What is Fatigue?
One of my favorites poets is the French poet Charles Baudelaire (1821-1867). In many ways, he was inspired by and followed in the footsteps of Edgar Allan Poe, who preceded him by about a decade, and Baudelaire even went so far as translating some of Poe’s works into French.
If you’re unfamiliar with either, they wrote stories and poems that are dark, sometimes including violence and murder, which would have been quite provocative at the time. Both authors have poems that are what’s considered prose poetry, which are poems that don’t follow a particular rhyme scheme or meter but still have many poetic elements to them, like imagery, metaphor, and repetition. They often read like particularly literary short stories.
One of my favorites of Baudelaire’s poems is “Every Man His Chimera”. In Greek mythology, a chimera is a three-headed hybrid beast, part lion, part goat, part serpent. In the poem, a narrator observes a train or parade of men walking by, all bent doubled over due to the “monstrous beast” on their backs—their chimeras—bowing them to the earth. As the narrator observes, none of the men are aware of what they carry, despite their “grave and wary faces.”
If you’re doing any kind of work, you’re generating fatigue, and that fatigue needs to be managed and dealt with so that you can recover and return to doing your normal, necessary, and expected amount of work. Fatigue management is at the core of exercise science, but some of us aren’t aware of just how much fatigue is weighing us down, like the men in Baudelaire’s poem.
It’s easy to clump all fatigue together and throw it all in the same bucket, but fatigue is as heterogeneous as breakfast cereals. Some fatigue is physical, and plenty is mental. Some of it is obvious, but, again, like the men in Baudelaire’s poem, other forms are invisible to us yet just as taxing.
In exercise science, “work” is physical force generation and power output. That’s to say, how much muscular and/or cardiorespiratory work we’re capable of doing—moving loads by lifting weights, power generation through something like sprinting, or even doing sustained endurance work, like jogging, cycling, or swimming.
Depending on your activity, you’ll be generating different kinds and amounts of fatigue. Fatigue usually scales with or runs parallel to whatever’s creating it, and the recovery period required is, of course, downstream and also usually scales or runs parallel to both the fatigue and the activity.
To be able to manage and mitigate fatigue—and thereby manage recovery and the subsequent workouts and training sessions—it’s first important to be able to be aware of it. To be aware of it, we should be able to define it.
So, what does “fatigue” even mean?
First, there’s a general definition. “Fatigue is defined as an internal homeostasis breakdown caused by an increase in energy production demanded by an external stimulus.” Then, there’s acute physical fatigue, which “can be generally defined as a decrease in physical performance related to a rise within the real/perceived difficulty of a task or exercise, as well as the inability of the muscles to keep up with the specified level of strength during exercises.”1
A shorter way to say all this is that fatigue is a decrease in force generation capacity2 and “a decrease in the expected or required power output.”3
You run a sprint a few times, and each one is going to get progressively slower because your muscles are not going to be able to produce the same amount of force with each subsequent one. You do some dumbbell curls, and at some point, you’re going to get close to muscular failure, the point where you can’t do any more reps, and on the next set you might be able to do the same number of reps, but it’ll likely feel more difficult. Again, this is acute physical fatigue.
Each exercise and workout/training session adds some acute physical fatigue, but the sessions are also adding some central nervous system (CNS) fatigue. “CNS fatigue can be defined as a decrease in the voluntary activation of muscles, directly related to a decrease in the frequency and synchronization of motoneurons, and a reduced drive from the motor cortex.” (See footnote 1.) That’s to say, your CNS homeostasis has been interrupted and is not able to coordinate with the muscles as well to produce the necessary force. If you’ve done some curls and find your hands are a little shaky afterwards, this is some CNS fatigue.
Acute muscular and CNS fatigue may last for a couple of hours or a couple of days, depending on your training status and the intensity of work done to cause it. The thing to remember is that acute fatigue accumulates with each workout, like the proverbial drops in a bucket, and it can lead to chronic fatigue, which “is a condition defined as a persistent tiredness lasting > months that is not ameliorated by rest.” Moreover, “Fatigue accumulation, if not resolved, leads to overwork, chronic fatigue syndrome (CFS), overtraining syndrome, and even endocrine disorders, immunity dysfunction, organic diseases and a threat to human health.”4
In addition to acute and chronic physical fatigue—muscular and CNS—fatigue is also mental. “MF [mental fatigue] refers to a psychobiological state characterized by a sensation of tiredness and lack of energy that an individual might experience after a prolonged high-demanding mental or cognitive task.”[2]
Interestingly, physical fatigue can lead to mental fatigue and vice-versa. In one paper, “What is mental fatigue in elite sport? Perceptions from athletes and staff”5, researchers conducted group discussions with 32 athletes to determine the factors that contribute to their mental fatigue and how it affects performance in their sport.
“Over-analysis of performance, an excessive amount of instructions or discussion on the field of play during training drills, and post-game reflections and media engagement[s]” and “external study or emotionally challenging work, media and sponsorship engagements, driving between commitments under time pressure, and managing relationship dynamics”, and, perhaps above all, media and fan scrutiny that is so ubiquitous and relentless on social media and the internet these days all contribute to mental fatigue.
Though the subjects here are elite athletes, they are a metaphor for all of us. Their situations are more concentrated and more visible, but most of these factors are things that we all experience at some point or another.
The athletes’ responses revealed some common themes and consequences of their mental fatigue on their physical performance.
“MF was perceived to be associated with athletes ‘going through the motions’; simply following an instruction, rather than performing activities with the required intensity. Both athletes and staff reported a perceived negative influence of MF on response and reaction time. Interestingly, a decrease in impulse control was discussed by staff and raised by athletes as a perceived impact of MF.”
Additionally, “decreased motivation and enthusiasm and increased levels of disengagement as reported by athletes and staff add to the previously established symptoms of low levels of energy and feelings of tiredness.”
Another common finding with mental fatigue is increased perceived exertion. “In exercise science, perception of effort is defined as the conscious sensation of how hard, heavy and strenuous a physical task is.”6
Even though we’re talking about fatigue as it relates to exercise activity, the fact is that any physical activity can cause physical and/or mental fatigue. I was recently speaking to a colleague about all this, and she admitted that washing dishes wears her out. Not only that, but the stress of needing to do it looms over her, which doesn’t help.
If you’re a knowledge worker, someone who has a cognitively demanding job, you’re going to accumulate a lot of mental fatigue, which is going to make you feel tired and definitely infringe on your desire to exercise. If you have a physical job, there might not be as much mental fatigue, but the physical fatigue can be enough to wear you down mentally, intellectually, and emotionally, leaving little energy available in other areas of life.
The body does what it can to manage fatigue, but it can only do what it can with the tools we provide. Most physiological systems in the body run on negative feedback loops, like a standard thermostat. Things get too cold, and the heater kicks on. It gets too hot, so it turns off. An example in the body is glucose regulation. Carbs hit the blood, and insulin comes in to take it out. Later, long after a meal is digested, blood sugar will start dipping, so glucagon will show up and start liberating stored energy.
By contrast, mental and physical fatigue reinforce each other, like a positive feedback loop. A classic example of a positive feedback loop is blood clotting at a cut. As some blood cells show up, others are called upon to join, and they don’t stop joining until the job is done. (I would say that the Burly Brawl in The Matrix Reloaded—where Neo fights almost countless Smiths—is also an example of a positive feedback loop.) That is, as you become more mentally tired and fatigued, your physical capacity for work will diminish. Likewise, when you’re physically fatigued and tired, your mental capacity for work will be reduced.
If there’s not a deliberate intervention to this cycle, it’ll be a downward spiral. Not that you’ll get to the point where you can’t do anything; you’ll just get to the point where you can’t do anything difficult without it feeling exponentially more so. Moreover, other downstream mental effects might start manifesting, like lack of motivation and “going through the motions”, even depression, apathy, and emotional outbursts. Physically, you’re likely to stagnate or even regress in performance, and you might end up with some kind of injury, which is definitely not helpful for mental and emotional improvement.
Of course, we’re aware of the acute—albeit good—stressors of our workouts and training sessions and the fatigue that they generate. And we’re aware that we all have fatigue generated from taking care of responsibilities with family, home, and work, but there are things going on that we may not realize.
Like the chimeras in Baudelaire’s poems, there are things that are weighing us down that we are not aware of, which is likely contributing to our mental and physical fatigue. We want to be aware of the fatigue we generate and accumulate so that we can manage it.
Next week, we’ll cover some bases for rest and recovery.
Tornero-Aguilera JF, Jimenez-Morcillo J, Rubio-Zarapuz A, Clemente-Suárez VJ. Central and Peripheral Fatigue in Physical Exercise Explained: A Narrative Review. Int J Environ Res Public Health. 2022 Mar 25;19(7):3909. doi: 10.3390/ijerph19073909. PMID: 35409591; PMCID: PMC8997532.
Gantois P, Lima-Júnior D, Fortes LS, Batista GR, Nakamura FY, Fonseca FS. Mental Fatigue From Smartphone Use Reduces Volume-Load in Resistance Training: A Randomized, Single-Blinded Cross-Over Study. Percept Mot Skills. 2021 Aug; 128(4): 1640-1659. doi: 10.1177/00315125211016233. Epub 2021 May 17. PMID: 34000894.
Debold EP. Recent insights into the molecular basis of muscular fatigue. Med Sci Sports Exerc. 2012 Aug; 44(8):1440-52. doi: 10.1249/MSS.0b013e31824cfd26. PMID: 22330018.
Wan JJ, Qin Z, Wang PY, Sun Y, Liu X. Muscle fatigue: general understanding and treatment. Exp Mol Med. 2017 Oct 6;49(10): e384. doi: 10.1038/emm.2017.194. PMID: 28983090; PMCID: PMC5668469.
Russell S, Jenkins D, Rynne S, Halson SL, Kelly V. What is mental fatigue in elite sport? Perceptions from athletes and staff. Eur J Sport Sci. 2019 Nov;19(10):1367-1376. doi: 10.1080/17461391.2019.1618397. Epub 2019 May 28. PMID: 31081474.
Martin K, Meeusen R, Thompson KG, Keegan R, Rattray B. Mental Fatigue Impairs Endurance Performance: A Physiological Explanation. Sports Med. 2018 Sep; 48 (9): 2041-2051. doi: 10.1007/s40279-018-0946-9. PMID: 29923147.