Karen Parnell

Guide to Heart Rate Variability (HRV) for Triathletes

December 23, 2021

Guide to Heart Rate Variability (HRV) for Triathletes

By Karen Parnell

Heart Rate Variability or HRV has been gaining popularity amongst athletes and triathletes and there are many reasons to consider using these readings to help with your training. This blog discusses what it is and why you may find it useful.

 What is Heart Rate Variability (HRV)?

Heart Rate Variability (HRV) is an accurate, non-invasive measure of your Autonomic Nervous System (ANS) – which responds to everything: how you exercise, recover, eat, sleep and perceive stress. Unlike basic heart rate (HR) that counts the number of heart beats per minute, HRV looks much closer at the exact changes in time between successive heartbeats (also called inter-beat intervals, RR intervals, NN intervals, etc).

The Autonomic Nervous System is a component of the peripheral nervous system that regulates involuntary physiologic processes including heart rate, blood pressure, respiration, digestion, and sexual arousal. It contains three anatomically distinct divisions: sympathetic, parasympathetic, and enteric. For HRV we will consider the sympathetic and parasympathetic systems.

Your “fight or flight” response is an automatic physiological reaction to an event that is perceived as stressful or frightening. The perception of threat activates the sympathetic nervous system (SNS) and triggers an acute stress response that prepares the body to fight or flee.

Your parasympathetic nervous system (PNS) is also referred to as the 'rest and digest' system as it functions to conserves the body's natural activity and relaxes the individual once an emergency has passed. The parasympathetic nervous system leads to decreased arousal.

To some extent HRV is counter intuitive as the inter beat times become more stable as your fight or flight mechanism kicks in. Hormones such as Adrenaline, Cortisol, Norepinephrine kick in, and your body gets ready to run or fight. This can be caused by the stress of being late for a meeting, getting married, your children getting poor grades, over training and many other factors.

It's good idea to record things that could cause your stress (mental and physical) and try to reduce or avoid them. When your stress levels are low the inter-beat intervals become shorter and longer - irregular so when this happens you can train harder that day.

Elite HRV - Heart Rate Variability

I use a free App called Elite HRV every morning to understand my Heat Rate Variability (HRV) which relates to your physical and mental stress levels. You will need a Bluetooth heart rate monitor to do this and a list of tested and recommended HRMs are listed in the App. I use a low cost HRM from Amazon and it’s detailed later. When you wake up you simple attached your heart rate monitor, open the App and take a reading for a few minutes. The App will then interpret the reading for you and give you a recommendation on how hard to train that day and how stressed you may be.

I use this along with how I feel to work out if I should train hard or easy that day or even have a rest day with guided meditation. I would not use this solely as my guide to training but with the help of the diary in the App you can start to understand your work, life and training stress triggers and think about reducing these stress inducers.

Below are some screen shots from the App to show you what data you will get:

In the App you get a score of between 1 and 10 for Sympathetic and Parasympathetic readings. You can see that the App will tell you what your reading means for example if your Sympathetic activity is elevated it may suggest active recovery and guided breathing. If your Parasympathetic activity is high, it may suggest recovery activities such as walking or light exercise.

On another day your morning readiness score may show a higher number like a 9 shown in the image below and it says you can train harder today.

These scores should not be used in isolation but used in conjunction with how you feel to make a decision about how hard you should train that day.

Heart Rate Monitors

You don’t need an expensive heart rate monitor to take the readings, for example I use the one below from Amazon. The heart rate monitor just needs Bluetooth to connect to the App.

CooSpo Heart Rate Monitor ANT + Bluetooth 4.0 Waterproof Sensor with Chest Strap

Your heartrate monitor (HRM) should be an electrical or electrocardiography (ECG) type which is usually in the form of a heart rate monitor strap. The optical or photoplethysmography (PPG) ones like the ones built into your sports watch or arm strap HRM cannot measure the inter-beat durations needed by the HRV App. You can tell an optical HRM from the green light it emits.


By considering your HRV daily score you can have a better idea if you are highly stressed or tending towards over training. Over training can lead to injuries or sickness, less motivation, poor concentration, loss of appetite, lowered immune system and inability to progress athletically.

Using the HRV App diary you can also see a pattern and identify stress triggers which you can learn to manage or avoid.

HRV may be a good reading to add into the mix so that you train well and meet your goals. 

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Karen Parnell is a Level 3 British Triathlon and IRONMAN Certified Coach, NASM Personal Trainer and Sports Technology Writer.

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