“You only have one muscle, Conor.” That’s what my crossfit coach used to say to me in his thick, Eastern European accent. It reminds me of the quote by John Muir, though with less personal significance (since I wasn’t as sweaty when I first heard it): “When we try to pick out anything by itself, we find it hitched to everything else in the Universe.”
Humans are complex, integrated systems—this is the one muscle. In other words, the heart is important. And so is the brain. Information passes from brain to heart and vice versa. Let’s start there.
Writing on the integration of the heart and brain goes back at least as far as Darwin. In 1872, Darwin commented on the way the brain affects the heart and the heart affects the brain through what’s now known as the vagus nerve. Darwin was onto something. If I had one single data point with which to quantify health, I’d take one that quantifies this heart-brain connection any day.
Why? Because regulation of the nervous system is important. Really important. In some sweet spot between overtraining and undertraining lies an optimal growth path. The health of the nervous system helps us find that spot. And these concepts don’t just apply to athletes. Since they’re really just intellectual athletes, cognitive high performers can benefit from this same language.
Author and intellectual bad boy Nassim Taleb popularized the term “antifragile” in response to a perceived gap in language. He was wondering what the opposite of fragile was--and he found the best term we have is “resilient.”
Resilience is a high watermark in clinical psychology. But some systems grow from disorder. Muscles, for example, through the process of “supercompensation,” not only deal with high stress events but become stronger afterwards. The term “antifragile” applies to the class of systems that exceed baseline after stress events.
So, to calibrate an optimal growth trajectory, we need a way to quantify the nervous system as a measure of antifragility. This is where heart rate variability (HRV) comes in.
HRV is a measure of the beat-to-beat changes in heart rate. We generally think of heart rate as constant since we throw around terms like beats per minute and max heart rate. But this is misleading. The time between heart beats varies quite drastically based upon nervous system function.
Going back to the vagus nerve—the brain/heart connection observed by Darwin. This is the longest nerve of the autonomic nervous system, with two primary branches.
One is the sympathetic nervous system, or our fight or flight stress response. The other is the parasympathetic nervous system, or our rest and digest system.
These two systems generally counteract one another, and generally work by exciting or inhibiting the nervous system.
Since one role of the autonomic nervous system is to influence the heart, HRV gives us a way to quantify the nervous system. When we inhale, we activate the sympathetic branch and heart rate increases. When we exhale, we activate the parasympathetic branch and heart rate decreases. This is why breathing is a staple across mindfulness traditions: it helps us regulate the nervous system in real time.
When I first started playing around with HRV, I strapped a sensor to a friend and seasoned meditator. I had him meditate while I watched his HRV rise, a sign of integration between sympathetic and parasympathetic function. This friend is also politically involved. So, to test my new metric du jour, I pushed the front page of the New York Times in front of him. His HRV plummeted. He suddenly became sympathetic dominant. The nervous system, acting in real time.
Some claim that HRV is the best single marker for disease because low HRV (that is, not much beat-to-beat variance in heart rate) is a primary predictor of all-cause death. Yes, that’s as grim as it sounds.
But if we flip that equation and talk about health rather than sickness, we find that HRV allows us to start to quantify antifragility. Let’s break that down into the physiological, emotional, and cognitive factors involved in self-regulation.
If we’re sympathetic dominant, our blood vessels constrict to divert blood from the gastro-intestinal tract and skin towards the muscles and lungs. Our heart rate increases and our pupils dilate. If we’re parasympathetic dominant, the inverse happens. Corresponding changes in the brain are seen in the appearance of noradrenaline and acetylcholine for sympathetic and parasympathetic activity respectively. In a nutshell, that’s the physiology.
The heart of emotional self-regulation (pun intended) is providing context-appropriate responses. It has been shown that individuals with higher levels of resting HRV produce more context-appropriate emotional responses. Here we start to see how this metric quantifies the emotional flexibility to respond to a broad range of environments in an appropriate way.
Finally, cognitive self-regulation refers to executive functions such as working memory, attention, inhibition, and flexibility. Study participants with higher levels of HRV performed better on standard tests of executive function.
When it comes to flow, we know that HRV increases in flow. We saw this in our work with Formula 1 drivers and we know this from elsewhere in the literature as well.
So I’ve been spending a lot of time on this idea for two reasons. In the moment, it provides a live status update of the body’s stress response. Over time, it also provides a metric for overtraining. It’s stupid simple to see if somebody is overtraining by looking at their HRV--and from our work with high performers we know that they have a strong tendency to overtrain.
And overtraining falls short of that elusive optimal growth trajectory.
So if you want to build a dashboard for your own optimal growth trajectory, HRV is a good place to start. It captures much of the complexity of that one integrated muscle. It’s a pulse check on antifragility.
Ok, I’m done with puns. Conor, out.
PS, want to find out more about our work with Formula 1? Check out the video here.
