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Heart Rate Variability (Part 1 of 2)

I’ve been intrigued by Heart Rate Variability (HRV) since I first heard about it from a friend 4-5 years ago. Apparently, the military uses HRV measurements to predict which special forces teams are most ready for last minute missions. From what I have gathered, HRV is the best indicator of performance amongst the different things monitored for highly trained soldiers. I thought this was fascinating and did not think much more about it until I started seeing the technology trickle down to wearables and sensors for pets and every day athletes. Although recovery and performance seems to be the focus of HRV technology providers, I was initially interested in implementing HRV due to the increasing incidence of Atrial Fibrillation (AFib) in endurance athletes as I think too much intensity in training is a factor. My passion and professional focus is on coaching cyclists but I also have 19 years of experience as a veterinarian working in an intensive critical care environment. I think that experience gives me a unique perspective. You may find it surprising that veterinarians use HRV along with other wearable metrics to help with the detective work when things ‘just ain’t right’ with the beloved pet that cannot tell us what is wrong.

What is this HRV anyway? I liked the following description on the website:

‘Heart rate variability or HRV is the physiological phenomenon of the variation in the time interval between consecutive heartbeats in milliseconds. A normal, healthy heart does not tick evenly like a metronome, but instead, when looking at the milliseconds between heartbeats, there is constant variation. ‘

“Higher HRV has been found to be associated with reduced morbidity and mortality, and improved psychological well-being and quality of life.”

I felt it was important to get into the tip of the iceberg of the science in Part 1 and here is some more info about HRV and AFib from another HRV company, ithlete. The study mentioned is also cited below for those interested. Part 2 will include my personal experience with a wearable I have been using for the last 6 months - WHOOP. Stay tuned.

So what is HRV telling us? It is an indirect measurement of the state of our Autonomic Nervous System. I’ll spare us the details from veterinary medical school and focus on the basics here as the autonomic nervous system is very complicated and fascinating.

The gist is that the autonomic nervous system has two parts that keep everything in the body balanced - the yin and yang, called the sympathetic and parasympathetic nervous systems.

The sympathetic nervous system can be thought of as our fight-or-flight response. We either fight or run, but both actions are powered by adrenaline (epinephrine) which comes from the adrenal glands. Pupils dilate, your mouth gets dry, heart rate increases, GI functions slow down, blood shifts to the muscles instead of the organs and your body is ready for immediate action. High intensity interval training (HIIT) and racing is powered by this response and I believe a little goes a long way as too much stress on the body leads to breakdown. Disease = dis-ease = too much stress. Increased sympathetic tone lowers HRV and can signal it is time to rest or ride easy until things settle down again.

The parasympathetic nervous system does the opposite: lowers heart rate, ramps up GI and organ function and brings everything back to homeostasis. In a healthy individual, both systems are constantly balancing each other out. Some stress on the body is good and keeps us strong, but most of the time we want the parasympathetic system to be doing its thing to keep us in a stress free state. Increased parasympathetic tone raises HRV and can be a signal that it is okay to take on a little stress in training.

It has been shown that the intensity of the exercise plays a large role in whether the sympathetic tone or parasympathetic tone is predominant (2). HIIT and anaerobic work triggers a sympathetic response whereas riding at or under an endurance pace (LT1, Mid-Coggan Zone 2 HR, Aerobic threshold (AT)) favors a parasympathetic response. Knowing this, along with HRV readings with athletes in training favors the popular polarized training model where 1 out of every 4-5 training rides is potentially classified as HIIT/anaerobic and the other rides are done at or below endurance pace. There is a real scientific basis for using a polarized model for performance and health and I think this is missed by many athletes and coaches that favor regular block or several interval sessions per week for a time crunched athlete. I believe this is too much stress on the body in the long run. That does not mean we cannot incorporate some times where we increase the stress load as that is necessary for improvement. If you are doing hard intervals more than 1-2 times a week regularly, then you may want to rethink your training plan and use HRV to help guide the density of your training.

Like with all things, AFib in endurance athletes is likely caused by many factors and I’m not saying that measuring your HRV will prevent it or diagnose it. This is not intended to be medical advice that you should get from your doctor. That said, the evidence at this time supports monitoring HRV to help with better training, recovery and potentially health for an endurance athlete. I’ll share my personal experience with HRV and implementing it with training in Part 2.

** This information is not to be taken as medical advice; consult your doctor with any health changes or concerns. And be sure to have routine physicals if you are undergoing any exercise or training plan.

1) Cardiac Autonomic Dysfunction and Incidence of Atrial Fibrillation

Results From 20 Years Follow-Up

Sunil K. Agarwal, Faye L. Norby, Eric A. Whitsel, Elsayed Z. Soliman, Lin Y. Chen, Laura R. Loehr, Valentin Fuster, Gerardo Heiss, Josef Coresh and Alvaro Alonso

2) [The effect of aerobic and anaerobic endurance training on the regulating function of autonomic nervous system and its significance].

[Article in Chinese]

Tian K1, Qin J, Huang L, Long M, Wu J, Yu S, Yu Y.

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