Polarized training involves athletes doing the majority of their training at a low intensity, while a much smaller proportion of it is hard, higher-intensity work. This balance (approximately 80% lower/20% higher) helps create the optimal conditions for adaptation—and it’s these adaptations that we want to see to get fitter, faster, and stronger.
Knowing when to time the higher-intensity sessions is key to getting the maximum gains from them—and central to this is having a sound understanding of your autonomic nervous system and how your body handles and responds to stress. In this article, we’ll look at how the autonomic nervous system works, how life stress and training stress affect us, and ways in which you can learn to reduce or remove stress from your life to be the best athlete you can be. After all, a stressed athlete who forces themselves to push through a tough workout is going to see far fewer gains than the athlete who knows when to rest and return to it another day. In my opinion, this can often be what separates the good athletes from the great. Equally, attempting to do a hard session a day after you’ve gone too hard in what was supposed to be an easy session may also compromise your ability to get full gains out of that hard session. Without proper recovery the high-intensity work simply cannot be as intense as it could be.
Stress is stress
We often hear the phrase “stress is stress,” meaning all stress (regardless of where it is coming from) has similar effects on our bodies and there is a physiological basis behind that claim.
In 1936, physiologist Hans Selye penned his General Adaptation Syndrome, which stated that no matter the source of the stress, whether environmental, physical, or psychological, our body, and more specifically, our autonomic nervous system, responds in similar and predictable ways. The initial reaction is alarm, then resistance or response to the stressor, and finally, if the stressor persists for too long or is too great for the body’s adaptation reserves, then we reach exhaustion. Importantly, as Selye discovered, stress is general, and all types of stress are processed by the same autonomic nervous system. This fact has particularly important ramifications to athletes who may be under other forms of (life) stress and not realize the impact that stress has on their training.
The autonomic nervous system
The autonomic nervous system (ANS) is the part of the nervous system that controls the body’s involuntary or autonomic functions, such as heart rate, digestion, respiration, and blood pressure. The ANS is divided into the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The sympathetic nervous system is responsible for the body’s “fight-or-flight” response, while the parasympathetic nervous system is responsible for the body’s “rest-and-digest” or “feed-and-breed” response.
The stress response is a complex series of reactions that is triggered by a stressor, such as an environmental, physical, or psychological stressor. The stress response is the body’s way of protecting itself from the harmful effects of stress and is mediated by the sympathetic nervous system and the hypothalamic-pituitary-adrenal (HPA) axis.
The stress response begins with the recognition of a stressor by the brain. The brain then triggers the release of hormones, such as adrenalin and cortisol, which prepare the body for the “fight-or-flight” response. The “fight-or-flight” response is a series of physiological changes that occur in the body in response to a perceived threat. The “fight-or-flight” response is mediated by the sympathetic nervous system and is characterized by an increase in heart rate, blood pressure, and respiration, and a decrease in digestive activity.
If the stressor is not removed, the stress response will continue, and the body will eventually reach a state of exhaustion. When the body reaches a state of exhaustion, the stress response is no longer effective and the body is vulnerable to the harmful effects of stress, such as the development of chronic diseases.
It is important to note that the stress response is a natural and necessary part of the body’s defense mechanism. The stress response helps the body to protect itself from harm.
Allostasis: the body’s way of handling stressors
The body is always trying to stay in balance, or homeostasis. Stressors threaten this balance and the body, when it is healthy, tries to respond and adapt to these stressors in order to maintain homeostasis. Sterling and Eyer (1988) called this process allostasis.
Famed neuroendocrinologist Bruce McEwen expanded on this idea explaining that too much allostatic load can lead to maladaptation and disease: “Hormones associated with stress and allostatic load protect the body in the short run and promote adaptation by the process known as allostasis, but in the long run, allostatic load causes changes in the body that can lead to disease. The brain is the key organ of stress, allostasis, and allostatic load, because it determines what is threatening and therefore stressful, and determines the physiological and behavioral responses.”
In other words, adaptation is promoted by a healthy level of allostatic load. For you, the athlete, the key takeaway should be that all stressors contribute to this allostatic load and that once a certain level of allostatic load is exceeded, and allostatic overload is reached, the body will no longer be able to adapt to and respond to the stress of training. More plainly, periods of rest and relaxation and the absence of significant external life stress is absolutely essential to improving.
Give the system time to adapt
All of the positive adaptations that we seek come not during the initial “alarm” phase when you are in the middle of a hard workout, but instead come when the workout ends, the stressor is removed, and the body has the time and energy to respond to the stress. The body needs this time and energy to repair the muscular damage done beyond the initial level of muscle strength so that, if the stressor is encountered again, you will be better prepared to increase your energy reserves in order to better withstand the stress for longer should it happen again. It follows that, if the athlete goes straight from one stressor (the workout) to another stressor (a stressful workday), the “alarm” is never removed and so the ability of the athlete to respond to that initial stressor is severely compromised. In terms of performance in sport, this can lead to sub-optimal performance, overtraining, and injury.
In looking at autonomic stress in the athletic context, Angeli, Minetto & Pacotti were among the first in a growing number of researchers to suggest that this combining of autonomic stressors—running from one stress to the next—as a key factor in athletic non-response and that overtraining syndrome is less a disorder related to poorly managed training loads and more a disorder related to poorly managed stress. From their 2004 paper:
“Overtraining syndrome reflects the exhaustion stage of Selye’s general adaptation syndrome, which is characterized by insufficient glucocorticoid response to increasing demands due to persistent physical and/or psychological stressors.”
In other words, when the overall stress pool of physical and psychological stress is too great, or continued for too long, the ability of the body’s glucocorticoid “stress response” system is impaired. Long before this reaches a clinical level (e.g., in the case of Chronic Fatigue Syndrome), the individual will typically exhibit diminished response to any given stressor, e.g., the stressor of physical training. A classic example of this is riders doing a stage race or big training camp see a depressed heart rate later in the event because their sympathetic system has been blunted.
Training stress is any stress that is added to the athlete’s life that is above and beyond their normal routine. In other words, training that adds fatigue (in addition to any fatigue that may be coming from other areas of the athlete’s life). In training terms, when we schedule a day that is beyond the athlete’s normal “chronic” load, we call this a loading day. When we schedule a load that is below the athlete’s chronic load, we call it a recovery day.
Importantly, normal routines can vary greatly from athlete to athlete and can even change on a daily basis, such as a busier day at work, additional family stress, etc. So, in addition to the athlete’s chronic level of training stress, we must also consider how today looks relative to their level of “chronic life stress.”
Providing there is enough adaptive energy within the system to respond to the stress, the body will adapt to the training stress and become more fit, meaning it will be able to handle a higher training stress in the future. When the body is low in adaptive energy, such as when it is dealing with other life stressors, the ability of the body to respond to training is compromised.
So, what’s the solution? The most important solution is to be aware of the general autonomic stress that you may be under and to take steps to manage it. The first step is to be aware of the signs and symptoms of stress. The second step is to take steps to reduce the amount of stress in your life. The third step is to take steps to not “stack your stressors” i.e., when life stress is high, dial down the training stress and vice versa.
Become more aware of signs and symptoms of stress
- Feeling anxious, irritable, or moody
- Feeling overwhelmed or like you are not in control
- Feeling like you need to drink more caffeine or alcohol to get through the day
- Feeling like you can’t focus or concentrate
- Feeling like you are not sleeping well
- Feeling tired all the time
- Feeling like you are sick more often
- Feeling muscle tension or headaches
Reduce the amount of stress in your life:
There are many ways to do this, but some of the most effective ways include:
- Identifying the sources of stress in your life and taking steps to reduce or eliminate them
- Taking breaks during the day to relax and rejuvenate
- Practicing relaxation techniques such as yoga, mediation, or deep breathing
- Getting regular exercise
- Eating a healthy diet
- Getting enough sleep
- Learning how to delegate tasks
- Learning how to say “no”
Don’t combine your stressors:
- Monitor your daily stress on a regular basis by running through this signs and symptoms checklist and considering your general stress state before determining how much training stress you want to add.
- If you are coming from a very stressful day of work, where your cortisol is running high and your body is in a pronounced “fight or flight” state, this is not a good time to prolong that ANS alarm phase by immediately moving to a second stressor of a high-intensity workout.
- To properly absorb and respond to the autonomic stress of a high-intensity workout, workout timing with respect to other stressors becomes key.
Along with the subjective assessment of how much stress the athlete is under, the addition of Heart Rate Variability (HRV) as an overall global “autonomic stress meter” can also be a really helpful tool to help determine your overall level of autonomic stress before determining how much training stress you would like to add on that day.
How to use HRV data to determine overall ANS stress
One of the best ways to gauge the overall stress on your ANS is to use heart rate variability. Heart rate variability (HRV) is a measure of the time between heartbeats. The more variability in that time, the lower the current stress on the ANS. The less variability between heartbeats, the higher the current stress on the ANS. By using HRV in this way, we get a “window” into the current level of autonomic stress to help us to determine if we want to add training stress on top of that.
When HRV is high relative to the athlete’s normal levels (i.e., current autonomic stress on the system is low):
The body’s adaptation reserves are high and in a great spot to absorb training with a higher than normal training load, i.e., to accept a loading day. These days will generally have a higher training stress than the athlete’s typical training stress and will often include some higher intensity training.
On the other hand, when HRV is low relative to the athlete’s normal levels:
The body is dealing with other stressors and the adaptation reserves are low. In these cases, response to training is compromised and a recovery day below the athlete’s typical training load is warranted. These days should have lower overall training stress than the athlete’s typical training stress and should include some lower-intensity training designed to boost the recovery of the ANS, i.e., easy training below the first threshold.
Stress is stress and never forget that your ANS is tasked with the heavy load of responding to all of the stressors in your life, regardless of where they might be coming from (training or otherwise). The ability of your ANS to continue to respond to stressors over the long term is contingent on you being kind to it, looking after it and giving it sufficient time to downregulate and build up energy for the next onslaught. This is where the polarized training approach can bring about the greatest benefits. By making sure that no more than 20% of your training each week is higher intensity, you give your body the best chance of managing stress and adapting to training. It is not overstating things to say that much of your long-term development as an athlete will come down to your ability to recognize and manage autonomic stress.
– Selye, H. (1936). A syndrome produced by diverse nocuous agents. Nature, 138(3505), 32.
– Sterling, P., & Eyer, J. (1988). Allostasis: A new paradigm to explain arousal pathology. In S. Fisher & J. Reason (Eds.), Handbook of life stress, cognition and health (pp. 629–649). John Wiley & Sons.
– McEwen, B. S. (1998). Protective and damaging effects of stress mediators. The New England journal of medicine, 338(3), 171-179.
– Angeli, A., Minetto, M., Dovio, A. (2004) The overtraining syndrome in athletes: A stress-related disorder. J Endocrinol Invest 27, 603–612.
– McEwen, B. S. (2007). Physiology and neurobiology of stress and adaptation: central role of the brain. Physiological reviews, 87(3), 873-904.
– National Institute of Mental Health. (2018). Stress: The different kinds of stress. https://www.nimh.nih.gov/health/publications/stress/index.shtml