Fatigue after a hard workout is a normal part of adaptation. The problem begins when the exhaustion does not pass after days of rest and interferes with training, work and sleep. The editorial team examined what lies behind chronic fatigue in athletes, what role the volume and intensity of training load play and how medications and supplements affect this condition.
What counts as chronic fatigue in sport
In sports medicine, the word "fatigue" covers very different states — from an acute drop in performance at the end of a workout to months of exhaustion when a person cannot perform even a familiar warm-up load. Chronic fatigue is usually spoken of when the feeling of exhaustion lasts for weeks, does not disappear after a few days of full rest and is combined with declining results.
It is important to distinguish this everyday and sporting term from the medical diagnosis of "chronic fatigue syndrome" (myalgic encephalomyelitis). For the latter, the classic criteria of the US Centers for Disease Control require at least six months of unexplained fatigue with characteristic accompanying symptoms, and the diagnosis is made only after other illnesses have been ruled out. In most athletes, persistent fatigue has a more specific cause.
Most often, chronic fatigue in sport is part of the overload continuum: from functional overreaching, which disappears within a few days, to non-functional overreaching and, ultimately, overtraining syndrome. However, roughly as often it is caused by factors outside the gym — lack of sleep, insufficient energy in the diet, iron deficiency, a past viral infection or psychological stress.
That is why the editorial team suggests looking at chronic fatigue not as a separate illness but as a symptom. The task of the athlete, coach and doctor is to work out exactly what this symptom is signalling, rather than simply "toughing out" a period of weakness or masking it with stimulants.
Training load as the main factor
Training is controlled stress. After a load, performance temporarily drops, and during rest the body not only recovers but also adjusts to a higher level. This principle of supercompensation works only when a balance is maintained between the stimulus and recovery. If new loads are placed on an unrecovered body, fatigue accumulates.
The risk of accumulation is highest during periods of sharp increases in volume: after a holiday, an injury, during training camps or when moving into a new weight or age category. Training-load monitoring studies show that it is not high volume itself that is more dangerous, but the spike relative to what the athlete has become accustomed to over the preceding weeks (Gabbett, 2016).
Another factor is monotony. When every training day has the same high intensity without easy days, cumulative stress grows faster than one would expect from the simple sum of hours. That is precisely why, in modern planning, easy days and unloading weeks are considered not "wasted time" but an obligatory part of the programme.
To assess load, both objective indicators (heart rate, power, tonnage) and subjective ones — the perceived-exertion scale, wellbeing questionnaires, sleep quality — are used. Monitoring reviews emphasise that subjective questionnaires often detect the accumulation of fatigue earlier than laboratory markers (Halson, 2014).
Non-training causes: sleep, energy, iron, infections
Sleep is the main tool of recovery. The expert consensus on athletes' sleep stresses that chronic sleep deprivation impairs cognitive function, mood, immune response and perception of effort (Walsh et al., 2021). Evening workouts, late flights, use of gadgets and caffeine in the evening are typical reasons why an athlete "trains correctly" but is constantly tired.
The second large group of causes is low energy availability. When a person systematically eats less than they expend, the body switches to an economy mode: levels of thyroid and sex hormones fall, and the recovery of bones and muscles worsens. The International Olympic Committee describes this state as relative energy deficiency in sport (REDs), and fatigue is one of its early manifestations (Mountjoy et al., 2023).
Iron deficiency is a classic cause of fatigue, especially in women, endurance runners and vegetarians. Depletion of iron stores can develop even before anaemia appears: haemoglobin is within the normal range while ferritin is already low. Loss of iron through sweat, micro-bleeding in the digestive tract and haemolysis under impact loads make athletes a risk group.
Finally, past infections should not be underestimated. After viral illnesses, in particular infectious mononucleosis or COVID-19, fatigue and reduced tolerance to load can persist for weeks. Returning to full training volume immediately after the temperature has gone is a common mistake that prolongs the period of weakness.
- sleep deprivation and disrupted sleep patterns;
- low energy availability (REDs);
- deficiency of iron, vitamin D, B12;
- past viral infections;
- psychological stress: study, work, competitive pressure.
Links to pharmacology and supplements
Pharmacological causes of chronic fatigue often go unnoticed, because athletes do not link how they feel with what they take. The most common example is stimulants. Caffeine, pre-workout complexes with high doses of stimulants, synephrine and other "fat burners" can mask fatigue during training but worsen sleep. The result is a vicious circle: slept poorly, took more stimulant, slept poorly again.
A separate risk group is people who use anabolic steroids. Exogenous androgens suppress the body's own "hypothalamus — pituitary — testes" axis. After stopping use, induced hypogonadism may develop, one of the main symptoms of which is pronounced fatigue, apathy, reduced motivation and libido (Rahnema et al., 2014). Such a state sometimes lasts for months.
Thyroid hormones, used non-medically for "cutting", and clenbuterol affect heart rhythm, sleep and metabolism. While taking them or after withdrawal, people often describe weakness, tremor, tachycardia and sleep disturbances. Diuretics can cause a loss of electrolytes — potassium, magnesium, sodium — which manifests as muscle weakness and cramps.
Fatigue can also be caused by entirely legal medications: beta-blockers, first-generation antihistamines, some antidepressants, isotretinoin, medications for treating insomnia. Alcohol, even in moderate amounts, worsens sleep architecture and recovery. That is why, at a doctor's appointment, it is important to honestly list everything you take, including supplements and illegal substances.
| Group of substances | Possible mechanism of fatigue |
|---|---|
| Stimulants, caffeine in high doses | Sleep disturbance, "crash" after the effect wears off |
| Anabolic steroids (after withdrawal) | Secondary hypogonadism |
| Thyroid hormones, clenbuterol | Tachycardia, insomnia, exhaustion after withdrawal |
| Diuretics | Loss of fluid and electrolytes |
| Alcohol, sedatives | Impaired sleep quality, after-effects |
How to distinguish "sports" fatigue from illness
Training-related fatigue usually has a clear dynamic: it builds up during hard blocks and noticeably decreases after an unloading week. If, however, even two or three weeks of reduced load bring no relief, that is a reason to look for another cause and consult a doctor.
The editorial team considers warning signs to be unexplained loss of body weight, night sweats, elevated temperature, enlarged lymph nodes, shortness of breath or chest pain during exercise, irregularities in heart function, and pronounced low mood. Such symptoms require examination regardless of the training context.
The doctor will most likely start with a detailed questioning about load, sleep, nutrition and medications, after which they will order basic tests: a complete blood count, ferritin, indicators of thyroid function, glucose, liver and kidney panels, and if needed sex hormones. We write about diagnosis in more detail in a separate article.
Self-medicating with "energy drinks" or new supplements without establishing the cause not only fails to help but can also mask a serious problem — from anaemia to diseases of the heart or endocrine system.
Editorial conclusions
Chronic fatigue in an athlete is a symptom, behind which there most often lies an imbalance between load and recovery, but not infrequently also sleep deprivation, insufficient energy, iron deficiency or a past infection.
Pharmacology can both cause fatigue and mask it: stimulants disrupt sleep, while withdrawal from anabolic steroids or hormones can trigger a prolonged period of exhaustion through hormonal disturbances.
The best protection is a gradual increase in load, planned rest, sufficient sleep and nutrition, and a timely visit to a doctor if rest does not help.
We also recommend reading our materials on the prevention and diagnosis of chronic fatigue, on overtraining and on tests for ferritin and thyroid hormones.
References
- Meeusen R, Duclos M, Foster C, et al. Prevention, diagnosis, and treatment of the overtraining syndrome: joint consensus statement of the European College of Sport Science and the American College of Sports Medicine. Med Sci Sports Exerc. 2013;45(1):186–205.
- Fukuda K, Straus SE, Hickie I, et al. The chronic fatigue syndrome: a comprehensive approach to its definition and study. Ann Intern Med. 1994;121(12):953–959.
- Gabbett TJ. The training–injury prevention paradox: should athletes be training smarter and harder? Br J Sports Med. 2016;50(5):273–280.
- Halson SL. Monitoring training load to understand fatigue in athletes. Sports Med. 2014;44(Suppl 2):S139–S147.
- Walsh NP, Halson SL, Sargent C, et al. Sleep and the athlete: narrative review and 2021 expert consensus recommendations. Br J Sports Med. 2021;55(7):356–368.
- Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57(17):1073–1097.
- Rahnema CD, Lipshultz LI, Crosnoe LE, et al. Anabolic steroid-induced hypogonadism: diagnosis and treatment. Fertil Steril. 2014;101(5):1271–1279.
Andriy Melnyk
A strength-sports coach and author of programs for beginner and intermediate levels. Writes about training planning.
