Low-intensity exercise and the stress response
Changes in cortisol and HRV following low-intensity exercise highlight its potential benefits
The stress response is much of what I spend my days thinking about, at least in terms of how we might be able to capture it using simple measurements of our resting physiology.
In practice, when we face a stressor (a perceived ‘threat’ or challenge), we have a physiological response that mostly involves the sympathetic nervous system and the endocrine system (hormones). The sympathetic nervous system is the infamous fight or flight, it prepares us for action (increasing heart rate and blood pressure, releasing adrenaline and noradrenaline via the adrenal medulla, providing increased blood flow to needed muscles, etc.).
At the same time, the hypothalamic-pituitary-adrenal (HPA) axis releases cortisol, a key stress hormone. While we cannot really measure sympathetic activity, these changes are often accompanied by a reduction in parasympathetic activity as it can be captured with HRV (e.g. rMSSD). Over time, the parasympathetic system will take over and get things back to the initial state, or fail to do so, in case of chronic stress (something we can also capture with HRV, when using good protocols).
Exercise is obviously a stressor, but the impact of this stressor on our body is highly dependent on its intensity (at least for people who exercise regularly), something I find very fascinating.
How so?
I was recently reading “Exercise Physiology: Theory and Application to Fitness and Performance”, a textbook by Powers and Howley which I had for a few years but never got around to reading until this summer.
In the book, the authors mention how circulating cortisol levels are reduced with low-intensity exercise. We can appreciate this in the figure below, which comes from Hill, E. E., et al. "Exercise and circulating cortisol levels: the intensity threshold effect." Journal of endocrinological investigation 31 (2008): 587-591.
While exercising at 60 or 80% of VO2max led to increased circulating cortisol levels, exercising at 40% of VO2max led to a reduction.
This reminded me of Seiler’s study which I often reference as one of the key HRV studies, basically covering the principles behind using HRV to guide training (Seiler, S., Haugen, O. and Kuffel, E., 2007. Autonomic recovery after exercise in trained athletes: intensity and duration effects. Medicine and science in sports and exercise, 39(8), p.1366. - something I discuss here), with the relevant figure being the one that follows, where HRV is suppressed after moderate or high-intensity exercise, but increased after low-intensity exercise:
We have also seen that in real life, outside of the lab, similar relationships can be seen in morning HRV data following workouts of different intensities, as reported in our research (Altini, M. and Plews, D., 2021. What is behind changes in resting heart rate and heart rate variability? A large-scale analysis of longitudinal measurements acquired in free-living. Sensors, 21(23), p.7932.).
See for example below how HRV is typically higher not only after rest days but also after self-reported low-intensity exercise:
These findings are not obvious, as exercise is a stressor, and we do not necessarily expect this type of response and a rebound above pre-exercise values in such a short time. However, changes in circulating cortisol levels and HRV following low-intensity exercise seem to highlight its potential benefits to reduce stress levels.
While individual variability certainly applies (e.g. in Seiler’s study, fitter individuals would re-normalize faster), I found it quite interesting that the same response (i.e. an increase in HRV) was present even after a session that was twice as long (e.g. 2 hours vs 1 hour, see Seiler’s paper for details).
In the three studies above, low intensity was defined in three different ways: 40% of VO2max, intensities below the first ventilatory threshold (VT1), and perceived exertion. If you are new to endurance exercise, you can also relate this intensity to the ability to hold full conversations without getting out of breath, while exercising. Alternative methods would be to look at lactate or heart rate, as I discuss here (this is what we can also call zone 2).
You can run these experiments for your own training, using the tools and protocols I describe in my blog: “Heart rate variability (HRV) analysis before and after exercise“, and see how different intensities (as well as environmental conditions, such as the heat) impact your post-exercise recovery. Fascinating stuff.
I’ve taken these measurements sporadically over the years, and as a result, I don’t shy away from easy, long training, especially given that the physiology substantiates my subjective feeling on this one :)
More low-intensity running awaits.
Take it easy!
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Marco holds a PhD cum laude in applied machine learning, a M.Sc. cum laude in computer science engineering, and a M.Sc. cum laude in human movement sciences and high-performance coaching.
He has published more than 50 papers and patents at the intersection between physiology, health, technology, and human performance.
He is co-founder of HRV4Training, advisor at Oura, guest lecturer at VU Amsterdam, and editor for IEEE Pervasive Computing Magazine. He loves running.
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Fascinating, Marco. I dug into the Seiler study, participants in the <VT1 conditions trained at 68% MaxHR / 61% VO2Max. Interesting to note that this is a level of intensity that, according to the first study you mention, does increase mean cortisol.
I'm going to experiment with taking HRV pre/post at different "easy" intensities and see if I can determine a personal "Easy Zone." Thanks for the inspiration!
N=1 here, but it's something I’ve noticed in myself. Since February, I've been training (cycling) in a more structured way for the first time in years, with a much greater focus on zone 2 work than before. I've found that on the days following these endurance sessions, my HRV tends to be higher and my resting heart rate significantly lower.
In recent years, I would just do whatever felt right in the moment, often leaving me tired and not fully recovered. As a result, I wasn’t doing any real endurance training, and my average HRV was lower while my resting heart rate stayed higher.
It's somehow comforting and encouraging to see that's backed by science as well.
What I'm really trying to say is: I feel fitter, less tired and more ready than ever and the numbers in your app show it when doing my morning reading.
Thanks for the good stuff, Marco!