We have released a new feature in HRV4Training Pro, Aerobic Endurance Analysis for runners and cyclists. In this post, we go over the background, cover how you can use this feature to track changes in aerobic endurance as you progress in your training and also provide additional details about how this differs from other estimates we provide, and how you can benefit the most from this feature.
In case you want to jump right in, and check out our latest feature, simply login at HRV4T.com with your HRV4Training credentials. We have a promotion and until December 9th you can get 15% off using code STAYAEROBIC at checkout.
What's aerobic endurance?
Aerobic endurance (or aerobic efficiency) relates to your ability to sustain a given workload. Endurance athletes tend to have high aerobic endurance, meaning that they can sustain a relatively high workload (for example pace or power), at a relatively low effort (typically measured in terms of heart rate).
To determine your aerobic endurance we compute the relation between output (pace or power) and input (heart rate). Intuitively, a lower heart rate for the same output (pace or power), when consistently shown over periods of weeks, translates into better aerobic endurance.
Similarly, a higher power or faster pace at the same heart rate, is linked to improved aerobic endurance. By analyzing the relationship between input and output for running or cycling activities, you can easily track aerobic endurance changes over time, as you progress with your training.
What's the difference with VO2max estimation?
If you are familiar with our work on VO2max estimation, you'll know that the same principle just explained, is also the principle behind VO2max estimates. In particular, the ratio between heart rate and pace or power is used as one of the predictors in the VO2max estimation model. You can learn more about VO2max estimation here.
What's the difference then? While VO2max is a good marker of cardiorespiratory fitness and aerobic endurance, the estimate depends also on parameters that have very little to do with actual aerobic endurance and performance, for example body weight. Losing weight will increase your VO2max without necessarily improving your aerobic endurance or performance.
Additionally, there are factors that can only be partially accounted for when estimating VO2max. A few examples are: running on trails or difficult terrains, which reduces pace and makes your data not really representative of your fitness, very short workouts where heart rate does not reach steady state, very long workouts where heart rate drifts, environmental factors such as hot days or training at altitude, etc. - the list goes on.
While many of these parameters are simply impossible to account for, what we can do is give you more control over what data is used to track changes in aerobic endurance. In particular, via the panel below you can filter workouts and environmental factors so that the resulting data is more representative of your aerobic endurance. You can also select how much data you'd like to use for each data point, for example selecting light smoothing, only this week of data will be used, while using average smoothing, which I recommend, uses 3 weeks of data.
The filtered workouts are also listed at the bottom of the page:
Blog post by Alessandra
HRV4Training is looking for brand ambassadors worldwide!
Are you passionate about sport and technology and have been using HRV4Training daily to improve your performance? We are looking for you!
We are going to select up to 10 brand ambassadors, and we accept applications in English, Italian and Spanish. Please read below for instructions.
Daniel Plews, age group ironman world champion and course record holder. First HRV4Training ambassador.
How can you apply?
To participate in the selection process, send us a couple of paragraphs about yourself, why you chose our app, what you love most about it and what do you expect from this collaboration (250 words max).
Don’t forget to add a picture of yourself in action and your social media profile/s.
What do we expect from the ambassador?
What do we offer to the ambassador?
Deadline: November 25th.
To apply please send a message to firstname.lastname@example.org
Full article at this link.
Blog post by Marco Altini
As previously reported we have added support for the CorSense sensor by Elite HRV.
CorSense is a sensor you can use rather than a cheststrap, and it is compatible with most Apple iOS and Android OS devices.
In this post, we'll show a few minutes of data collected under different conditions, highlighting how the sensor is very accurate in detecting RR intervals and can therefore be used reliably for HRV analysis.
Data was acquired using the CorSense sensor and a Polar H7 (previously validated with respect to ECG here), both connected to a different device running the HRV Logger app, which is an app that simply records everything coming from the sensor plus additional features.
During data acquisition, we collected data a few minutes while breathing freely, and a few minutes while deep breathing, to elicitate higher HRV due to RSA. You will see in the plots below visually the effect of deep breathing as we get greater swings in RR intervals.
A final note on data synchronization: data cannot be perfectly synchronized because it is not timestamped by the sensors. What we can do is either to log real time and then to split data in windows based on when data was collected, then compute HRV features on these windows or to sum up RR intervals over time. For this analysis we went with the second option and also tried to visually align the data streams.
Blog post by Marco Altini
We have released a new feature in HRV4Training Pro, Functional Threshold Power (FTP) estimation for cyclists. In this post, we go over the background, cover how you can use this feature to plan and track your training and progress and also provide additional details about the underlying algorithms that we developed to estimate FTP and their accuracy, as well as some of the current limitations.
In case you want to jump right in, and check out our latest feature, simply login at HRV4T.com with your HRV4Training credentials.
HRV4Training can now be used to read HRV data from the Health app, convert that data (SDNN) to Recovery Points (a more readable metric), and analyze your physiology similarly to what we normally do when you measure using the phone camera or an external Bluetooth sensor. You can learn more about this feature in this blog post, while below we go more practical on how to use your Watch with our app.
Due to some limitations in the way apps can communicate with the Apple Watch, you need to follow the following steps in order to gather meaningful data:
If you do not get your data in Health right after using the Breathe app, try to synch your Apple Watch and it will show up a few seconds afterwards.
Always remember that context is key, so while the Apple Watch writes somewhat random HRV numbers also during the day or night, that data could be affected by artifacts, and it is always decontextualized.
To properly interpret physiology, data must be acquired under standard, reproducible conditions, and the best way to do so is with a measurement as soon as you wake up, or with a night long measurement (not just a minute or two over a night).
What are Recovery Points? A more human friendly HRV score. For more information, read this.
How can you use SDNN instead of rMSSD to generate Recovery Points? SDNN captures physiological stress similarly to rMSSD hence it can be used in a similar manner. For more information, read this.
How accurate is the Apple Watch in measuring HRV? Very accurate, provided you stay completely still and use the Breathe app to take a measurement. For more information, read this.
When should I use the Breathe app to take a measurement? First thing in the morning.
How much time do I have after measuring with the Breathe app, to fill in my tags in HRV4Training? You have one hour. When you tap 'read from Health' we always check only the last hour, and see if we can find any HRV scores in the Health app, then take the last one. For this reason, we highly recommend reading data right after you have measured.
Why can I see only Recovery Points and Heart rate instead of all HRV features when using the Apple Watch to measure? The Apple Watch does not provide us with RR intervals that could be used to compute different features, but only with SDNN. Hence, other features cannot be computed, apart from the Recovery Points that we show in the app.
Should I use the Watch or the camera? While the watch provides accurate data under ideal conditions, the metrics reported are limited with respect to the ones we can compute directly using the camera or a bluetooth sensor. Similarly, having no access to RR intervals or raw data, we need to trust Apple to correctly identify and remove potential artifacts. Hence the camera still remains our preferable method, unless you find it uncomfortable or are experiencing any issues.
Heart Rate Variability (HRV) features: can we use SDNN instead of rMSSD? A data-driven perspective on short term variability analysis
One really good thing about the sports science community, is that we have settled on what feature to use when we talk about heart rate variability (HRV).
As many of you know, HRV can be computed in many different ways, starting from our basic unit of information, the RR intervals (beat to beat differences in instantaneous heart rate). The sports science community through the work of many, including Martin Buchheit, Daniel Plews, Paul Laursen, Andrew Flatt, Martin Esco, Fabio Nakamura and a few others, in the past 10-15 years, settled on rMSSD as the most meaningful and practical feature to use in applied research and real life, when working with athletes.
Why rMSSD? Well, first of all, most sports scientists are physiologists, they know what they are talking about when considering physiological processes in the human body, and it turns out what also came up from all these studies, is that there is mainly one thing that can be measured using short term HRV features: parasympathetic activity. Without going into another primer on HRV (see this post if you are looking for one), parasympathetic activity represents our body's rest & recovery system, and can be captured in terms of HRV: a stressor might for example induce a physiological response in terms of reduced parasympathetic activity, which translates into lower HRV as the nervous system modulates heart rhythm in response to such stressor. Parasympathetic activity acts quite fast, in the matter of seconds. How do we capture these fast changes? rMSSD, due to how it is computed (just math), captures fast changes in instantaneous heart rate, hence it reflects very well parasympathetic activity. It's also easy to compute and standardized, hence we can be certain we all talk about the same thing, which is a good starting point.
Wonderful, we have a feature that everybody agrees on, and has also a clear link to how physiology works. All problems are solved and we can use HRV4Training or our favorite HRV app to gather data, compare results, and learn a bit more about how our body responds to training and life stress.
Well, not so fast.
Blog post by Marco Altini
I have helped Strava developing their current Relative Effort, a metric used to quantify training effort, combining intensity and duration. You can read Strava's official launch blog post here as well as an interview I gave here if you are interested in learning a bit more about how this metric was developed and how you can use it effectively.
Blog post by Marco Altini
Sharing a podcast I've recorded recently. Thank you Mikael for inviting me to discuss HRV4Training, physiology and human performance on the Scientific Triathlon podcast: https://scientifictriathlon.com/tts144/
In this podcast, we skip the basics of HRV (that hopefully are clear by now!) and go directly into how you can use the data, in particular covering:
Will wrap that up in a blog post soon, but you can see some screenshots here should you be interested: https://www.strava.com/activities/1798124065/overview
Since launching HRV4Training, the easiest and most cost-effective solution to acquire high quality physiological data, in particular HR and HRV at rest, we published a fair amount of work. From the validation of the camera-based measurement, to acute day to day changes in physiology (heart rate and HRV) in response to training, to methods to estimate VO2max from workout data, methods to estimate running performance and the relation between HRV, training load and injury in Crossfit. Transparency and solid scientific grounds are what we believe in, which is why we started documenting to the public and validating our work since day zero.
Most importantly, HRV4Training gave the opportunity to universities not associated with us, to collect physiological data easily in the wild, potentially leading to additional insights. More and more universities have started publishing papers based on data collected using the app and web platform in the past few years, and we'd like to cover the last two that came to my attention recently. Both papers are authored by Sara Sherman, who was a master's student at the University of Alabama, under the supervision of Michael Esco, who needs no introductions.
Sara is currently pursuing a PhD at the University of Illinois-Chicago and will be providing her input alongside mine in this overview of her work.
The two studies authored by Sara cover two different topics, both investigated in a population of thirty-one NCAA Division I female rowers at the University of Alabama, Tuscaloosa.
Let's break down this section into measurement time and menstrual cycle, and discuss the two studies separately.
Just a quick announcement that all new iPhones are already compatible with HRV4Training, short video on an iPhone XS below.
As usual, please double check our camera based measurement best practices, to make sure you'll collect high quality data on which meaningful analytics can be derived.
We have just added support for the CorSense. CorSense is a sensor you can use rather than a cheststrap, and it is compatible with most Apple iOS and Android OS devices.
Now you can use it with HRV4Training, too.
Blog post by Marco Altini
We have just released our latest feature in HRV4Training Pro: half marathon and full marathon running time estimates. In this post we go over how these prediction models work.
In particular, this work is an extension of our previously published analysis (see "Estimating running performance combining non-invasive physiological measurements and training patterns in free-living” which was accepted for publication at the 40th International Engineering in Medicine and Biology Conference, full text here, while another blog post explaining the paper can be found here).
In the published work we built models able to estimate running performance (10 km time) using 2 years of real world data from more than 2000 individuals, including morning physiological measurements obtained using HRV4Training, workouts acquired from Strava and TrainingPeaks, anthropometrics and training patterns.
Next week it's HRV4Training 5 years birthday, and we are doing a small giveaway.
You can win either free access to HRV4Training Pro for one year (more info on the web platform can be found here: https://www.hrv4training.com/pro--teams.html), or a singlet (see pic below).
To participate, simply write a post or blog on HRV4Training (anything you like or find useful for your training or lifestyle in general), and tag us (or email it to us) either on Instagram or Facebook (or your blog!) - you have until next week (August 18th).
Just make sure we know you wrote it :)
Marco & Ale
Just a quick update that we have just added support for several Android phones that struggled with HRV4Training, in particular the ones with multiple cameras such as the Huawei P20 Pro.
Please make sure to double check this list of practical tips for your measurement. and in particular making sure your phone is not doing many other operations during the measurement, maybe give it a try even in airplane mode to ensure there is not much background network activity, in case you are experiencing problems.
Blog post by Marco Altini
In the past few months we’ve been busy building HRV4Training Pro, a web platform for individuals and teams aiming at better understanding how different stressors affect their body, so that adjustments towards better health and performance can be made.
In this post, I’d like to cover the main approach behind our new platoform, deriving from the past 5 years of learnings. Since we launched the first and only validated camera-based Heart Rate Variability (HRV) measurementa couple of years back, we had the opportunity to learn a lot through continious iterations and feedback from our community as well as from top scientists in the field.
From the average guy just like myself, to elite triathletes that I occasionally enjoy slowing down, HRV4Training made it extremely easy for everyone to gather meaningful data points linked to physiological stress.
So, what’s HRV4Training Pro about?
Blog post by Marco Altini
Scosche recently announced their latest sensor, the Rhythm24. The new sensor builds on the previous Rhythm+, and adds a lot of new functionalities, including an heart rate variability (HRV) mode. For a detailed overview of the many functionalities of the sensor, check out DC rainmaker's review here.
In this post, we will focus on only one of the many new features this sensor brings, which is the ability to send accurate RR intervals, which can be used for heart rate variability analysis once connected to an app such as HRV4Training. We've been very happy with a few preliminary tests, and would recommend using this sensor to everyone that had trouble with the camera based measurement (for example on some of the non-compatible Android phones) or simply prefers to rely on an external sensor.
You can get the sensor at this link or clicking below:
If you've been following some of our previous validations, you know that most wristbands, while accurate for heart rate analysis, cannot be used for HRV, as they heavily filter the signal (a procedure typically needed to acquire a more stable signal, less affected by motion artifacts, and therefore able to guarantee a more reliable heart rate during exercise, which is the main target application of such sensors).
Let's look at some data.
Publication: "Estimating running performance combining non-invasive physiological measurements and training patterns in free-living"
Our latest work, titled "Estimating running performance combining non-invasive physiological measurements and training patterns in free-living" was accepted for publication at the 40th International Engineering in Medicine and Biology Conference.
In this work we build models able to estimate running performance using 2 years of real world data from more than 2000 individuals, including morning physiological measurements obtained using HRV4Training, workouts acquired from Strava and TrainingPeaks, anthropometrics and training patterns.
In particular, we provide insights on the relationship between training and performance, including further evidence of the importance of training volume and a polarized training approach to improve performance.
More details, at this link.
This post is mainly motivated by countless emails (and a few negative reviews) that we have received regarding the possibility to integrate the Apple Watch in HRV4Training, which clearly highlight some misinformation around the topic of both HRV and the Apple Watch itself.
Simply put, at this stage, the Apple Watch cannot be used for reliable and meaningful HRV analysis using third party apps.
Interested in learning why? Read on.
We have released a new feature in HRV4Training, lactate threshold estimation. In this post, we go over the background, cover what we mean as lactate threshold, how you can use this feature to plan and track your trainings and progress and also provide additional details about the underlying algorithms that we developed to estimate lactate threshold and its accuracy, as well as some of the current limitations.
Lactate threshold estimation in HRV4Training and actual 10 km race time.
TrainAsONE is your very own AI personal running coach that helps to keep you fit, healthy, and injury free whilst training to reach your goal. Designed for runners of all abilities, award-winning TrainAsONE constantly adapts to your lifestyle and every run your perform (or miss). Powered by advanced artificial intelligence and big data analysis of over 11 million kilometres of running, an indicator of its success is the weekly tally of personal bests and approaching 100 podium places enjoyed by their users to date.
We recently collaborated with TrainAsOne and added their platform to the list of our supported integrations, allowing all TrainAsOne users to push HR and HRV data via HRV4Training.
Similarly to other integrations, you will have to go to Menu / Settings and enable the integration, then login with your TrainAsOne credentials, so that we can push HRV data each morning automatically, right after the measurement.
You can also manually push data for any past day from the History page, by tapping a measurement bar, and selecting 'Push to TrainAsOne' - data will show up under Settings / Health and Metrics in TrainAsOne.
We hope you'll enjoy the new feature. Please see a few screenshots below.
Heart rate variability (HRV) is a well-understood phenomenon allowing us to monitor objectively physiological stress.
However, historically HRV analysis has been poorly standardized, leading to difficulties in properly designing and implementing studies as well as difficulties in comparing studies outcomes. The ease of access to HRV data today often obscures the complicated nature of understanding and correctly interpreting the information provided and underlying physiological processes.
When designing HRV4Training we had to make several choices that I'd like to highlight here as transparency and clarity can only move this field forward.
In this (updated) post we cover the basics of HRV and in particular the physiological mechanisms behind it, as well as best practices and a brief introduction to data analysis. I hope what follows can give some clarity to the ones interested in learning more about physiological stress and how their body responds to it.
What is heart rate variability (HRV)?
Each beat of our heart is triggered by an electrical impulse that can be easily recorded by an electrocardiogram (ECG), one of the most common ways to monitor heart activity. However, our heart doesn't beat at a constant frequency. When we talk about heart rate variability (HRV), we are interested in capturing the variability that occurs between heart beats.
Let's look at 60 seconds of ECG data. This is some data I recorded on myself using the ECG Necklace, a research prototype we developed when I was working at imec, a few years ago. The device is a small sensor connected to 2 ECG leads:
In technical jargon, the differences between beats are called RR intervals. The name derives from the fact that the shape of the ECG signal at each beat has been assigned letters (namely the QRS complex). For more on the QRS complex you can just have a quick look on the Wikipedia page, however the only relevant point here is that R represent the peak(s). Going back to my ECG, if we zoom in a bit and look at only 10 seconds, we can see clearly there are differences between intervals; some are shorter, other longer:
Another way we can look at these differences is by plotting an histogram of the RR intervals. Basically we stack up RR intervals that are of similar duration. This way it's actually much easier to see how the values are distributed over a rather wide range. For this plot, I used again the full minute of data from the first plot:
It was April 12, 2017, 5 days out from the 2017 Boston Marathon when my coach (Adam St. Pierre Carmichael Training Systems) first introduced me to HRV4Training.
As a physician, runner, and gadget geek I was immediately intrigued by this quantitative metric of stress. Both personally and professionally my stress over the past ten years of life has been both manageable and predictable. My professional time was spent working shifts at my local hospital. While my shifts were often stressful, I was able to leave my stress where it belonged (at work). When I was off I was off, and could sleep well knowing that my partners would provide great care for the patients I had handed off to them. After a tough shift, I would intuitively know that my body and mind needed rest. Easy runs after hard shifts were the norm.
As life would have it, one month after learning about HRV4Training my career took a dramatic change. I was laying awake at 3AM at the hospital when I came across a posting on my Facebook feed asking if I would be interested in founding an online primary care practice especially for runners called SteadyMD. I was immediately hooked and found myself on a plane bound for St. Louis to meet with the founders of the company. The day after my meeting in St. Louis I embraced entrepreneurship and began monitoring my HRV.
My life of predictable stress as a physician, parent, and athlete quickly morphed into a life of unpredictable stress by adding the additional role of startup entrepreneur. Many of my friends are founders of companies. For years they have shared their stresses with me, but it wasn’t until recently that I could relate to what they had been going through.
Challenge has always been my main motivator in life. Easy tasks don’t engage me. Medical school, mountaineering, big wall climbing, backcountry skiing, marathons, and raising a family have all been my objectives. Starting a business is hands down the hardest thing I have done to date.
As an entrepreneur my work is no longer confined to the walls of the hospital. My sleep has become more erratic with late nights responding to emails followed by early mornings waking with epiphanies and excitement. Despite my increased workload, balance is of paramount importance to me. Falling into the trap of neglecting family, career, and workouts is not an option. HRV tracking has been a useful tool to achieve this balance.
Metrics aside, spending 60 seconds every morning checking HRV provides a valuable opportunity to build self-awareness. Learning to listen to your body is an invaluable skill. I use the HRV testing interval as a daily ritual to check in with myself and simply be mindful of my situation.
My HRV consistently drops after busy hospital shits, nights of restless sleep, and periods of consistent hard training. As a runner I train with heart rate and power. Many argue that these metrics are inferior to pacing based on perceived exertion. I agree that self-awareness is the ultimate measure, but for most people objective quantitative metrics are a means to help better understand ones body.
Since partnering with SteadyMD I have found utility in using HRV4Training as a means to coach my patients to a healthy stress/rest balance. In the recent book Peak Performance by Magness and Stulberg the authors present a growth equation that is defined as growth=stress+rest. HRV is a great way to quantify the rest portion of the equation. HRV provides me as a physician a window into my patients’ progress towards growth.
For years I have seen athletes over train and suffer the consequences of deteriorating performances, injuries, and even development of chronic medical conditions.
My goal at SteadyMD is to keep my patients healthy and performing to their potential. To learn more about my innovative primary care practice for runners that provides you access to me from the convenience of your Smartphone go my website www.steadymd.com/hrv or follow me on Facebook, Instagram, or Twitter @steadymdrunning
Final Surge is the training and coaching platform that empowers athletes and coaches to reach fitness and performance excellence like never before. Ideal for runners, cyclists, and multisport athletes and coaches in sports like triathlon, duathlon, XTERRA and more. Final Surge is used by athletes, coaches, teams and clubs of many sports around the world where performing your best is what it’s all about. FinalSurge.com, and the Final Surge iOS and Android app, have all of the features you need to track and analyze your training, from communicating with coaches and planning future workouts to synchronizing workout, activity, and GPS data from your tracking devices. We recently collaborated with Final Surge and added their platform to the list of our supported integrations, allowing all Final Surge users to push HR and HRV data via HRV4Training.
Similarly to other integrations, you will have to go to Menu / Settings and enable the integration, then login with your Final Surge credentials, so that we can push HRV data each morning automatically, right after the measurement.
You can also manually push data for any past day from the History page, by tapping a measurement bar, and selecting 'Push to Final Surge' - data will show up under Daily Vitals in Final Surge.
We hope you'll enjoy the new feature. Please see a few screenshots below.
Today's Plan is one of the leading platforms used by cyclists to track and optimize training, including a sophisticated training system, coaching tools and analytics as well as many integrations with other sensors and tools out there.
As many HRV4Training users are also Today's Plan users, we have received many requests to integrate the two platforms. Today's Plan provided great support in the past few weeks so that we could add their platform to our integrations, and provide all HRV4Training users with the possibility to push their physiological data to Today's Plan.
Similarly to other integrations, you will have to go to Menu / Settings and enable the integration, then login with your Today's Plan credentials, so that we can push HRV data each morning automatically, right after the measurement.
Note that you will be able to push data to Today's Plan only if you are a premium user on their platform, otherwise we are not allowed to push data.
You can also manually push data for any past day from the History page, by tapping a measurement bar, and selecting 'Push to Today's Plan' - data will show up under Calendar in Today's Plan.
We hope you'll enjoy the new feature. Please see a few screenshots below.
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and try the HRV4Training app!
1. Context & Time of the Day
3. Paced breathing
4. Orthostatic Test
5. Slides HRV overview
6. rMSSD vs SDNN
1a. Acute Changes in HRV
1b. Acute Changes in HRV (population level)
1c. Acute Changes in HRV & measurement consistency
1d. Acute Changes in HRV in endurance and power sports
2a. Interpreting HRV Trends
2b. HRV Baseline Trends & CV
3. Tags & Correlations
4. Ectopic beats & motion artifacts
5. HRV4Training Insights
6. HRV4Training & Sports Science
7. HRV & fitness / training load
8. HRV & performance
9. VO2max models
10. Repeated HRV measurements
11. VO2max and performance
12. HR, HRV and performance
13. Training intensity & performance
14. Publication: VO2max & running performance
15. Estimating running performance
Camera & Sensors
1. ECG vs Polar & Mio Alpha
2a. Camera vs Polar
2b. Camera vs Polar iOS10
2c. iPhone 7+ vs Polar
2d. Comparison of PPG sensors
3. Camera measurement guidelines
4. Validation paper
5. Android camera vs Chest strap
6. Zoom HRV vs Polar
7. Apple Watch and HRV
8. Scosche Rhythm24
9. Apple Watch
1. Features and Recovery Points
2. Daily advice
3. HRV4Training insights
4. Sleep tracking
5. Training load analysis
6a. Integration with Strava
6b. Integration with TrainingPeaks
6c. Integration with SportTracks
6d. Integration with Genetrainer
6e. Integration with Apple Health
6f. Integration with Todays Plan
7. HRV4T Coach advanced view
8. Acute HRV changes by sport
9. Remote tags in HRV4T Coach
10. VO2max Estimation
11. Acute stressors analysis
12. Training Polarization
13. Custom desirable range / SWC
14. Lactate Threshold Estimation
15. Functional Threshold Power(FTP) Estimation for cyclists
16. Aerobic Endurance analysis
1. Intro to HRV
2. HRV normal values
3. HRV by sport
4. HRV, strength & power
5. AngelSensor & HRV
6. HRV 101: How to
7. Top 5 most read articles
8. HRV normalization by HR
9. How to use HRV, the basics