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Why Heart Rate Variability (HRV) is a Key Parameter in Rewire’s Readiness Score

If you are an athlete (of any level) you have most likely tracked your workouts in a number of ways. From subjective metrics such as the rate of perceived exertion (RPE, or how hard a workout feels), to distance, duration, power, heart rate, pace or speed, as well as compound metrics such as Strava’s Relative Effort or Training Peak’s TSS, these can all serve a purpose in our quest to quantify the stimulus we apply with training. 

However, an equally important (or maybe more important?) question we want to answer is the following: how are you responding to training? After you went out for your session, did your body bounce back from that homeostatic disruption? How long did it take? Are you ready for another high intensity session or should you take it easy another day or two?

Being able to answer these questions can help us avoid a potential state of negative adaptation and hinder performance outcomes in the long term. Here is when Heart Rate Variability (HRV) comes to the rescue. 

What is HRV?

HRV is a term that refers to ways to summarize in a number the variability between heartbeats. The variation between heartbeats results from the activity of the autonomic nervous system in response to stress. As the body is continuously re-adjusting to maintain a state of balance, called homeostasis, heart rate, blood pressure, glucose level, hormones, etc. — react to the challenges we face and the autonomic nervous system works to keep everything in balance so that we can function optimally (e.g. do not develop chronic conditions, or improve our performance). Heart rhythm (and therefore HRV) is regulated by the parasympathetic branch of the autonomic nervous system, the one in charge of rest and relaxation. Hence, measuring HRV is an effective way to capture how the body is doing while trying to maintain a state of balance in response to different stressors (training, lifestyle, etc.).

Example of a few seconds of ECG data, including detected beats. The time differences between beats are called RR intervals and are the basic unit of information used to compute HRV. We need several RR intervals to be able to compute your HRV. This is why HRV needs to be computed over a certain amount of time, typically between 1 and 5 minutes.

​In particular, a reduction in certain HRV features typically means that parasympathetic activity is reduced, and therefore we have not fully recovered or in general, there is more stress in our lives. At rest, the body is predominantly parasympathetic, which is why HRV analysis today is mostly focused on identifying reductions in parasympathetic activity, captured by features such as rMSSD (the root mean square of successive differences in RR intervals). The use of rMSSD is motivated by physiological mechanisms: the vagus nerve acts on receptors signaling nodes to modulate pulse on a beat to beat basis while sympathetic activity has different pathways with slower signaling. Hence beat to beat changes captured mathematically by rMSSD reflect parasympathetic activity, also called vagal influence. 

This means that when we train or face other stressors, HRV is typically reduced at the acute level (during and right after the stressor). Additionally, if the stressor is particularly large (say, a hard race), or if we are responding poorly to a series of stressors (for example a block of high intensity sessions), HRV can remain suppressed for several days or longer. This is a typical sign of negative adaptation, something we can avoid by better managing and adjustring training, based on our unique physiological response. 

What about non-training related stressors?

One of the key aspects of measuring HRV and using it to gauge readiness or guide training, is its ability to track your stress response regardless of the source. What does this mean? No matter if stress comes from training, work, getting sick, poor lifestyle, or some unexpected event, it will have an effect on our ability to cope with additional stressors and perform. HRV is an overall marker of stress and will be affected by pretty much any factor that has an influence on your autonomic nervous system, making it a great tool for training management.

How can you measure your HRV with Rewire?

HRV forms a part of Rewire’s Readiness Assessment and can be measured live in the background from your Bluetooth Heart Rate Monitor. It can also be read from a health app like Oura or Apple Health. We have seen how HRV is a global marker of stress and also how it is typically impacted acutely by any sort of stressor. This comes at a cost: we cannot just measure HRV anytime and use the data reliably, as HRV will typically reflect changes in heart rate modulation due to a myriad of transitory stressors we might not be really interested in (e.g. having coffee, or walking up the stairs). Timing of the measurement becomes key if we want to assess baseline physiological stress in response to larger acute and chronic stressors, and use this data for daily adjustments. 

Measurement time

Almost the entirety of research up to date has been carried out with morning HRV measurements, hence this is typically the preferred protocol and also what is implemented in Rewire, where you can also set reminders to help you make the morning readiness assessment a part of your daily morning routine

The Readiness Assessment should be taken first thing in the morning, while in a rested physiological state. While in the past subjects in clinical studies were asked to go to the lab, avoiding eating, drinking and exercising in the 2 hours preceding a measurement, waiting between 10 and 30 minutes before the measurement to get back into that relaxed state, things are much simpler now due to the technological improvements that allow users to measure simply using their phones. Ideally, measurements should be taken as soon as a person wakes up, while still in bed. The morning routine, or having a standard measurement protocol should sound familiar in many situations, for example measuring weight before breakfast, measuring blood pressure in standard conditions (sitting, arm position, etc), and similarly, assessing readiness to determine the impact of training and lifestyle on physiological stress and recovery needs.

Body position and measurement duration

In terms of body position, lying down, sitting or standing are good alternatives, but in case you do not lie down, make sure to wait a few seconds before measuring, and use the same body position each day. Several studies have also shown that for time domain features representative of parasympathetic activity, such as rMSSD, the most commonly used metric in today’s tools, 60 seconds are sufficient. 

Measurement Frequency

Measuring Readiness daily is best to obtain useful data as it establishes a strong baseline for HRV. It also means you can check in regularly with your readiness, allowing you to make smarter training and recovery decisions. Since Rewire also collects a range of cumulative data points such as training load and mental load, checking in daily ensures that there is a more complete data set involving the highs and lows of your training and work. Measuring daily is also often easier to remember, since it can form a part of your regular morning routine.

What to do (and not to do) while measuring 

During the assessment, movement should be avoided, but there are also other aspects that can trigger artifacts and require a little more attention. In particular, yawning and swallowing should also be avoided, the latter for example causes a sort of instantaneous bradycardia that can affect the measurement. 

Breathing

HRV is affected by breathing. The question of using controlled or paced breathing or breathing naturally needs to be analyzed in the context of our target application, which is measuring physiological (chronic) stress first thing in the morning, longitudinally within an individual. One of the main reasons behind using paced breathing is that it is supposed to make the measurement more reliable and improve measurement repeatability. In our experience, this is not the case and letting people breathe freely feels much easier to most. In our tests we have highlighted how self-paced and paced breathing result in the same differences between consecutive measurements, hence proving that one way or the other is as effective. Thus, Rewire does not use paced breathing as part of the readiness assessment. 

How can you use the data to adjust training?

At the beginning of this blog, I covered the physiological underpinnings of HRV measurement as well as key aspects of data collection: context and best practices. By following best practices meaningful data points truly representative of physiological stress can be collected. As technology today allows for easy data collection, many of the basic physiological mechanisms behind applied use of HRV (for example the acute drop in HRV after hard workouts) have been successfully identified in user-generated data. These types of analysis provide further evidence of the effectiveness of today’s technologies in capturing individual responses to stress. ​It’s important to remember that physiology is complex, and while acute stressors (such as a hard workout) and the resulting HRV changes are often repeatable and easy to understand, there might be other factors behind the relationships that we are seeing (or not seeing) in our data. No stressor acts in isolation, there’s always something going on with our lifestyle, training, health, and so on. 

Let’s look at how we can use the data to adjust training. In the past decade, we have seen how HRV has been used to capture changes in training load, fitness and performance. In a landmark study, Kiviniemi et al. proposed a first protocol to guide training based on HRV readings, and analyzed changes in training load and VO2max in recreational runners following an HRV-guided program, compared to controls following regular periodization. The authors state that the basic idea of HRV guided training was to decrease the training stimulus when HRV decreased and maintain training stimulus high when HRV remained the same or increased. Often, HRV-guided training results in lower frequency of high intensity exercises compared to the control group. This is a common theme as most protocols aim at avoiding the application of too strong a stressor (e.g. a hard session) when the athlete is not physiologically ready (e.g. when HRV shows high stress present on the body). Based on this data, HRV guided training may adjust both the timing and amount of high-intensity exercises at individual level. Yet, in these studies, performance for the HRV-guided group improved, showing how the timing of the high intensity sessions does matter. Rewire uses HRV as well as other objective and subjective measures to provide you with training and recovery guidance. Readiness-guided training aims at providing the most appropriate training stimuli in a timely manner, when the body is ready to take it, so that positive adaptation will occur, leading to better health and performance outcomes.

After the initial studies by Kiviniemi et al. most researchers shifted their approach to one less coupled to day to day variability and acute stressors, trying to look at medium and long term trends and more significant stressors that might affect physiology chronically. With the new approach, we do not really care if a single daily score is below baseline, what we care about is that the baseline itself does not go below normal values. Intuitively, for the baseline to go below normal values, we need quite a few “bad days” (low HRV scores), therefore adjustring training less often and only when a stronger negative response is present.

​Vesterinen et al. were able to show improved performance for the HRV-guided group using this protocol. In particular, the number of high intensity workouts was lower for the HRV-guided group, but despite the lower amount of high intensity exercise, the group was able to improve running performance over a 3000 m time trial. In a similar study, Javaloyes et al. examined the effect of training prescription based on HRV in road cycling performance. After 4 weeks baseline measurements, 17 well-trained cyclists were split into two groups, HRV-guided and traditional periodisation group. The training program lasted another 8 weeks, and performance measures were taken before and after the 8 weeks in both groups. In the study, the HRV guided group improved peak power output (by 5%)  and 40 minutes time trial performance (by 7%), while the traditional periodisation group did not improve in any metric. The authors conclude that daily training prescription based on HRV could result in a better performance enhancement than a traditional periodization in well-trained cyclists.

Rewire‘s algorithm builds HRV into it’s readiness scores alongside a range of additional subjective and objective measures to provide a holistic approach to readiness. HRV impacts both Rewire’s Overall and Physical readiness score and training recommendations are provided accordingly. Rewire also provides a Personalized Recovery Session that isolates particular weak points in your state and selects a session tailored to your goals for that day. This combination of training and recovery recommendations allows you to perform at your best.

Wrap up

In this blog, we have covered the basics of HRV, and why it matters. We have also provided useful tips and best practices for your morning Readiness Assessment, so that you can collect high-quality data representative of changes in baseline physiological stress, using the Rewire app.

Needless to say, HRV is not the only relevant marker to quantify readiness, and should be integrated with information related to training load as well as subjective metrics such as stress, frustration or muscle soreness, all aspects that might independently indicate potential issues. Rewire offers an integrated approach to readiness that combines all of these parameters to provide you with a comprehensive view of your readiness to train.