Heart Rate Variability: What is it and How Should Endurance Athletes Use It?

How many tools are in your training and racing toolbox these days? Truth be told the market is saturated with gadgets, new technologies, new philosophies, algorithms, and promises to make you better, faster, fitter. As educated consumers, you need to evaluate new products coming to the market and decide their personal worth to your personal training and racing toolbox. Heart Rate Variability (HRV) is a recent addition.

Although (HRV) started as the means to monitor cardiac arrhythmias, over the past several years HRV has been commandeered by the athletic community as a training and recovery metric. Following the release of research articles, several companies started creating their own tools, applications, and proprietary algorithms for HRV monitoring geared towards athletes looking for ways to continually improve.Discerning athletes want to know, however, whether this new piece of information is actually useful.

So what is Heart Rate Variability (HRV) and what is all the fuss about?

Heart rate variability is simply the measure of inter-heartbeat variation for a given period of time. What this means, is that although your heart might beat at 60 beats per minute, those beats are not happening perfectly every second on the second. There is variation, and that’s a good thing. More variation generally means that the parasympathetic branch of your nervous system, the “rest and digest” branch of the autonomic nervous system, is doing its job as a regulator in your body. As it adjusts subconsciously to internal and external stimuli it’s reflected in the inter-heartbeat variation. Essentially, HRV monitoring gives us a window into how well our autonomic nervous system is working. This, in theory, could give us a way to measure how physiological stress, training load, and fatigue are effecting how our bodies may be recovering from a race or a particularly hard work out or training block. What makes HRV monitoring so appealing is that it is a time efficient, inexpensive, and non-invasive tool.

Generally speaking, once you’ve established your baseline for HRV you have a tool that allows you to monitor positive and negative trends in fitness, fatigue, and readiness to compete. It should be noted that HRV is a measure best used for comparison internally and not across a population, or rather, athlete to athlete. This is due to it being a highly personal metric that doesn’t correlate well between subjects especially in one day, single value, measures.

Most often higher HRV values correspond with a well-functioning nervous system because it means that your body is reacting to stimuli and constantly making micro-adjustments. On the other hand, when HRV measures are low, or are decreasing to below baseline, this is generally a sign that your nervous system is fatigued and may be responding in a more sluggish fashion to that same stimuli. However, this is where it gets complicated… increases or decreases in HRV are not always so cut and dry.

For example:

  • An increase in HRV with a decrease in RHR (resting heart rate) generally means an athlete is coping well with training.
  • An increase in HRV with an increase in RHR is generally a sign of accumulated fatigue unless it at the very beginning of a short training block.
  • decrease in HRV with an increase in RHR is generally a sign of accumulated fatigue unless an athlete is tapering and then it could be a positive sign for readiness to perform.
  • decrease in HRV with a decrease in RHR is generally a sign on prolonged low-intensity high volume training and if it cannot be reversed with rest could be a sign of being in an overtraining state.

Additionally, athletes can also see a dip to below HRV baseline values for a day or two after an especially taxing workout or race, and an increase during times of illness with the immune system is in overdrive. The exact opposite of what you would expect if HRV monitoring were a simple tool. Essentially, understanding how HRV changes day to day and over time on an individual level are important. Especially when positive and negative “scoring” days are not always what they seem. This makes HRV a tricky, but not impossible, metric if you have the time to put the measures into context.

So if you are going to use HRV what are the most effective ways to make it a good tool to have?

  • The No One-Day Rule.
    HRV measures work best for evaluating trends over time and the use of rolling averages. As mentioned above do not get hung up on one low reading or one high reading as these values do not always spell out a completely clear story. Interpretation is essential.
  • Take HRV readings immediately upon waking in the morning.
    Research (*link: https://www.ncbi.nlm.nih.gov/pubmed/16531900) has shown that when monitoring rested versus over-trained athletes there is little difference in overnight readings, but waking up is often enough stress to show who’s nervous system is not working as well (ie who is more fatigued). This also means it is important to stay awake during the readings in either a reclined or semi-reclined position upon waking in the morning.
  • Take frequent readings.
    To most accurately use HRV, you are going to be evaluating trends over time. Most applications work best with daily recordings as they are looking for slight changes from your normal baseline. This also means consistency is key. Try to take your HRV measure every day, at the same time, in the same position to rule out confounding variables. This means recording at least 5 days a week and not just after hard workouts, long efforts, and races.
  • Don’t use it alone.
    Like most training metrics, HRV is best used in conjunction with other information. The best tools to use with HRV will generally be resting heart rate, training load, and some subjective measure like sleep quality. Some applications account for this automatically by syncing with TrainingPeaks or Strava, but if using solo you’ll want to be cognizant of context.
  • Clinicians use 5-minute measures.
    If the application you are using gives you an option to chose the length of the reading, go longer. Most applications will give you an option of a 1-minute reading or an open/5-minute reading. The longer the reading the more clean data you can get. In many clinical studies researchers will actually take 10-minute long readings in order to obtain a really clean and consistent 5-minute chunk to analyze
  • Trust your technology.
    We are fortunate that over the past couple of years smartphone technology has advanced exponentially, in particular the cameras. Advances in cellphone cameras allow us to now use photoplethysmography to accurately assess HRV without any extra gear. Some Android phones or older phones do not have this capability, but most applications can also be paired with most bluetooth capable heart rate monitor chest straps. Keep in mind, however, that wrist-worn monitors, like GPS watches with build in HR monitors, are not accurate enough to monitor interbeat variation required for HRV measures.

So… is that a Yes or a No? Should you add HRV to the data you’re tracking or not bother? It really comes down to whether you’re the type of athlete who can be consistent with how you gather data and how frequently. Sporadic measurements don’t provide nearly as much value, and measurements taken at different times of day, in different body positions, can also make it more difficult to get an accurate and useful picture from the data. If you’re going to be consistent, then go for it!

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