February 01, 2022

What is Heart Rate Variability? How important is it for cycling?

We take a deep dive into a trendy, but often misunderstood metric in endurance sports

Words: Nick Busca

Photos: Getty Images; SWpix; Alejandro Javaloyes; Jasmijn Muller

Many of us have heard of ‘HRV’ or Heart Rate Variability. But how many of you really know what HRV is? More importantly, does it actually help to optimise your training schedule and racing outcomes? Or is it just another number and better to focus on something else instead?

In this guide, we answer some of the most frequently asked questions around a trendy and often misunderstood metric.

What is HRV?

Your heart rate does not beat with a constant rhythm, but it has variations between heart beats (Photo: Getty Images)

Heart Rate Variability is the physiological variation (in milliseconds) between consecutive heart beats. If our bodies didn’t each have unique control mechanisms, our hearts would beat at a constant rhythm — but that’s not the case in the real world.

“The heart has its pacemaker, and there’s a spot (called the sinoatrial node) that generates electric potentials that initiate contractions, resulting in heart beats,” explains Marco Altini, founder of HRV4Training and consultant for the one of the most popular tools for measuring HRV – Oura ring. (For reference, Altini’s credentials are impeccable. He has a Ph.D. in data science, a degree in computer science engineering, and a master's in human movement sciences and high-performance coaching. In short he knows his stuff.)

“If you had no other mechanism to modulate heart rhythm, your heart would beat at approximately 100 beats per minute,” he explains. “However, if you’ve measured your heart rate before, you'll have noticed that doesn't happen. Something else is affecting the heart beats and producing a variation.”

What affects HRV?

HRV is due to the work of the autonomic nervous system (ANS). Photo: INEOS-Grenadiers.

HRV is due to the autonomic nervous system (ANS), the peripheral nervous system that controls involuntary processes like heart rate, blood pressure, respiration and digestion. The ANS works to keep our body in a state of balance called homeostasis, which is critical for optimal functioning. The body maintains this balance by controlling daily stressors through the hypothalamus, a region of the brain that processes information and transmits signals to stimulate or relax the rest of the body.

Within the ANS, there's a difference between the sympathetic (or fight-or-flight response) and parasympathetic (rest-and-digest) nervous systems. Heart rhythm and HRV are regulated by the latter.

If, for example, the activity of the parasympathetic system is reduced, our body is under stress and our HRV reduced. On the other hand, if you’re resting, the parasympathetic activity increases, and the HRV score is higher.

What’s an HRV score?

A screenshot of the HRV Training app. Photo: HRV4Training.

It’s possible to detect your heart rate through different devices, like heart-rate straps, optical monitors in watches, rings and phone cameras (the efficacy of the latter has also been validated).

After a period of data collection, an app on your phone calculates an individual HRV score based on the data history and subjective feedback (from questions like: How did you sleep? Did you consume alcohol? Do you feel sore?).

“HRV can be calculated in different ways, starting from RR intervals (beat-to-beat differences),” explains Altini. “Companies and researchers now all tend to rely on the same feature: rMSSD." rMSSD means root mean square of successive differences between normal heartbeats — but don't worry about the meaning, just see it as the metric behind the numeric score.

The rMSSD is a feature associated with short-term changes in heart rhythm. It has been considered a reliable measure of parasympathetic activity — and the only one through which researchers have been able to capture physiological processes mathematically. rMSSD can also be compared across studies, it’s time-invariant (either a shorter or longer window gives similar results) and is reliable even for measurements that last only 60 seconds.

Exercise physiologists Alejandro Javaloyes and Manuel Mateo-March performing field tests with team Movistar. Photo: Javaloyes archive

"We use the LnRMSSD [a version of rMSSD] to guide training. If the LnRMSSD has the 'Smallest Worthwhile Change' [usually 0.5 x standard deviation], then we reduce training load," says Alejandro Javaloyes, an exercise physiologist for the Spanish Cycling Federation and the Miguel Hernandez University in Elche. Together with his colleague Manuel Mateo-March (also involved with the Federation and the University, and formerly with Movistar from 2012 to 2020), they have been using HRV for several years now.

And, for example, if they had prescribed a hard workout (like a 5-hour ride with threshold intervals), but they notice that LnRMSSD is outside the optimal range, they reduce training to something easier, like 3 hours easy riding.

Alejandro Javaloyes performing a VO2max test with the Spanish Cycling Federation. Photo: Javaloyes archive

When the HRV scores fall under standard fluctuations, it means that there is "too much stress, and more probabilities of non-functional overreaching [over-training]," says Javaloyes.

When do you measure HRV?

HRV4Training app works through the camera of your smartphone and the technology has also been scientifically validated. Photo: HRV4Training.

You typically measure your HRV score first thing in the morning or a device does it while you sleep. However, the most important thing is being consistent over time.

“Once we’ve collected accurate data at the right time, we need to be able to interpret that data,” says Altini.

The inherent, daily variability of HRV scores is something that HRV4Training and other apps consider in their readings. Their software interprets the HRV fluctuation according to normal baselines, historical values and subjective feedback. A software programme that simply interprets a high HRV score as good, and a low one as bad, would be inaccurate and misleading.

Why should you measure it?

HRV scores can help athletes and coaches to tailor training programs according to individual stress levels. Photo: Getty Images.

Proponents say HRV provides a better physiological understanding of one’s level of stress and fatigue. The rationale is simple: although you feel good, if you have a low HRV trend your sympathetic system may be still recovering from stress and you might benefit more from a day off than a high-intensity session. On the other hand, if you have a positive HRV reading and your trend is good despite feeling tired when you wake up, perhaps you're ready to go hard.

Cycling coach and ultra-racer Jasmijn Muller has been using HRV and RHR (Resting Heart Rate) for a couple of years now and suggests that her athletes use them.

"Within the coach platform, I can see their HRV trends, but also assess this against their baseline and if the changes are trivial (e.g. a few too many drinks at the weekend) or something more serious is underlying," she explains. “In one of my riders, it proved particularly useful to spot what was eventually diagnosed as a positive COVID case.”

Cycling coach and ultra-racer Jasmijn Muller. Photo: Muller archive

Through HRV, she can also evaluate her athletes’ readiness to perform, if their training load is appropriate and if they’re responding well to training. “For example, during a classic base training block (low intensity), I would expect to see an increase in HRV, whereas during a HIIT block I would expect to see a temporary decrease in HRV.”

Similarly, you can use HRV trends to guide your training: if the daily HRV is outside a normal variation, a lower-intensity session would be more beneficial. But if HRV is high and within the usual range, high-intensity training is okay.

Muller uses HRV4Training in combination with the OURA ring, but she also says that testing it through the phone camera in the morning is easy as "for many people, their phone is the first thing they grab anyway when they wake up”. But she also uses it because it’s “scientifically and independently validated”.

Considerations for female athletes

Some apps and HRV readings can help coaches and athletes to tailor training around the menstrual cycle. Photo: SWPix

Some of the HRV platforms on the market also have features that would be beneficial for female athletes in particular.

“In several of my female athletes who still have a regular ovulatory menstrual cycle, I can see their RHR is higher during the luteal phase (second half of the menstrual cycle), and HRV is generally a bit lower, indicating that parasympathetic activity is slightly suppressed and recovery from training may take longer,” says Muller.

With this information, often she’d then reduce the amount of high-intensity sessions and increase the number of rest or easy days.

“I recommend all my female clients to use the WILD app [an app specifically developed for female athletes] to log their periods and hormonal symptoms at any time of the month,” she says. “Tracking these subjective scores via the platform helps me to adjust training if needed and stimulate better performance.”

Why might you not want to measure HRV?

Sometimes, though, less data is better. As the Planche des Belles Filles in 2020 has proved to everyone (Pogačar rode it on feeling and without any tech support). Photo: Getty Images.

If you’re already recording other data, adding yet another measurement to the mix may result in additional stress. Adding more testing procedures is innately cumbersome and can increase the cost of training (although some apps like HRV4Training only cost £10 and work through your phone).

That said, many coaches and athletes have been using it successfully — its science is getting more robust and the technology is easier to use. So don’t count HRV out just yet.