this dimension it can also give you some guidance as to what type of load it is most ready for.

    The (marginal) downsides to this approach are…

    1. After doing your test, you need to devote a couple of extra minutes to analyzing the file.
    2. It requires more interpretive skill in file analysis in terms of artifact correction. By the way, I can only assume that the algorithms in the apps are set to identify and eliminate artifacts at the maximal level to avoid confusion so I don’t see this as a negative. In fact, as you develop your interpretive skill the inclusion of this step will lead to more sensitive analysis of your HRV data.
    3. Like the 920 method, you’ll then need to keep track of your long term data somewhere to make identification of your red flag days easy. I’ve offered a spreadsheet here to simplify that process.

    My take: as a serious athlete, it’s your job to get to know your body as well as possible. This is five minutes a day very well spent!

Whichever method you choose, HRV has a lot to offer in terms of keeping you healthy and helping you get the most out of your training. I am, in general, not an early adopter of the latest trends or the latest technology but this is one area where I would encourage you to make a bit of an exception and explore. Particularly, in the time limited world of working age-group tri, I can see huge, perhaps even decisive, advantage to the incorporation of HRV in your training.

Train smart

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