This post is in response to A Vagus Nerve Survival Guide to Combat Fight-or-Flight Urges by Christopher Bergland
Sebastian Kaulitzki/Shutterstock
Medically accurate illustration of the vagus nerve.
Source: Sebastian Kaulitzki/Shutterstock

This Psychology Today blog post is phase two of a nine-part series called "The Vagus Nerve Survival Guide." The nine vagal maneuvers featured in each of these blog posts are designed to help you stimulate your vagus nerve—which can reduce stress, anxiety, anger, and inflammation by activating the "relaxation response" of your parasympathetic nervous system.

Tonic levels of aerobic exercise stimulate your vagus nerve and lower stress responses associated with "fight-or-flight" mechanisms. Tonic levels of low, moderate, and vigorous physical activity also improve heart rate variability (HRV), which is the measurement of variations within beat-to-beat intervals.

Why Is it Important to Seek a "Tonic Level" of Daily Physical Activity?

When it comes to the dose and intensity of your daily exercise regimen, there is one important caveat: Overtraining or doing too much cardiovascular exercise lowers HRV and reduces vagal tone. Therefore, I purposely use the phrase “tonic levels” of physical activity to emphasize the importance of working out at a level of exertion that makes you feel good while making sure to avoid the pitfalls of exercise fanaticism. That said, everyone is going to have slightly different levels of Rated Perceived Exertion (RPE) while doing various intensities of aerobic exercise. What feels easy for you may seem intense to me, and vice versa. 

Additionally, there's an ongoing debate about how vigorously someone needs to work out to improve feelings of subjective and overall well-being. Unfortunately, there still isn’t a clear consensus amongst experts. Some studies say moderate-to-vigorous physical activity (MVPA) is best, while others say low-intensity exercise may be better. 

All animals (including humans) seek pleasure and avoid pain. Therefore, if exercise is viewed as a disagreeable or “painful” experience, odds are that you will avoid this behavior. Based on the universal concept of the "Pleasure Principle," it's important to find a physical activity that you enjoy and then to exercise at a “tonic level” of intensity that makes you feel good. Of course, this level of intensity will probably vary from day to day and at various stages of your lifespan. 

I am a retired ultra-endurance athlete (aka "exercise junkie") who spent decades pushing my mind and body to extreme limits by doing things such as breaking a Guinness World Record by running 153.76 miles nonstop on a treadmill in 24 hours and winning the Triple Ironman (7.2-mile swim, 336-mile bike, 78.6-mile run) three years in a row. Needless to say, I know the pitfalls of exercise fanaticism and overtraining very well. Fortunately, I survived that phase of my life without any long-term injuries and practice what I preach when it comes to seeking a "tonic level" of exercise these days.

Luckily, when I was younger and relentlessly pushing my body to its absolute physical limits, my City Coach co-founding partner and exercise physiologist guru, Jonathan Cane, taught me the importance of monitoring HRV as an ultra-endurance athlete. 

Coach Cane insisted my HRV stayed consistently high, which was a marker that my parasympathetic nervous system was robust and that my vagal tone was strong. Pulling the reins in on my tendency towards exercise fanaticism allowed me to stay right on the cusp of overtraining without ever burning out due to the toxic stress byproducts (such as chronically high cortisol levels) associated with a hyperactive “fight-or-flight” sympathetic nervous system response.

In the 1990s, it became well known amongst kinesiologists and sports medicine experts that HRV was an invaluable tool for achieving peak performance as a marathon runner, Ironman triathlete, or ultra-distance athlete of any sport.

Below are a few scientific studies that offer some empirical evidence that supports the connection between moderate “tonic levels" of physical activity and improved HRV as a marker for robust parasympathetic responses and healthy vagal tone for people of all ages and various demographics:

In 1992, a study, “Comparison of 24-hour Parasympathetic Activity in Endurance-Trained and Untrained Young Men,’ was published in Journal of the American College of Cardiology. The researchers concluded,

“Parasympathetic activity is substantially greater in trained than in untrained men, and this effect is present during both waking and sleeping hours. These data suggest that exercise training may increase parasympathetic activity over the entire day and may, therefore, prove to be a useful adjunct or alternative to drug therapy in lessening the derangements of autonomic balance found in many cardiovascular diseases.”

In 1993, Ronald E. DeMeersman of Columbia University’s Applied Physiology Laboratory published a report, “Heart Rate Variability and Aerobic Fitness." DeMeersman concluded,

“Heart rate variability, a noninvasive marker of parasympathetic activity, diminishes with aging and is augmented after exercise training. The physically active group had significantly higher fitness levels which were associated with significantly higher levels of heart rate variability when compared with their sedentary counterparts. These findings provide suggestive evidence for habitual aerobic exercise as a beneficial modulator of heart rate variability in an aging population.”

In 2004, another study, “Moderate Physical Exercise Increases Cardiac Autonomic Nervous System Activity in Children with Low Heart Rate Variability,” was published in the journal Child's Nervous System. The researchers concluded, “Our data suggest that the 12-month moderate exercise training has a positive effect on cardiac ANS activity in the children who initially had low HRV.”

The last clinical study that I’m going to reference in this blog post is a comparative analysis, “Heart Rate Variability and Physical Exercise: Current Status,” which was published in the journal Herz: Cardiovascular Diseases. The authors of this meta-analysis conclude,

“HRV is currently used for the noninvasive assessment of autonomic changes associated with short-term and long-term endurance exercise training in both leisure sports activity and high-performance training. Furthermore, HRV is being investigated as a diagnostic marker of overreaching and overtraining.

A large body of evidence shows that, in healthy subjects and cardiovascular patients of all ages (up to an age of 70 years), regular aerobic training usually results in a significant improvement of overall as well as instantaneous HRV. These changes, reflect an increase in autonomic efferent activity and a shift in favor of enhanced vagal modulation of the cardiac rhythm. Regular aerobic training of moderate volume and intensity over a minimum period of 3 months seems to be necessary to ensure these effects, which might be associated with a prognostic benefit regarding overall mortality.”

Technically, measuring your heart rate variability (HRV) with scientific precision requires some high-tech gadgetry. But, there are also a variety of low-tech options that measure RR intervals from Polar, Mio Alpha, LifeTrak, FitBit, and many other manufacturers. The key to all of these HRV tools is to understand that you are your own universe when it comes to monitoring your HRV. Once you have established a baseline, don’t compare your numbers to anybody else. Simply keep track of where your HRV is in relation to your baseline.

Since retiring from athletic competitions, I don’t use any gadgets to monitor my HRV. If you want to make the investment in an HRV monitor, it can be a valuable tool....but you can also just use common sense and lifestyle markers (such as overuse injuries, changes in appetite, irritability, insomnia, etc.) to gauge the signs of overtraining and use intuition and gut instincts to predict that your HRV may be low based on everyday psychological signals that you are stressed out.

Hopefully, having some empirical evidence on the link between HRV, stimulating your vagus nerve, and reducing the stress responses of your sympathetic “fight-or-flight” nervous system will serve as a source of motivation that inspires you to seek a dosage and intensity of physical activity that fits your lifestyle and makes you feel good. As I mentioned earlier, this Psychology Today blog post is "phase two" of a nine-part "Vagus Nerve Survival Guide" series. Please stay tuned for upcoming posts.  

References

Goldsmith RL, Bigger JT Jr, Steinman RC, Fleiss JL. Comparison of 24-hour parasympathetic activity in endurance-trained and untrained young men. J Am Coll Cardiol. 1992 Sep;20(3):552-8. DOI: https://doi.org/10.1016/0735-1097(92)90007-A

Ronald Edmond De Meersman. Heart rate variability and aerobic fitness. American Heart Journal. Volume 125, Issue 3, March 1993, Pages 726-731. DOI: https://doi.org/10.1016/0002-8703(93)90164-5

Nagai, N., Hamada, T., Kimura, T. et al. Moderate physical exercise increases cardiac autonomic nervous system activity in children with low heart rate variability. Childs Nerv Syst (2004) 20: 209. doi: 10.1007/s00381-004-0915-5

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