Determining fatigue and state of readiness has long been a holy grail for endurance coaches. Accurately gauging these aspects would allow athletes to push harder when they are more ready and back off when they are more fatigued. Among the dizzying array of scores and assessments out there, Heart Rate Variability is one that seems most promising to me. It’s not a new metric, but there’s new and evolving evidence for its utility and value for athletes.
Assessing Readiness and Fatigue
Various philosophies have tried to encapsulate readiness and fatigue. Early in my career, we used a resting heart rate philosophy. When resting heart rate went up, that was an initial sign of fatigue. When resting heart rate went down, that was a sign some positive adaptation was taking place. Technology started adding sophistication to this simple morning measurement. Systems like the Omegawave emerged as early as the 2000’s. They promised that by looking at heart rate, DC potential, heart rate variability and other resting metrics we would have a better gauge on when an athlete is ‘ready’ or not. Fast forward to today and devices like Whoop and Oura offer a frictionless way to collect resting data and alchemize it in an attempt to answer the question, “Should I work out hard today or not?”
What started as a simple morning measurement taken with your fingertips and a stopwatch has been muddied by proprietary algorithms, inconsistent collection practices, inaccurate collection methods and naive interpretations (i.e., higher/lower is better, more data is better). Recently, people fixated on Heart Rate Variability (HRV) as a primary means to gauge ‘fatigue’. The quotes are intentional to emphasize the vagueness of the word. Nevertheless, many of the recovery and readiness scores we see today use HRV as one or even a core component of their algorithm.
Meet Marco Altini, PhD.
With that as a backdrop, I asked Marco Altini, PhD to lead a coaching education session for CTS Coaches and arm them with the latest information on HRV. If you don’t know Marco, you should. He is quickly becoming the most sought-after expert in wearables and HRV. His app, HRV4Training, is an essential tool for many elite and everyday athletes. Bottom line, follow Marco if you want to know about this stuff (his blog, in particular, is a go-to resource for me).
Heart Rate Variability is a tool I’m actively using with most of my athletes. Below are the practical points and some interpretations I took away from the session.
Heart rate variability is simply the measure of inter-heartbeat variation for a given period, reflected by what is known as the R-R interval (see figure below). Although your heart might beat at a rate of 60 beats per minute, those beats do not happen perfectly every second on the second. There is variability, and the variability of the timing between each heartbeat is important.
Measuring heart rate variability provides a noninvasive window into how well your autonomic nervous system (ANS) is functioning. The ANS is a regulator with the important job of helping to maintain homeostasis. Steadiness (less variability) is not necessarily the goal because when you are fatigued, your autonomic nervous system cannot adjust to internal and external stimuli as effectively. Within this line of reasoning, we can utilize HRV to monitor training load, fatigue, and potentially how well you are adapting to training. Negative adaptations to long-term training can be associated with reductions in HRV. An increase in fitness (positive adaptation) would be associated with increases and/or stability in HRV indices.
Requirements to Use Heart Rate Variability Effectively
To utilize HRV you must be able to commit a few things. In total, they take 2-3 minutes out of your morning routine.
- Establish a baseline. HRV does not provide valuable information with one singular reading. Instead, it carries the most weight when you can monitor positive and negative trends. As with many other metrics in exercise physiology, for performance, context is key.
- Take effective readings. In addition to daily readings, it’s important to also utilize the best measurements possible. Although nighttime averages presented by Whoop and Oura are growing more useful, I still prefer to have athletes take readings first thing in the morning, after going to the bathroom, and while in a seated position. Read more about why these specific conditions are important
- Understand HRV is not cut and dry. Like any training metric, HRV is not perfect. An increase in your HRV (indicating more variability between heartbeats) is not always positive. A decrease (indicating less variability) is not always negative. Several times throughout Marco’s presentation, he emphasized that part of the issue with using HRV are ‘naive interpretations’, namely the fallacy of ‘higher is better’. Utilize context and monitor trends over time. If you start using HRV, commit to gathering data for at least 4 weeks before making big decisions.
- Combine with sleep and subjective data. In its best context, HRV should be combined with sleep and subjective data on soreness, motivation, and your interpretation of performance. This provides more useful clues as to whether and how to adjust future training.
The Value Proposition
Going back to the intro to this article, we must understand WHAT we are actually going to do with HRV data.
Throughout the first half of Marco’s presentation, one theme remained clear: acute depression in HRV is an indication the body will not be receptive to hard and/or high intensity work. Great! Now we have an indicator telling us (in conjunction with other variables) to go easy! But can the opposite also be true? Can HRV be an indicator of ‘readiness’ or the body’s ability to accept and adapt to increasing workloads? I posed this question to Marco and the answer was a tepid ‘probably not’. He elaborated by explaining that although long range stability and perhaps gradually increasing HRV are indicators of long-term positive endurance adaptations, acute positive changes in HRV likely cannot be used to prompt a big training stimulus, like a back-to-back block or a really long run.
OK, so now we know the fundamental value proposition: By using HRV, you are trying to prevent the negative. You have data that either tells you that you are going to have a poor workout, or not adapt to it. Remember that. I’ll bring it up again later!
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You are trying to prevent a big effort from turning into a nothing burger or, even worse, creating maladaptation. This framework is important because it creates the right context for the following new information.
Practical Use Case for Heart Rate Variability
With the benefit of better information, here’s how I use HRV myself and with athletes I coach.
- Take HRV in the morning, after using the bathroom, and in a seated position. You can either use a HR monitor or the camera on your smartphone (personally, I use the latter).
- Go through HRV4Training’s subjective questionnaire (see screenshot below). I don’t customize any of the tags, but you can if you prefer.
- I ask athletes to text me if you they see a yellow or a red flag (HRV4Trainnig uses a green, yellow, red flag system to continue training, limit intensity and rest).
I like this framework because it’s easy (takes 2-3 minutes in the morning), consistent, and actionable. The questionnaire uses a ‘better/worse/same’ scoring system, which I find more reliable than a 1-10 system in this context. I care more about whether your muscle soreness is better or worse today than it was yesterday, not necessarily an 8 out of 10. The subjective questionnaire, as well as the HRV scores, automatically port into Training Peaks. Then, I can see everything side by side and decide on future training.
I take all three points of data into the decision-making process: HRV, subjective score, and upcoming training. From that, the athlete and I make one of three decisions:
- Keep training the same
- Rearrange the training. Namely, swapping an upcoming hard workout for an easier one
- Eliminate a hard workout and replace with an easy run or take a day off
I’d like to emphasize that at the end of the day there’s a human (me!) analyzing the information, considering the context, and then making a decision. I don’t let the algorithm determine the athlete’s destiny. We make that decision collectively by weaving together the physiological data, subjective scoring, and the context of training.
In total, I find that I typically alter 1-2 workouts about every 2 months. That might not seem like a lot. However, consider that substituting an easy workout for a hard one might prevent a few days of declining performance after you’ve dug yourself too big of a hole. Then you can see the value proposition of making small changes compounds by preventing further damage.
One gigantic caveat
By now I may have convinced you that pressing your finger against your iPhone camera is the easiest and most effective way to determine whether you should keep or skip your hard workout. Good, you’ve been paying attention. Before you do that, though, there’s one big caveat.
If you are going to use HRV to advise training, remember that it’s essentially going to tell you whether or not your next workout will stink. That’s the value proposition I mentioned earlier. HRV is going to tell you when you are likely to perform poorly or not adapt to the training. It is not going to tell you when you are ready for a PR!
That caveat is most important before a race. What if your HRV goes down the tubes the morning of the biggest race of the year, say Ultra Trail du Mont Blanc (UTMB)? This happens so much that many Olympic coaches have started to blind their athletes to the data in advance of the Olympics or World Championships. Life stress, sleepless nights, and anxiety can all negatively affect HRV, no matter how good your taper was. Just ask Katie Schide, winner of the 2022 UTMB and one of the athletes I’m honored to coach. Below is her HRV graph in the several days leading up to UTMB. The big dip with yellow bars are Aug 25th and Aug 26th the, day before and the day of UTMB.
Viewing her graph on the morning of August 26th, if it had been anything other than race day for UTMB, the reasonable decision would have been to skip the ‘workout’ planned for that day. Obviously, that was not a viable option. The point is, if you are using HRV, be realistically prepared for that scenario. Before your race, you can blind yourself to the data and live in a temporarily oblivious state. Or, you can do what Katie did and say ‘f*ck it’ and go win the biggest ultramarathon on the planet!