What have Jennifer Aniston, Mike Woods, Prince Harry, Will Smith and Tadej Pogačar in common? You guessed it, they’re all users of products that measure heart rate variability (HRV) – in the case of our fab five, smart rings. Israel-PremierTech’s Woods is an Oura Ring athlete, while Ultrahuman sponsors Pogačar’s UAE Team Emirates outfit. Then, of course, you have HRV readings from Whoop and the likes of Garmin and Polar. Your HRV’s more than likely being measured as you read this feature. But how can it benefit your performance? And how does it benefit the superhumans who battle with 30,000km of racing and training each year when they have a support team to analyse the data and act on those readings? Well, you’ll soon find out thanks to those in the HRV know, starting with the most fundamental question of them all: what exactly is HRV?
What is HRV?
“HRV is an indirect measure of our stress response when measured at the right time and according to best practices,” explains Marco Altini, founder of the popular HRV app HRV4Training and generally considered one of the world’s leading experts on HRV. “As we face a stressor, typically our autonomic nervous system shows increased sympathetic activity and reduced parasympathetic activity (our ‘recovery system’).
“While we cannot measure non-invasively parasympathetic activity, the autonomic nervous system impacts heart rhythm in ways that we can measure. For example, reduced parasympathetic activity after a stressor typically results in a slightly higher heart rate and reduced HRV. In short, cardiac activity changes in response to stressors in ways that can be captured with an HRV measurement.”
HRV examines the heart rate variation from beat to beat. That’s because even when the heart rate is stable – say 55bpm – the gap between beats isn’t exactly one second. In fact, it varies between beats. This links back to the autonomic nervous system, as greater variations between beats (increases in HRV) are associated with parasympathetic activity (rest and recovery), while, as Altini states, reductions in the variations between beats (decreases in HRV) are associated with the sympathetic activity (fight or flight).
So, HRV’s pretty damn important. It’s also influenced by a number of factors, including your hydration status, how well fuelled you are and the quality of your sleep. That’s because your autonomic nervous system regulates just about every organ in the body, so if something is wrong, your body will attempt to deal with the problem, which will affect your HRV. And it’s this all-seeing-eye of assessment why so many teams are using it.
How the pros use HRV
“Yeah, we use HRV on our team,” says Stephen Barrett, coach to riders such as Sam Bennett at Decathlon AG2R La Mondiale. “For us, I’d say HRV data’s most usable during training camps and when the riders are undertaking big blocks of focused training. But, the most important thing with HRV is that each athlete must collect enough data to set their baseline. Once you’ve gained enough data, you can then spot increases and decreases in a rider’s scores and react accordingly.”
And it’s that reacting and analysing that, as Tudor Pro Cycling coach James Spragg told us, isn’t unequivocal in its actioning: “There’s just too much noise in the HRV data, so we don’t use it.”
Perhaps surprisingly, for someone who has a vested interest in HRV, Altini agrees that HRV is only of benefit if used very precisely. “Context is always king when it comes to HRV as this a marker that’s highly sensitive – in other words, any stressor can impact it – but not specific (i.e. from the physiological data alone, it’s not possible to determine which stressors caused a change),” he says. “That said, measuring HRV might help us make some adjustments in our lifestyle if we see, for example, a consistent suppression below our normal range in response to behaviours over which we might have control. They include alcohol intake, travel and sleep routine.”
(This author can certainly concur with points one and three. I’m currently typing with a Whoop wrapped around my right wrist, and my HRV scores were crippling low after a weekend at a music festival; that said, the subsequent headache and brain fog were another two key parameters that my body was under self-induced stress and attack!)
“In a cycling context, HRV is mostly useful to make small adjustments to training intensity, but it cannot possibly replace a training plan,” says Altini. “That means if your device is advising a hard session based on a physiological reading, that’s incorrect, given that it has no context on other performance limiters that aren’t captured by the data and are key to cycling, like muscle soreness. Instead, it’s useful in support of a training plan.
“Keep in mind that HRV isn’t about what you can do on a given day – unless you are sick, you can typically perform well ‘on a low HRV score’ – it’s mostly about what stress you can assimilate and respond positively to.
“If it’s race day and your HRV’s low, there’s nothing to worry about, as you can still perform. During a training block, though, going hard in a situation of low HRV might not result in the hoped-for adaptations, as you’re doing a workout when already in a sub-optimal physiological state. But just remember that HRV, when measured at the right time, is a marker of your stress response, not a marker of stress. Hence, a low value doesn’t simply mean that you’re under stress, but that you’re unable to respond positively to that stress within a reasonable time.”
Good for training, not for the Tour
In 2024, a paper entitled, ‘Daily cardiac autonomic responses during the Tour de France in a male professional cyclist’, featured in the journal ‘Frontiers in Neuroscience’, analysed the HRV response of a rider not only during the Tour as the name suggested but throughout the season. The background context of the study highlighted the uncertainty of HRV’s usefulness in a cycling scenario: “HRV is a common means of monitoring responses to training, yet in professional cycling, one may question its usefulness, particularly during multi-day competitions such as Grand Tours.”
The researchers recorded the beat-to-beat variations of the diminutive 57kg professional, who’d recorded top-10 finishes at the Tour de France and Vuelta a Espana [answers on a postcard to who it is, please. The researchers were predominantly French] and discovered what, exactly?
Firstly, HRV dropped during periods of high stress, like altitude camps. Though not surprising, it does give the data-crunching support team empirical evidence that an athlete is in stress, which then hammers home the importance of recovery techniques, like sufficient fuelling and massage. It also gives the rider the figures to support what he or she probably feels inside and, as Altini mentions, could force a change in intensity.
“For most of our riders, we prescribe a training plan for the week ahead, but for our higher-performing guys, it’ll be for three days,” says Barrett. “That means we can be more precise and modify if needed. Before the Giro d’Italia this year [where the team won two stages thanks to Valentin Paret-Peintre and Andrea Vendrame], we’d held an altitude camp on Mount Etna. We monitored training most days and noticed after a hard three-day block, where the quality of the sessions was very good, that some of the riders’ HRV was blunted. We’d achieved what we wanted to achieve in those three days, so we eased off a little and reduced to a two-day block to reassess again after a shorter time period. It’s not a drastic change, but it does help decision-making.”
The second conclusion of the Tour study was that monitoring HRV during a Grand Tour isn’t that useful as even if a rider’s registering a low HRV, there’s not an awful lot you can do about it. Yes, a domestique might be able to change roles for an easier day. But what if you’re a GC rider? A low HRV doesn’t mean you can go easy. The study also highlighted the importance of protocol on accurate HRV results when citing limitations of the study.
“During a multi-stage race such as the Critérium du Dauphiné or the Tour de France, the cyclist sleeps in a new hotel every night. Therefore, it’s likely that the changed conditions (e.g. hotel room and temperature) increase the inter-day variability in the HRV responses,” the authors wrote. “In addition, although the cyclist was closely followed by the team nutritionist and the team doctor for hydration and medication, respectively, these factors were not accurately measured. However, we strongly believe that the three- to four-fold decrease in HRV during competition periods was mainly caused by the extreme loads and only marginally impacted by other factors such as nutrition, hydration or medication.”
Perfect protocol
When it comes to HRV protocol, the gold standard is to analyse a long strip of an electrocardiogram undertaken at the doctor’s. Not that practical. However, as many of you know, in recent years, numerous companies have launched apps and wearable heart rate monitors that aim to achieve something similar. The accuracy of these methods remains under scrutiny, but the technology is improving.
“A few of the wearables that are on the market today can provide reasonably accurate HRV data when collected in the absence of movement, i.e. during the night. In my experience, this is the case for Oura, Whoop, Garmin and Coros,” says Altini. “There are also studies looking at spot checks with Oura and HRV4Training (the app I develop), both using the fingertip and phone camera and paired to a chest strap, showing that both systems are accurate (and more accurate than other apps or wearables).
“Given that many endurance athletes tend to have a GPS watch, like a Garmin, I consider a dedicated wearable unnecessary. This wasn’t the case a few years ago, but recent versions are as accurate as dedicated wearables.”
Accurate in the right scenario, adds Altini. “The link between HRV and parasympathetic activity is valid only at rest and when there are no other confounding factors,” he says. “This is key because even swallowing saliva or drinking water will impact your HRV in ways that aren’t representative of your stress response. That’s why HRV must be measured first thing in the morning, at rest or during the night, and cannot be used as a marker of stress outside of these well-defined and rather controlled settings (despite what’s currently marketed to users of wearable technologies).”
Altini says that simply consuming more carbohydrates for dinner than normal can skew the results as it raises heart rate and will be picked up as a negative response by wearables in the evening. “Often, evening data is more tightly coupled with our behaviour than our stress response and, therefore, is less useful for daily guidance,” he says.
Part of, not the whole, picture
Barrett agrees that morning’s best and reveals that he and his team are a proponent of Altini’s HRV4Training app. “If we’re using that or the Elite HRV app, we’ll ask them to collect a measurement first thing and in the lying position. If the riders are wearing a Whoop or Oura ring, it’s done automatically. The most important point is it’s ticked off at the same time every single day, first thing in the morning.”
“In fact, nearly as important for us is that we use HRV along with several different metrics like the Training Stress Score in TrainingPeaks, rate of perceived exertion, even factors like hydration status and oxygen saturation,” he adds. “For me, it’s key that you have all of these metrics, not just one single point, and that you let it guide your decision, not make it for you.
“You should also avoid being too impulsive. Once we have baseline readings, I’ll see how a rider’s data trends over a certain number of days. If the scores are consistently low, I’d certainly reconsider the training content that day. But I wouldn’t on one data point as that could simply be a bad night’s sleep. A lot of the HRV focus in the past has been an acute focus, in other words, on one day. For me, it’s best to look at the chronic – the results over a certain amount of time. Yes, we pay attention to the acute, but the chronic is a clearer sign of adaptation and state of being.”
So, those readings on your wrist do have meaning and can impact your riding performance. But you need data, trends and patience, plus ideally further data to cross-reference with. And don’t forget subjectivity because how you feel is, well, how you feel.
All in all, listen to Altini and his HRV guidance. “Make sure not to confuse HRV with the made-up scores wearables provide you. HRV is your actual physiological response, and, as such, it’s much more informative about the state of your body than any attempt to capture your recovery or readiness that combines incomplete and decontextualised information. Pay attention to meaningful changes in physiology – e.g. HRV and resting heart rate outside of your own normal range. This data might help you improve self-awareness and make small and meaningful adjustments. The data is there for this reason – to aid self-awareness, not to replace it.”
