When Stress Piles Up
Using Heart Rate Variability (HRV) to manage individual responses to training and lifestyle stress
In this post, I’d like to show how you can use a simple morning measurement of your resting physiology to gather useful information about your body’s response to training and lifestyle stressors.
In particular, we’ll look at two case studies using data from me and Alessandra in the two months leading to the New York City marathon, while dealing with additional non-training related stressors (work, university exams, etc.).
We’ll see how stress piles up and how the contribution of training and lifestyle choices has a cumulative impact that is reflected in your body’s physiological state (your HRV). Hopefully, the case studies will be helpful to better understand how you can apply similar principles to your own case so that you can better manage stress towards improved health and performance.
Before we dive into the case studies, let’s just cover a little theory so that everything, later on, is easier to understand. If you are familiar with HRV, just skip the next section.
What’s Heart Rate Variability (HRV) and why does it matter?
HRV is a term that refers to ways to summarize in a number the variability between heartbeats.
Right, but why do we care?
For a simple reason: HRV is the only practical, non-invasive and cost-effective way we have to measure the activity of the autonomic nervous system (well, part of it, see later). Our body is continuously re-adjusting to maintain a state of balance, called homeostasis. Our heart rate, blood pressure, glucose level, hormones, etc. — react to the challenges we face and the autonomic nervous system works to keep everything in balance so that we can function optimally (e.g. do not develop chronic conditions, or improve our performance).
Heart rhythm (and therefore HRV) is regulated by the parasympathetic branch of the autonomic nervous system, the one in charge of rest and relaxation. Hence, measuring HRV is an effective way to capture how our body is doing while trying to maintain a state of balance in response to different stressors (training, lifestyle, etc.).
In particular, a reduction in certain HRV features (for example rMSSD) typically means that parasympathetic activity is reduced, and therefore we have not fully recovered. If you are interested in learning more about these aspects (theoretical basis, measurement protocol — which is key! — and applications), check out this deck.
The data shown below was recorded using a 60s morning measurement while lying down in bed, right after waking up, in a relaxed state. Tools of the trade:
Marco: exams, post-injury training
In my case, the two months leading to the marathon have been quite stressful, but hardly because of training (an important detail on this will follow). Some context first: last September I started a new Master’s degree, and therefore the past months were quite busy (running a business, going to lectures, doing assignments, and trying to prepare a marathon). In particular, at the beginning of October university started taking much of my time, with exams approaching around the 20th.
You can see in the plot below how “lifestyle stress” (third row), a subjective annotation in the HRV4Training app that I use to track how stressed I feel at a given point in time, gets quite a bit higher than my normal (above normal values, derived from my previous 2 months of data).
You can also see that training does not change much in this period (second row), as I am unable to increase load due to my previous injury firing up frequently. While training load is not high, training does have an impact on me here psychologically. When I go out I sometimes file pain, the marathon is approaching, and as any runner knows, no matter how slow we are, we all have our goals and have to deal with them mentally.
Despite no changes in training load, I felt very stressed with exams approaching (must be getting too old for that!), and you can see how my HRV goes down, below normal values, and stays there for several weeks.
Post exams, I have a bit of stress relief (highlighted by higher HRV around October 28th for three days), right before flying to NYC and getting all stressed for the marathon. Obviously, running a marathon is hard, and preparing it while recovering from an injury is far from ideal. While I’m aware this is not the smartest choice, what are the odds that both I and Alessandra get extracted in the NYC marathon lottery together? (I did the math, about 0.81%). So while I’ve paid attention and trained the minimum possible, I still went out for my sessions and long runs, hoping to be able to run the distance without additional issues.
In the last week shown above, you can see how weeks of HRV below normal values, and chronic stress due to university exams and self-imposed expectations or fears related to the marathon disappear really quickly. After the marathon I have pretty much no stressor bothering me, and this is reflected in a rapid increase in HRV (reduction in stress) back to normal values.
The drop I experienced during the weeks preceding the race brought my HRV baseline to a two years low (!) and honestly got me worried a bit. I gave priority to sleep and other recovery measures, but I also felt really overwhelmed by everything I was doing (I’m an anxious person). On the other hand, it was nice to see that I was able to bounce back quickly once stress was gone. Clearly, it was all in my head.
What this tells me is that I certainly need to work on better coping strategies when different stressors pile up (meditation, anyone?).
On to the next case study.
Alessandra: business trips, the first marathon
In Alessandra’s case, we also have a few important stressors. In terms of training, this was her first marathon, and obviously it comes with a little self-doubt and anxiety. Alessandra also started a PhD earlier this year and is running the business with me, meaning there are frequently up and downs in work-related stress. Additionally, she teaches and travels a lot to give lectures and workshops all around the world, making traveling an important aspect to consider.
Let’s look at the data. We can see below two different situations in which training and lifestyle stressors are managed differently, resulting in a good physiological response in one case, and a negative one in the second case (but after the race).
In particular, during an earlier business trip to Belgrade (shown in red in the first plot) we have a decrease in HRV for a few days. We can see how Alessandra also subjectively annotates these days as stressful (the third plot, increase in ‘lifestyle stress’).
Yet, as we knew about the trip, here we kept training load very low to avoid additional stress that could impair recovery and hinder longer-term adaptations and performance. This is a simple principle used by most HRV-guided research protocols: when your body is already stressed, it is not a good time to add high-intensity training.
In the second part of the figure, we can see how once again there is high lifestyle stress (traveling to the first marathon), and this time, of course, there is a huge spike in training load (the race!). The combination of the two triggers a large reduction in HRV, below normal values.
While it is fairly obvious that after a race some recovery is needed, each of us is different, and as shown in these examples, different stressors can pile up. Hence, looking at the data post-event can be a useful objective method to determine when physiologically we are back to normal.
The data above should clearly show how important context is.
It makes little sense to analyze HRV just in relation to training or to measure it without properly contextualizing data. This is why HRV4Training includes a simple questionnaire after the measurement and provides the visualizations shown above to let you explore your data in relation to all other factors (travel, subjective parameters, training load, etc.).
Reality is that stress is cumulative and comes from many sources, and of course there can be periods in which training is the major stressor, and in that case we can see stronger relationships between training load and HRV (which does not mean reduced HRV! positive adaptation results in a stable or increased HRV over time, see this post for more details), but normally, many factors play a role.
These case studies show that we cannot isolate training and lifestyle stress or think that training is not affected by everything else going on at any given moment in our professional or personal life.
A simple marker such as HRV, measured in a well-defined context using a validated app (first thing in the morning while in a rested state), can capture stress deriving from all sources and help us make meaningful adjustments to maintain things in check.
In an era in which it seems way too easy to get out of touch with our body and subjective feeling, I hope our work does not come across that way. On the contrary, a morning check via an objective assessment of your resting physiology combined with a short questionnaire where you have to just stop and think, for a few seconds, about how you are feeling physically and mentally, can help us get in better tune with our body and pay a little more attention to stress. It all starts with a bit more awareness.
A few useful links:
- A primer on Heart Rate Variability
- Another case study
- Overview page in HRV4Training Pro
- HRV-Guided training
Marco holds a PhD cum laude in applied machine learning, a M.Sc. cum laude in computer science engineering, and a M.Sc. 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.