Posts

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.

Podcast #4 – Diving into Rewire’s Mindset Recovery System

On this episode, Ed Gibbins is joined by Sun Sachs (CEO and Co-founder) and Dr. Walter Staiano. The Rewire team discuss what our Mindset Recovery System involves, its uses, and the science behind the protocols it uses. 

Stay tuned at the end of the podcast for a demo of our Mindset Recovery System featuring guided box breathing and binaural beats specifically designed for relaxation.


Podcast #1: Talking Brain Endurance Training (BET) with Walter Staiano

On this episode, Ed Gibbins sits down with Dr. Walter Staiano to talk about Brain Endurance Training. Walter has a Ph.D in Psychobiology of Perception of Effort and Exercise Tolerance and is the leading researcher and expert in the practical application of Brain Endurance Training (BET). His research has been featured in best-selling sports performance books including Alex Hutchinson’s ‘Endure’ and he’s worked with Olympic and World Championship teams as well as the British Military.


What is Brain Endurance Training?

BET is a cognitive training specifically designed to increase your resilience toward the sense of fatigue and perception of effort. 

How does BET work?

It works on the principle that the sense of fatigue (either physical or mental) develops in certain areas of the brain and these areas can be targeted by specific cognitive tasks and can induce an adaptation in the brain which will make the athlete more resistant to fatigue. Sense of fatigue is a key component in sport because it critically alters performance by increasing the perception of effort in completing a specific action (both physical and mental).

You were involved in some of the earliest studies on Mental Fatigue, how did you get involved in the field and is there anything that inspired you to do so?

For the most part of my undergraduate and master degrees, I focused my attention on the physical side of fatigue (muscle fatigue). However, I realized that multifaceted topics such as fatigue require multidisciplinary approaches to completely grasp the concept. In this context, by the end of my master degree, I decided to shift my attention toward mental processes of fatigue. It’s there that I met Professor Marcora and I decided to embarque in a journey to understand the link between the mental aspect of fatigue and physical performance. This set the base for what became an innovative and successful line of research across the world on the effect of mental fatigue on physical activity and sport performance. What inspired me the most was that we proved scientifically that exercise exhaustion, in particular in endurance events not mainly caused by physiological factors, instead is the results of complex brain processes that resulted in an increased perception of effort which lead to earlier exhaustion. 

Why is the perception of effort an important component? 

The research so far suggests that rating of perception of effort or perceived exertion (RPE) is a crucial component and a determinant in sport performance. The sensation of how heavy and strenuous a task is perceived is a valid and reliable tool (as good as heart rate).  Marcora et al. (2009) ‘Mental Fatigue Impairs Physical Performance in Humans’ and subsequent work from that group provides compelling evidence for that. RPE is, as well, a simple and reproducible tool for assessing training load in sport as well as military use. 

How do you know BET is working? How does it transfer into sport performance?

For many years, science has demonstrated, and it is well documented, that mental fatigue affects physical performance in several sports. More recently there is evidence that proves using a task that induces mental fatigue as a “training stimulus” to overload the brain will create an adaptation that can be beneficial when translated into sport performance. As a matter of fact, it is very well established in science that the brain can be trained as much as the body and that it is playing a key role in improving sport performance.

There is a lot of controversy in science about the transfer of cognitive drills into the specific sport. So it is sometimes difficult for coaches and sport scientists to understand how much cognitive training can actually help and transfer benefits in a specific sport. BET, however, does not work specifically on improving a specific sport capacity, instead it targets the individual’s ability to become more resilient and resistant to mental fatigue, which has a negative impact on physical performance.

Your studies have investigated the effect of BET on sports ranging from cycling to football. What have been some of your key findings and are there any findings that have surprised you?

In recent years I have collected data using BET training with football and cycling in particular. Some of the key findings that I found really interesting is that BET boosts as much the cognitive performance and the physical performance of the athletes. In the physical domain, it is effective in boosting generic capacities such as Yo-Yo tests as well as more specific ones like sprint and changing of directions. What surprised me is that it seems BET is more effective when athletes need to push while in a fatigued state. That means that this type of training really improves someone’s ability to push their limit by the end of a competition in the case of multiple events on the same day or multiple day events.   

Who can benefit from BET?

Basically everyone who is interested in increasing their level of performance and ability to be more effective in sport or in life and to be more resilient to fatigue. So it can be used by athletes to boost their physical performance, by military personnel or corporate employees to become more effective in taking decisions while in a fatigued state. So the applications are actually very vast. 

In what ways do you see BET assisting your athletes?

BET creates an additional workload outside the physical training routine and it challenges the athlete’s mental ability to tolerate stress and, when able to adapt, increase their resistance against mental and also physical fatigue. You can do these exercises wherever you are and outside regular training hours, as long as you have a mobile phone or an iPad at any time of the day. After a while, it looked that the brain found a way to adapt and handle this additional workload.

What is the most important thing you think athletes need to understand about BET?

Pain or effort perception doesn’t develop in the muscle, but in the brain and so it is worthy to train this part of the body to find a competitive edge.

It is also important that athletes understand that BET is a hard workout and is based on prolonged periods of highly demanding cognitive tasks so it is not a fun game to do for a minute or two. It has to be structured and it can be overloading as much as a high-intensity physical training session. 

There are some athletes like David Goggins that put a big focus on the mind. Essentially for them, the goal of a workout is to suffer physically to train their mind. What do you think the benefit of BET is over and above just pushing yourself and suffering?

This is a great example of how for decades athletes (as well as military personnel) have used physical training to build a mental resilience toward pain or discomfort. Indeed, this is a great and also scientifically proven method (as mental and physical exertion are linked). BET can become a great supplement training to build more resilience without taxing the body (which could lead to injury or overtraining).  

What can we learn from athletes like this and apply in our BET sessions?

Athletes have a huge drive toward improvement and to go beyond their own limit. They have learnt to deal with a lot of pain, high effort and insane levels of discomfort. As such, they explore every possible solution to gain the winning edge they need to perform better. In the last two decades (thanks to scientific and technological advancement), the brain has become the ultimate area to explore in the quest to be faster, stronger and better. In this context, BET is the result of years of research that brought to life a type of valid and reliable training method that can indeed help athletes get the edge they are looking for.  

Do not pray for an easy life; pray for the strength to endure a difficult one.

Bruce Lee

Mentioned Studies

A Randomized Controlled Trial of Brain Endurance Training (BET) to Reduce Fatigue During Endurance Exercise” 
Walter Staiano; Michele Merlini; Samuele M Marcora
Medicine & Science in Sports & Exercise, 2015

“Mental Fatigue impairs physical performance in humans”
Samuele Marcora; Walter Staiano; Victoria Manning
Journal of Applied Physiology, 2009

“Impact of 4-week Brain Endurance Training (BET) on Cognitive and Physical Performance in Professional Football Players”
Walter Staiano; Michele Merlini; Chiara Gattoni; Samuele Marcora
Medicine & Science in Sports & Exercise, 2019

A 101 Guide to Binaural Beats

Binaural beats are a form of brainwave entrainment that have been shown to have positive effects on stress, anxiety (1,2), focus (3), motivation, confidence and meditation (4). Binaural beats work when two different frequencies are heard, one in each ear. This creates a third tone, the binaural beat, whose frequency is the difference between the two other tones, e.g. if the tone in one ear is 400Hz and the other is 410Hz, the binaural beat is 10Hz. This binaural beat is shown to have a positive impact on the user’s mindset. It is important to note that stereo headphones are required to achieve a binaural beat since when using a speaker or non-stereo headphones the frequencies are already mixed outside the brain and hence no binaural beat is created.

The various tones of binaural beats affect the user differently. The following tones are used in the Rewire Mindset Recovery System:

  • 0.5 – 3.5 Hz – Delta wave for deep sleep
    • In a 2018 study, participants who received this frequency during sleep entered deep sleep quicker and for longer (5). This allows participants to gain more of the benefits of deep sleep including physical recovery.
  • 4.0 – 6.5 Hz – Theta for meditation/sleep
    • A 2017 study showed that even listening to a 6Hz binaural beat for just 10 minutes induced the user’s brain into a state similar to that achieved during meditation (4).
  • 7.0 – 12.5 Hz – Alpha for relaxation/dreams
    • In a 1990 study, alpha wave binaural beats were shown to have a positive influence on the user’s relaxation (6).
  • 13.0 – 38.5 – Beta for Activity            
    • It has been shown that beta wave binaural beats can positively affect vigilance performance and mood (3), and a recent 2019 study showed that beta wave binaural beats have a positive impact on long term memory (7).

Binaural beats also have a positive effect in counteracting the negative effects of mental fatigue. A recent 2020 study by Walter Staiano, Rewire’s Scientific Advisor, showed that binaural beats reduce the negative effect of mental fatigue (8). This makes binaural beats an important part of mindset recovery and pre-competition preparation in sports to minimise the negative effects that mental fatigue is shown to have on endurance performance (9,10).

We have put together a demo of our mindset recovery system featuring theta wave binaural beats which you can watch and download below.

You can also hear what binaural beats sound like raw and play around with different tones here.

References

1.        Padmanabhan R, Hildreth AJ, Laws D. A prospective, randomised, controlled study examining binaural beat audio and pre-operative anxiety in patients undergoing general anaesthesia for day case surgery. Anaesthesia. 2005; 

2.        Garcia-Argibay M, Santed MA, Reales JM. Efficacy of binaural auditory beats in cognition, anxiety, and pain perception: a meta-analysis. Psychol Res. 2019; 

3.        Lane JD, Kasian SJ, Owens JE, Marsh GR. Binaural auditory beats affect vigilance performance and mood. Physiol Behav. 1998; 

4.        Jirakittayakorn N, Wongsawat Y. Brain responses to a 6-Hz binaural beat: Effects on general theta rhythm and frontal midline theta activity. Front Neurosci. 2017; 

5.        Jirakittayakorn N, Wongsawat Y. A Novel Insight of Effects of a 3-Hz Binaural Beat on Sleep Stages During Sleep. Front Hum Neurosci. 2018; 

6.        Foster DS. EEG and Subjective Correlates of Alpha-Frequency Binaural-Beat Stimulation Combined with Alpha Biofeedback. 1990; 

7.        Garcia-Argibay M, Santed MA, Reales JM. Binaural auditory beats affect long-term memory. Psychol Res. 2019; 

8.        Axelsen JL, Kirk U, Staiano W. On-the-Spot Binaural Beats and Mindfulness Reduces the Effect of Mental Fatigue. J Cogn Enhanc. 2020; 

9.        Marcora SM, Staiano W, Manning V. Mental fatigue impairs physical performance in humans. J Appl Physiol. 2009; 

10.      Lopes TR, Oliveira DM, Simurro PB, Akiba HT, Nakamura FY, Okano AH, et al. No Sex Difference in Mental Fatigue Effect on High-Level Runners’ Aerobic Performance. Med Sci Sport Exerc. 2020;Volume Pub. 

Simple Ways to Improve your Sleep for Athletic Performance

The quality of our sleep has huge implications for athletic performance. Sleep is essentially the time when physical and mental recovery occurs. With good cognitive function and physical readiness being required for us to perform at our peak, it is obvious to see how it is important that we have good quality sleep to perform at our best.

Start listening to our circadian rhythms

“We are the supremely arrogant species; we feel we can abandon four billion years of evolution and ignore the fact that we have evolved under a light-dark cycle. What we do as a species, perhaps uniquely, is override the clock. And long-term acting against the clock can lead to serious health problems”

Professor Russell Foster

Our sleep patterns our guided by circadian rhythms, which essentially act as our body clock and determine the appropriate time for hormone release, which in the case of sleep is melatonin. With the invention of the light bulb and screens has come the ability to overcome this natural body clock. The presence of blue light reduces the secretion of melatonin, increasing alertness and keeping us awake. This delays the onset of sleep and reduces the amount of time that we spend asleep. By cutting out screens as you prepare to go to sleep you can ensure that your onset of sleep is faster and thus your time in bed is more efficient.

Think in cycles not hours

“Eight hours sleep is an average amount of sleep people get per night, and it somehow seems to have become a recommended amount – for everyone. The resultant pressure put on getting this is incredibly damaging and counterproductive to getting the right amount of sleep that we individually need”

Nick Littlehales

To maximise the quality of our sleep we should quantify sleep in terms of cycles, not hours. One sleep cycle lasts approximately 90 minutes and hence if we can time our sleep to wake up at the end of a cycle we can wake up at the lightest point of our sleep and feel refreshed and ready to start the day.

We should also not be too concerned about the amount of sleep we get each day but instead over the whole week. In his book “Sleep”, Littlehales suggests that we should be getting 35 cycles per week, averaging 5 cycles a day, which works out as 7 hours 30 minutes. This approach is much more achievable than consistently hitting 8 hours which can be quite pressuring and stressful and a cycle approach is reflective of how we actually sleep. Littlehales says that this approach reduces the stress hormones released from struggling to sleep and allows us to get effective rest and recovery.

Apps like Sleep Cycle ensure that your alarm wakes you up at the lightest point of sleep possible meaning that you wake up feeling refreshed and ready to start the day.

Be more consistent

Our body adapts to the time we fall asleep and thus by being more consistent, we can fall asleep quicker and ensure the time that we spent in bed is efficient. Not only is our sleep more efficient when we are consistent, but we also get more slow-wave and REM sleep – the times when physical and mental recovery occurs respectively. This allows us to maximise the benefits of training and perform at our best. 

Laura Kline, Rewire Athlete, tells us that by developing a consistent routine she has been able to ensure she gets adequate sleep. “By 8:00 I have my magnesium drink and try to limit my screen time. I aim to be in bed by 9:30 – I find that following a set schedule makes a difference as my body knows it’s time to shut down.” Laura says that by doing this she can typically fall asleep within minutes and on the inevitable days that she can’t follow her routine she notices a difference the next day. 

Optimising our sleep is not necessarily about getting more sleep, but about making the time that we spend in bed as efficient as possible. Only by working to improve the way that we sleep can we truly allow for sufficient recovery to develop our athletic performance.

Further Reading:

Blue light from light-emitting diodes elicits a dose dependent suppression of melatonin in humans
West et al.
Journal of Applied Physiology, 2011

Sleep: Change the way you sleep with this 90 minute read
Nick Littlehales

New Feature: Sleep Consistency – Why We Track it, How Do You Compare?
Whoop

‘Arrogance’ of ignoring need for sleep
James Gallagher