Road cycling has always had a fascination with data, from speed and heart rate to power and sleep quality. As the amount of data available has increased over the years, riders, coaches and sports scientists have been able to better understand the effect of training on the body and how to optimise a rider’s performance.
In recent years many of us have adopted wearable tech as part of our daily attire with premium devices monitoring something called Heart Rate Variability (HRV). But what is HRV and how can it help you train and recover more effectively? To dive into the science behind HRV I caught up with Dr Marco Altini; he’s the founder of HRV4Training, an app that provides HRV data insights for athletes, and holds advanced degrees in data science, computer engineering and sports science.
So what’s the value of HRV? As Altini explains, “For endurance athletes, tracking HRV helps to understand how the body is responding to stress and recovery.” That data – and an understanding of what it means – allows an athlete to guide and adjust their training load to ensure that the work amounts and intensity will drive the desired adaptations.
What is Heart Rate Variability?
Before we can look at how to use HRV data first we need to understand what it is measuring. Heart Rate Variability (HRV) refers to the variance between the duration of each heartbeat. Unlike standard heart rate data that measures the number of beats per minute, HRV looks closer at the composition of each heartbeat. It is the small fluctuations in the intervals between each beat that HRV is concerned with. These intervals are influenced by the autonomic nervous system (ANS), which controls autonomous bodily functions like heart rate, digestion and breathing.
The ANS is divided into two branches: Firstly there is the Sympathetic Nervous System (SNS) which is more often referred to as the “fight or flight” system. The role of the SNS is to prepare your body for physical activity by increasing your heart rate and reducing HRV. The other branch of the ANS, Altini explains, is the Parasympathetic Nervous System (PNS) or the “rest and digest” system which promotes recovery and relaxation. The PNS has the role of slowing the heart rate and increasing HRV. As Altini explains, “We cannot measure ‘stress’ and we cannot measure parasympathetic activity, but we can measure how our heart rhythm changes due to the modulation of the nervous system.”
HRV is measured in milliseconds with a higher HRV value indicating that your body is in a state of recovery and is well-rested. A high variability means that your body can adapt to the loads being put on it by all manner of bodily functions. Conversely, low variability is a sign that your body is stressed and cannot easily adapt to a change in stress.
The body has a stress budget that it can handle without any effect on your HRV outside of its normal range. Simple bodily functions like eating and walking – even swallowing saliva – all put stress on your body in a way that can affect HRV. However, your body can bounce back from these stresses and return to a normal HRV in a matter of minutes to hours. The more stress that is put on your body the more time it can take for HRV to return to normal. And if you are continuously stressed more than your body can sustainably deal with you will find that your HRV will start to drop as it struggles to bounce back from the cumulative effect of all that stress.
Where the stress comes from is irrelevant. HRV is unable to distinguish what is causing the body’s stress response – whether a hard training session, a poor night’s sleep, or stress at work. “We can only take so much stress. It all adds up and it all matters, not only from training,” says Altini. Since the body cannot differentiate training stress from other emotional or physical stressors, even on low volumes of training it is possible to exceed what your body is capable of productively adapting to at stressful times.
These stressors are not always obvious; for example, you might not feel the effects of a bad night’s sleep or a looming work deadline, but this all takes a slice of your body’s stress budget. The more of this that is consumed by other stressors the less is left for training before you are in debt and need to prioritise recovery to operate at your best.
How can you really measure HRV and what does the data tell you?
A great deal of the latest generation of smart wearable tech like Whoop bands, the Oura ring, and smartwatches from brands like Apple and Garmin all claim to measure HRV using the optical scanner that sits against your skin.
Technically, Altini explains, this is not entirely correct. “Optical sensors in smartwatches and wearables measure PRV, or Pulse Rate Variability, which tends to be very close to HRV but is also influenced by other factors like blood pressure,” he says. Even though PRV and HRV are not technically the same, Altini goes on to say that “Many modern wearables can provide reasonably accurate data when measuring PRV during the night.”
The easiest way to collect true HRV data is an HRV-capable chest strap. Altini recommended Polar’s HRV-capable straps, which can be paired to a phone or smartwatch by Bluetooth or ANT+. “We validated them in our research. Pairing them to an app like HRV4Training (which Altini created) and measuring for one or two minutes after waking up is the simplest and most effective protocol in my opinion,” he says, adding that the reading should be taken after going to the toilet if this is needed first thing in the morning.
Some wearable tech captures HRV data and converts it into a training readiness metric typically displaying daily HRV as a percentage or as a score. Although this can be easier to interpret on the surface, Altini thinks that this form of metric should be approached with caution. As he explains, “The approach used by most wearables is where there is no normal range and only made-up scores are provided, typically on a scale between 0-100 or in percentage.”
But this way of displaying HRV doesn’t take into consideration the fact that everyone does have a normal range with a measurement anywhere in this band considered normal. In reality, he describes the proper use of HRV as “less engaging and quite boring, as a good physiological response simply means that our HRV is within our normal range.” If you are following a well-structured plan that is tailored to your current level then for the most part there shouldn’t be any red flags or necessary changes day to day.
When starting to integrate HRV into your training it is crucial to pick one method of measuring your HRV – wearable tech with optical measurement or chest strap – and stick with it. Comparing an HRV reading from a smartwatch to a chest strap monitor will open up the potential for error which will compromise its validity. It is also worth noting that most wearables will measure HRV throughout the night when you sleep. This is done to measure at a time of rest much like trying to capture an accurate resting heart rate value. Although this is most devices’ default, measuring throughout the night is open to more deviation than measuring specifically in the morning.
When asked why measuring at night can be problematic Altini went on to say, “These measurements may be more influenced by behavioural factors such as a late dinner or heavy training session late in the evening more than morning readings.
“This is why morning measurements are preferred, particularly for athletes who want to monitor subtle physiological changes and use the data for actionability and to determine their ability to positively respond to training on a given day.” Accurately replicating the same conditions you test under each day will provide the most reliable and meaningful data.
HRV can both be used as a trend analysis tool and as a daily assessor of training readiness. A steady increase or decrease in HRV is more meaningful than a single high or low reading that could be a result of a particularly hard training session, a stressful day, or even just a bad night’s sleep. Altini says, “A single suppression might be associated with a transitory stressor that is quickly gone, while how the data is trending over several days or weeks is better associated with our chronic state.”
As your HRV begins to deviate away from the normal range it is important to spend some time analysing why this might be, both from a training and a broader life perspective. HRV doesn’t discriminate between stress factors so although your training intensity may be fine in isolation, in the context of a more stressful work week things might need adapting.
At this point it can seem like a higher HRV value is always better however evidence suggests that this is not necessarily always the case. A blog post from Altini discusses the relationship between abnormally high HRV compared to your normal range and overtraining. This is especially apparent when an athlete has overtrained through high volumes of low-intensity aerobic exercise. When an athlete is in this state the parasympathetic system is hyperactive trying to regain balance, resulting in a higher-than-normal HRV.
How can HRV tracking make you a better cyclist?
The first step is to monitor and record your HRV daily; the more consistently you capture data the more useful it becomes. It can be particularly advantageous to keep a log of your training and other factors such as sleep quality, mood, and stress/anxiety levels to provide more context when looking back over historical data.
If you are new to HRV data collection or you are changing your protocol it is important to gather a baseline reading. A 2020 study from Antonio Granero-Gallegos et al. details that “HRV-guided training starts with a preparation period of about four weeks, which serves as a standardized data collection phase to obtain the baseline HRV values.” Only once you have this standardized data is it possible to assess your HRV against this baseline.
Understanding HRV makes it a powerful tool for monitoring stress and recovery. HRV data can give an objective insight into how your body is processing the total stress it is under. Tracking HRV over time allows you to see trends from when you are both fresh and recovered and also stressed and require rest. Having this knowledge opens up your training to be guided by your HRV. This can help to avoid overtraining, allowing riders to adapt their load to ensure optimal performance and recovery.
Altini says, “HRV can be a valuable tool for making small adjustments to training plans. It should not replace structured training, but it can complement it by providing insights into how well the body is recovering and how we are responding to training.
“For example, if HRV is consistently lower than usual over several days, this might suggest accumulated fatigue, and reducing training intensity or incorporating more recovery days could help. On the other hand, if we train hard and our HRV is stable, it is a great sign that we are responding well to training, and will likely adapt to the stimulus.” Looking at your HRV data in the morning should not be used as a barometer to assess if it is a hard or an easy training day. Instead, it can complement a well-thought-out plan by providing insights into how well the body is recovering and how it is responding to training.
HRV data is not just a catch net to prevent overtraining; it can also tell us how well we are positively responding to training. If you are increasing your training load and your HRV remains stable this means that the training is being well-received by the body and it is likely to adapt well to the stimulus.
Importantly, low HRV can offer clues earlier than your own subjective evaluation might. Altini stresses, “Athletes can still perform well even with low HRV.” Again, it’s a measure of how much stress you can accumulate. So when in a heavy training block a lower than normal HRV is indicative that your body isn’t fully recovered. This means that although you could push on and complete a hard training session it would likely not yield the desired adaptations.
This is where Altini believes that HRV-guided training can represent a significant benefit to longer-term training adaptations. “Multiple studies have shown that reducing training intensity on days in which HRV was suppressed eventually led to similar or better performance outcomes with respect to not adjusting training,” he says, “despite training less hard over the duration of the study (since some sessions were skipped due to low HRV, either acutely or chronically).”
Small changes, big impact
In short, HRV provides us with a way to implement small changes to a well-designed plan and to gather feedback on the body’s response. In many cases, athletes who find themselves in a state of what seems like overtraining are rather simply under-recovering. Consistently trying to maintain a high training load whilst already in a state of stress is a fast track to overtraining and a decline in performance, and keeping track of your HRV and comparing it against historical trends can act as an early detector against overtraining, allowing you to reduce intensity and prioritise recovery.
As convenient as it might be to simply plan your recovery days ahead of time following a simple principle of two or three days of training followed by an easy recovery day, the human body doesn’t always benefit from following this pattern. How your body responds to training can vary as other stressors could massively impact how well you recover from day to day. Using HRV to be more fluid with training and recovery will allow for the balance to be optimised.
Lastly, it is worth considering HRV within a greater holistic approach to incorporate it into other metrics such as heart rate, power, sleep, and motivation levels (where, again, a daily journal can provide insight). Although HRV can be an incredibly useful asset it is best used in collaboration with other training data to provide a clearer map of your training path.
Having access to HRV data can be incredibly powerful and shines some light on your body’s response to stress. It can help cyclists fine-tune their training, optimise their recovery schedule, and avoid overtraining. By regularly monitoring your HRV and understanding what it tells you about your body, you can make smarter training decisions which can lead to an increased performance on the bike. If you already have access to a smartwatch or heart rate monitor that can measure HRV, taking some time to understand it could be the next step to elevate your training and performance.
Did we do a good job with this story?