Issue #14: Heart Rate Variability

✍️ Author’s Note

Welcome to Threshold Lab, where each week I break down one training topic, dig into the science, and give you actionable protocols to use immediately. I’m Stephen Pelkofer, an aspiring HYROX Elite 15 athlete with a men’s pro personal best of 59:41 and men’s pro doubles best of 51:39. Let me read the research, listen to the experts, and then give you the tools to make it work for you.

🏃 Beginner HYROX Endurance Program

I recently partnered with Christian Harris (former Crossfit Games athlete) Move Fast Lift Heavy to build an endurance program that is ideal for beginner to intermediate endurance and HYROX competitors. This is a 12 week program with simple but proven progressions that will get you ready for race day.

The program starts at about 80 minutes of running and builds up to 3-3.5 hours by the end, with a HYROX specific block in the 6 weeks leading up to a race. You can purchase the program on TrainHeroic here! You can also check out all of the MFLH programs available here.

👨‍💻 Introduction

Topic: Heart Rate Variability (HRV)

For endurance athletes, metrics like pace and power are familiar training guides. But there’s another number on your smartwatch or ring that could help fine-tune your training: heart rate variability (HRV). HRV is a measure of the difference in the time interval between consecutive heartbeats. While your wearable device might report your heart rate as 60 beats per minute (bpm), that doesn’t necessarily imply a uniform one-second gap between each heartbeat (humanperformancealliance.org). 

Can monitoring this metric help recreational runners and HYROX competitors train smarter and avoid burnout? In this issue, we dive into a recent meta-analysis on HRV-guided training and how you can apply the insights for better performance and fatigue management.

Main Reference today: Lundstrom C. J., Foreman N. A., & Biltz G. (2023). Practices and Applications of Heart Rate Variability Monitoring in Endurance Athletes. Int. J. Sports Med., 44(1), 9–19. DOI: 10.1055/a-1864-9726

⚡ Fast Finds for the busy reader

• HRV-guided training shows promise: Athletes who adjusted their workouts based on HRV saw slightly greater improvements in aerobic fitness and race performance than those sticking to a fixed plan.
  

• Better recovery, less fatigue: Athletes using HRV avoided the deep fatigue or overtraining funks that can halt progress.
  

• Trends trump single readings: Don’t obsess over one day’s HRV score. Day-to-day values bounce around with stress, sleep, and hydration. Instead, watch the trend. If your HRV plunges and stays low for 3-4 weeks, that’s a red flag for overtraining (humanperformancealliance.org).

🔬 Deep Dive

What did the meta-analysis look at? To test if HRV can truly guide better training, researchers combined results from multiple studies on endurance athletes. The recent meta-analysis pulled data from 8 studies (2007-2020) with a sample size of 199 athletes. Participants ranged from untrained and sedentary people to well-trained runners, cyclists, cross-country skiers, and other endurance athletes. In each study, one group followed a predetermined training plan, while the other used daily HRV readings to adjust training intensity (for example, doing a lighter session or extra rest if that morning’s HRV was significantly below their normal baseline) [1].

Key findings: After several weeks of training, generally both the HRV-guided groups and the fixed-plan groups improved performance, but the HRV-guided athletes often did slightly better. On average, those using HRV saw a bit more improvement in VO₂max and endurance test results than the predefined-plan athletes. The effect size was small in favor of HRV-based training, but it was consistent across studies. Notably, the HRV-guided approach significantly boosted the athletes’ resting HRV (parasympathetic tone), whereas the fixed training groups saw smaller gains in HRV. In practical terms, the HRV-focused athletes were not only getting fitter; their bodies also showed signs of better recovery and adaptability. Perhaps the most important note: fewer athletes in the HRV-guided camps hit the wall or regressed during training. The review reported a lower likelihood of negative responses (stagnation or overtraining symptoms) when using HRV to personalize training load.

Fatigue and performance balance: One reason HRV-guided training can help is by catching fatigue early. HRV serves as a flag for increased stress on the body – a sustained drop in your baseline HRV is often an early warning sign. In the studies analyzed, the HRV-guided groups would back off intensity or insert recovery days when their numbers trended down, helping them avoid that drained, overtrained state. Meanwhile, athletes on rigid plans might unknowingly push through low-HRV days, increasing their risk of stagnation. By using HRV as somewhat of a “check-engine light”, the guided-training athletes managed to mitigate fatigue while still making fitness gains.

Best practices for HRV monitoring: The experts are quick to caution that it’s not about any single day’s HRV reading. There are many factors that can impact your HRV number: poor sleep, dehydration, alcohol, stress. The review emphasizes establishing a reliable baseline and using rolling averages to inform training decisions. A smart way to do this might be collecting daily HRV data under consistent conditions for a few weeks to figure out your typical range. Once you know your baseline (say your 7-day average HRV usually hovers around 50 ms), you can notice meaningful deviations. A one-off low value isn’t cause to panic or cancel a workout, you might just be temporarily stressed. But if your 7-day average is significantly down compared to your baseline, it’s wise to dial back intensity for a bit. Conversely, a sustained upward trend in baseline HRV over months could be a sign your training is improving your aerobic fitness and recovery capacity (humanperformancealliance.org). As usual in the threshold lab, the key is to focus on trends… not single data points.

Are our devices accurate? As we’ve discussed in previous issues of the Threshold Lab newsletter, our devices aren’t always 100% accurate. I did not do a deep dive into this yet, but here is a promising study about the accuracy of the Oura ring for HRV and resting heart rate: Accuracy Assessment of Oura Ring Nocturnal Heart Rate and Heart Rate Variability in Comparison With Electrocardiography in Time and Frequency Domains: Comprehensive Analysis. You shouldn’t let your watch or ring determine how you tackle every single day, but these devices will keep getting better and more accurate as the tech keeps improving each year.

Time for some lab notes ⬇️

🧪 Lab Notes

In every issue of this newsletter, the “Lab Notes” are going to be the protocols that you can apply to your training and routine right away. The goal of this section is to translate the science into actionable steps for the reader, whether you’re a recreational runner/HYROX competitor, or someone pushing the limits of their peak potential.

• Build your baseline & track trends: Log your HRV daily and monitor the 7-day rolling average (most fitness apps will display this). After 2-3 weeks, you’ll know your typical baseline range. Don’t overreact to one-off fluctuations – focus on the trend relative to your norm. For example, if your long-term baseline is 50 ms, seeing 45 one morning isn’t alarming by itself, but a sustained drop into the 40s over several days might be meaningful. On the flip side, an upward trend over a training block (averaging 55-60) could indicate improving fitness.
  

• Adjusting training based on HRV signals: If your 7-day HRV average drops ~10% or more below baseline and you’re feeling run-down, consider it a sign of accumulated fatigue. Practically, that could mean swapping a hard interval session for an easier aerobic run, or taking a rest day, to allow recovery.
  

• Combine HRV with your subjective feel: HRV is a useful tool, but it shouldn’t override common sense. Always cross-check your numbers with how you feel. If your HRV is slightly low but you feel fresh, stick to your plan and routine. Conversely, if your HRV is telling you you’re stressed and you subjectively feel fatigued, that’s probably a clear signal to prioritize recovery.
  

• It’s more complicated for elite or highly trained athletes: If there are any elite athletes reading this… Welcome! And please reach out to me with training topics you are interested in). Studies on elite athletes show mixed results. HRV can remain low even when performance is low. The research refers to this as “HRV Saturation” when the resting heart rate is very very low (common trait of extremely fit people) [2].

Remember, the goal isn’t to maximize your HRV every day, but to keep it in a healthy range as you progressively overload and recover. Getting real time data and feedback is great – learning to combine this with how you subjectively feel and adjusting based on that is a skill you can develop over time. That’s it for this edition of the Threshold Lab. Happy training!

References

[1] Buchheit, M., Flatt, A. A., & Voss, S. C. (2022). Practices and applications of heart rate variability monitoring in endurance athletes. International Journal of Sports Physiology and Performance, 17(8), 1107–1118. https://doi.org/10.1055/a-1864-9726

[2] Plews, D. J., Laursen, P. B., Stanley, J., Kilding, A. E., & Buchheit, M. (2013). Training adaptation and heart rate variability in elite endurance athletes: Opening the door to effective monitoring. Sports Medicine, 43(9), 773–781. https://doi.org/10.1007/s40279-013-0071-8