Overtraining

Geschatte leestijd: 18 minuten Overtraining can lead to a wide range of mental and psychological complaints. In this article, we will discuss the causes and symptoms of the different stages of overtraining and how to prevent this ‘career killer’.

What is overtraining?

Athletes train to improve. To get better (faster, stronger), the load per training session must be increased. This is only possible if there is enough time for rest and recovery between workouts. Overtraining is seen as a training intensity and volume that actually leads to deteriorated performance and requires days to weeks of rest for recovery [1,2]. Overtraining doesn’t necessarily have to be negative. In fact, when I walk into the gym, this is the goal. A heavy workout today will result in muscle soreness tomorrow or the day after. If I were to train the same muscles again tomorrow, I would expect worse performance. But that’s why I have a training schedule that provides the right amount of rest for recovery. This type of overloading with sufficient rest and performance improvement is also called ‘Functional Overreaching’.

Overtraining syndrome

If the overload is extreme, coupled with insufficient rest and external stressors, eventually overtraining syndrome can develop [2]. Overtraining syndrome can also arise from chronic inflammation and its effects on the central nervous system such as worsened mood, general fatigue, and neurohormonal changes [3,4,5]. Between functional overloading and overtraining syndrome, there is another intermediate category called non-functional overreaching. When we talk purely about athletic performance, you can summarize those three stages as follows (according to the European College of Sport Science) [2]:

What is overtraining?

Athletes train to improve. To get better (faster, stronger), the load per training session must be increased. This is only possible if there is enough time for rest and recovery between workouts. Overtraining is seen as a training intensity and volume that actually leads to deteriorated performance and requires days to weeks of rest for recovery [1,2]. Overtraining doesn’t necessarily have to be negative. In fact, when I walk into the gym, this is the goal. A heavy workout today will result in muscle soreness tomorrow or the day after. If I were to train the same muscles again tomorrow, I would expect worse performance. But that’s why I have a training schedule that provides the right amount of rest for recovery. This type of overloading with sufficient rest and performance improvement is also called ‘Functional Overreaching’.

Overtraining syndrome

If the overload is extreme, coupled with insufficient rest and external stressors, eventually overtraining syndrome can develop [2]. Overtraining syndrome can also arise from chronic inflammation and its effects on the central nervous system such as worsened mood, general fatigue, and neurohormonal changes [3,4,5]. Between functional overloading and overtraining syndrome, there is another intermediate category called non-functional overreaching. When we talk purely about athletic performance, you can summarize those three stages as follows (according to the European College of Sport Science) [2]:

• Functional overreaching: Decreased short-term performance. Improved performance after a rest period of several days to weeks
• Non-functional overreaching: Decreased performance over a longer term. Required recovery time of weeks to months. No improved performance, no permanent damage.
• Overtraining syndrome: Decreased performance for at least 2 months. Heavier symptoms than non-functional overreaching. Presence of a third factor (not related to other illness). ‘Possible end of a sports career’.

The difference between non-functional overreaching and overtraining syndrome is not always clear. The severity of the symptoms is also not always a good indication. Especially the required duration of the recovery period can indicate the difference. Where I further refer to ‘overtraining’ in this article, I mean both non-functional overreaching and overtraining syndrome. However, I will try to specify them as much as possible.

Symptoms of overtraining syndrome

In addition to the direct effects on athletic performance, we see an increase in the number of bodily processes negatively affected by non-functional overreaching and overtraining syndrome. Ranging from the mentioned (neuro) hormonal changes and psychological complaints to problems with the immune system. Below we see an overview of possible symptoms. These are divided into ‘parasympathetic changes’ and ‘sympathetic changes’. This refers to the parts of the autonomic nervous system that can respectively put the body in ‘rest mode’ or in the ‘action mode’. The symptoms of overtraining syndrome can be divided into ‘uppers’ and ‘downers’ in this way. ‘Uppers’:

• Restlessness and insomnia
• Quickly irritated and agitated
• Increased heart rate
• Increased blood pressure

‘Downers’:

• Fatigue
• Depression
• Decreased heart rate
• Decreased motivation

In a 2011 study among young elite athletes, this division was not made, but we get some more details about the reported symptoms [15].

Stress and overtraining

An important requirement to classify overloading as overtraining syndrome is the presence of an external stress factor [6,7,8]. Psychological and/or social stress factors associated with physical pressure. The extent to which someone is stress-resistant would therefore also affect the likelihood of overtraining syndrome [6].

How often does overtraining syndrome occur?

Not often. Overtraining syndrome seems very rare although exact figures are unknown. This is partly due to the use of different definitions for the term overtraining syndrome and the associated symptoms. Non-functional overreaching occurs more frequently, as you would expect. According to a 1987 study, non-functional overreaching occurred at least once in the life of 60% of ‘elite’ runners and 30% of ‘non-elite’ female runners [9]. I assume that the term ‘elite’ dates back to a time when top athletes wanted to avoid the label ‘professional athlete’ because of Olympic aspirations. According to a 2000 study, ‘overtraining’ occurred in 35% of adolescent swimmers [10]. ‘Staleness’ has been measured in various studies. A term that refers to the body’s inability to cope with physical and psychological stress, resulting in breakdown rather than recovery. This characteristic of both non-functional overreaching and overtraining syndrome occurred in 5 to 30 percent of swimmers during a season [11,12,13]. According to a 1998 study, this was 15 percent for British elite athletes [14]. A more recent 2011 study concluded that overtraining occurred in about 30% of adolescent elite athletes [15]. This occurred an average of two times per career and lasted an average of 4 weeks per incident. In this study, terms like non-functional overreaching, overtraining syndrome, and ‘staleness’ were lumped together.

Overloading per type of sport

According to the same 2011 study, there was also something to say about the type of sport and the associated risk of non-functional overreaching or overtraining syndrome. Some of the conclusions were different from what you might expect. The risk was found to be significantly higher in individual sports (37%) than in team sports (15%). This was also evident in an earlier study from 2001 (48% vs. 30%) [16]. In both studies, the simple explanation for this difference was found in the amount of training. Individual elite athletes were found to train on average 6 to 7 days and for an average of 2 hours longer per day than team athletes. In addition, individual athletes appear to be much more focused on their sport than team athletes. They spend much less time on things like school and hobbies [10,15,16]. Researchers call this “a risk of developing a one-dimensional person”. If your entire identity depends on your athletic performance, you can imagine that the pressure is considerably higher. Especially when those performances start to decline due to pressure and stress subsequently increases even further.

“Sports with lower physical demand, higher risk of overtraining”

It is therefore not surprising that there is more frequent occurrence of non-functional overreaching and overtraining syndrome in individual athletes. However, I mentioned that there were some remarkable conclusions in the 2011 study regarding the likelihood of both per type of sport. Overtraining appears to occur more frequently in sports with lower physical demand (34%). We have already mentioned that external stress factors can play an important role. Some studies with sports with relatively low physical demand show that these external factors can weigh heavily. For example, two studies among young golfers [16,17].

Not being able to train hard means less control and less stress

This explains why non-functional load and overtraining syndrome can occur in sports with low(er) physical demand. However, it does not explain why it would occur more frequently than in sports with higher physical demand. After all, we also saw that individual athletes are more often overtrained, among other things due to the higher number of training hours. I do not find the explanation in the studies. But I can think of a likely factor that can at least partially explain this. Apparently, the difference in stress due to psychological factors is greater here than the difference in physical demand. Perhaps that relationship is causal. Perhaps the psychological stress is higher precisely because the physical demand is lower. For example, as a boxer, you can train harder and heavier, thereby taking control over your performance in the ring. As a darts player, you cannot throw increasingly heavier darts. You have fewer opportunities to increase your training intensity and therefore less control over your performances. A lack of control over your circumstances is one of the key characteristics of negative mental stress. But I should stress that this is just my theory.

Overtraining between men and women

Even when we look at the differences in the likelihood of overtraining between men and women, psychological stress factors play an important role. Recently, we explained here that women recover faster from physical exertion than men. Presumably because of some kind of built-in protection. A limiter that ensures that women can relatively tap into less of their maximum capacity than men. An evolutionary protection of women’s reproductive functions. This would therefore mean that overtraining (under the same conditions) should occur less frequently in women than in men. However, the opposite is true according to the 2011 study. The only explanation the researchers can give is the possibly extra high pressure on young female athletes [15,19]. The struggle through conflicts between the women’s ‘cultural role’ and the role as an athlete.

What happens in the body during overtraining?

There are several hypotheses about what exactly happens in the body that subsequently leads to the wide variety of symptoms. It will not be surprising that it proves difficult to identify a single process as the cause.

• Glycogen hypothesis
• Central fatigue hypothesis
• Glutamine hypothesis
• Oxidative stress hypothesis
• Central nervous system hypothesis
• Hypothalamus hypothesis
• Cytokine hypothesis

Glycogen hypotheses

The first hypothesis is still the simplest. ‘An empty tank’. Glycogen is the form in which muscles store glucose/sugar as a source of fuel. Low glycogen levels have been associated with poorer athletic performance, but also with increased oxidation (burning with the help of oxygen). Additionally, we also see decreased levels of BCAAs (Branched Chain Amino Acids), which in turn affects muscle recovery. However, glycogen does not seem to play a major role in overtraining. There are many cases known of swimmers who consciously ate too few carbohydrates for a study. They became more tired during training with low glycogen levels. However, their performance did not deteriorate to the extent that corresponds to overtraining [20]. Too few carbohydrates/glycogen does not automatically lead to overtraining. The reverse also holds true: Sufficient carbohydrates/glycogen does not automatically protect against overtraining. As shown in a study from 1995 [21].

Central fatigue hypothesis

The neurotransmitter serotonin can be considered a downer due to its ‘inhibitory’ effect. Premature ejaculation in men is explained by some due to a shortage of serotonin. A shortage of this downer then makes it too difficult to reduce excitement. Conversely, an excess of serotonin can take away desire, not only in sex. Serotonin is produced from tryptophan. Tryptophan and BCAAs ‘compete’ during training for access to the brain [22-24]. However, during training, the amount of BCAAs in the blood is reduced due to increased burning. This results in more tryptophan entering the brain, which is converted into serotonin. This hypothesis is quite plausible based on certain studies. Increases in unbound tryptophan have been associated with fatigue, presumably due to increased serotonin production in the brain [25-27]. Studies in which serotonin levels were artificially increased in athletes resulted in deteriorated performance [28]. “`html Another possible culprit is not the amount of serotonin itself, but an increased sensitivity to it. Well-trained athletes often have lower sensitivity to serotonin, while the increase in this sensitivity has been observed in overtrained athletes [26]. Unfortunately, there are still few studies that have looked at serotonin concentrations themselves. Many studies looked at serotonin precursors such as tryptophan. Moreover, things like mood and fatigue are subjective, so the role of serotonin in overtraining needs further investigation.

Glutamine hypothesis

The amino acid glutamine serves various important functions. For example, it plays a crucial role in the immune system, but is also involved in glucose production. Lower glutamine levels after training could, according to some studies, cause upper respiratory tract infections in overtrained athletes [1,29,30]. Glutamine concentrations in your blood can be temporarily reduced by long workouts (longer than 2 hours) [31,32]. Low concentrations have been observed primarily in overtrained athletes [1,29]. This could be a sign of excessive glutamine consumption or the inability of overworked muscles to produce glutamine [31]. However, there are still many questions about the role of glutamine and the potential benefits of supplementation. A study from 2002 found that the reduced function of the immune system could not be prevented with glutamine supplements [31]. However, earlier research from 1997 among marathon runners showed that supplementation with glutamine could reduce the number of inflammations [33]. However, the fact that prolonged efforts such as a marathon can increase the risk of upper respiratory tract infections does not automatically mean that there is overloading or overtraining. In a study among swimmers, it was found that ‘only’ 13% of overtrained athletes reported having had upper respiratory tract infections [29]. Among non-overtrained swimmers, this was 56%.

Oxidative Stress Hypothesis

During physical exertion, oxygen is used to generate fuel in the muscles. The free radicals created by the oxygen in damaged muscles are also necessary to regulate their recovery [34]. However, when oxidative stress caused by a pathological condition (including overtraining) becomes disturbed and chronic, inflammations, muscle fatigue, and painful muscles can occur [35]. Oxidative stress at rest appears to be higher in overtrained athletes and also increases when they train. Athletes engaged in endurance sports train to absorb and use as much oxygen as possible as an energy source. The production of citric acid is an indicator of the body’s ability to oxidize and should increase during endurance sports. In a study with overloaded rats, the production of citric acid actually decreased [36]. This led to the suspicion that overtrained athletes have a reduced response to the oxidative stress induced by training. This would make them more vulnerable to oxidative damage.

Autonomic Nervous System Hypothesis

Disruptions in the autonomic nervous system could explain some symptoms of overtraining syndrome. Especially when it comes to lowering the sympathetic system (‘upper’) and dominating the parasympathetic system (‘downer’). Such an imbalance, where the body’s rest mode predominates, can cause many of the previously discussed physical and mental symptoms of overtraining [1,4,37]. A decreased function of the sympathetic (‘upper’) system is determined, among other things, by catecholamines measured in nocturnal urine [1]. Catecholamines can be neurotransmitters such as noradrenaline and dopamine. In some studies, the measured levels were lower as physical exertion increased, only to return to normal levels afterwards [1,4,18]. However, this is not evident in all studies [38]. Moreover, a decreased function of the sympathetic system can also be caused by the sensitivity of organs to these catecholamines rather than the quantity itself [37].

Heart Rate Variability

Another way to measure the function of the autonomic nervous system is by looking at variations in heart rate, the so-called heart rate variability. Your heart rate is controlled by the sinus node. This is a piece of muscle fibers in the heart that in turn is controlled by the autonomic nervous system. That your heart rate shows variations is a good thing. It adapts depending on the situation and need. The parasy mpathetic nervous system will aim for a lower heart rate in calm conditions while the sympathetic nervous system will increase it for action. In a Finnish study from 2002 among overtrained athletes, it was found that heart rate variability upon awakening was lower than in the control group [39]. Other research that used heart rate variability concluded that the effects of intensive training are reversible. The imbalance between the sympathetic and parasympathetic systems could be restored by a week of rest [40].

Hypothalamus-pituitary-adrenal (HPA) axis hypothesis

The so-called hypothalamus-pituitary-adrenal (HPA) axis is the chain involved in the regulation of various hormones such as testosterone and cortisol. Studies with endurance athletes have measured (subtle) changes in this chain. Overtrained athletes may also have adjustments in the levels of these types of hormones [1,4,37,41]. However, no patterns have yet been found in these changes. They are also highly individual and dependent on many other factors besides physical exertion [1,41].

Cytokine hypothesis

Finally, the cytokine hypothesis. Cytokines were briefly mentioned in the article on muscle growth mechanisms. These signaling molecules come into action during the deliberate damage you cause during training. They respond to local inflammation and play a crucial role in recovery and overcompensation after training [4,42]. This is part of the process by which muscles are better prepared for their task next time. Overloading combined with insufficient rest could exacerbate these inflammations, but also cause them to become chronic [5]. This creates an imbalance between the damage incurred and the muscles’ ability to repair it.

Overtraining Syndrome: Cytokines as Underlying Cause?

As mentioned, it is difficult to pinpoint a single cause. However, the cytokine hypothesis seems to come closest.

Cytokines and Glycogen

The decreased glycogen stores could, for example, be a consequence of the disrupted cytokines due to training. Cytokines can influence parts of the hypothalamus and drastically decrease appetite. As a result, less enters the body, leading to less glycogen being produced [4]. Cytokines can also disrupt the transport of glucose to and within the muscle cell. This can result in less glycogen being stored with the glucose that is available in the blood.

Cytokines, Tryptophan, and Serotonin

As mentioned, the uptake of tryptophan by the brain and increased sensitivity to serotonin have been associated with increased fatigue and depression. According to the cytokine hypothesis, however, tryptophan levels are lowered by systemic inflammation such as that seen in overtraining syndrome [4]. This is because tryptophan is used to produce proteins involved in inflammation. In addition to increased tryptophan levels, decreased tryptophan levels have also been associated with symptoms of depression.

Glutamine deficiency due to cytokines

Glutamine deficiencies can also occur earlier because, like tryptophan, glutamine is used to produce proteins in response to inflammation. Chronic inflammation also increases catabolic processes where the body must use resources to provide energy. This involves more glucose consumption and more protein breakdown. Glutamine is needed, as mentioned, to create new glucose (‘gluconeogenesis’). However, it is also needed to support the increased excretion of nitrogen by the kidneys, caused by increased protein burning [1,4,31]. Thus, cytokines can increase glutamine consumption in two ways.

Problems with the cytokine hypothesis

The cytokine hypothesis attempts to provide a comprehensive explanation for many of the symptoms of overtraining syndrome. Systemic inflammation is said to underlie overtraining. Parallels are drawn between overtraining and other stress-related conditions. However, the cytokine hypothesis also has serious shortcomings. There is scarce evidence of increased cytokines in overtrained athletes [4]. Some studies showed acute increases in certain cytokines after training but did not look at long-term effects [42-44]. A study of overtrained cyclists found no changes in the concentration of two cytokines. It was considered that this could be because cycling mainly emphasizes concentric muscle contractions. It is precisely eccentric contractions that lead to the deliberate damage caused by training more quickly. However, another study did find increased cytokines in cyclists [43]. It should be noted that this did not specifically concern overtrained cyclists. This was true for most studies on cytokine levels after training.

Testing for overtraining

The diagnosis of overtraining syndrome can be made based on the mentioned symptoms (and excluding other causes of these symptoms). However, there are also tests that can be performed to confirm suspicion. Various metabolic values can be measured, including kidney function and levels of magnesium, potassium, and blood sugar. A complete blood test can also provide insight, as well as iron levels, creatine kinase, and thyroid hormone levels. Cortisol levels at rest do not appear to differ between overtrained and non-overtrained athletes, while research into differences in testosterone shows various results. The testosterone:cortisol ratio does not yet appear to be a good indicator of overtraining.

What to Do in Case of Overtraining?

The treatment of non-functional overreaching and overtraining syndrome is essentially simple: rest. You should distinguish between relative rest and absolute rest. Relative rest involves reducing training volume and intensity. Depending on the athlete’s motivation, this may be preferable to absolute rest. For many athletes, absolute rest can be a stressor in itself. Training at a lower intensity may therefore be a more acceptable solution. It is advised to gradually increase the training volume first and then the intensity. For example, starting with 5 to 10 minutes per day and slowly building up to an hour. Only then can the intensity increase [25]. In the case of overtraining syndrome, two weeks of such relative rest may prove insufficient. This may be indicated by performance. In such cases, only absolute rest can provide a solution [25]. In the end, the best approach in terms of physical and mental rest should be assessed for each sport and athlete individually.

Preventing Overtraining

Keeping track of mood can be a way to prevent functional overreaching from turning into non-functional overreaching or overtraining syndrome. This can be done, for example, using mood questionnaires [2,46,47]. In a study of swimmers, the number of “burnouts” was found to be reduced from 10% to zero. This was achieved by adjusting the training program based on responses given in the Profile of Mood States survey. The training load was simply reduced when the score in the survey decreased [46,47]. Athletes should be informed mainly about the risks of overtraining and early symptoms such as increased fatigue at the same load. I hereby try to make my modest contribution, resulting in overtrained fingers.

Summary

Functional overreaching is a temporary deterioration in performance followed by better performance after rest and recovery. In non-functional overreaching, training intensity and volume lead to deteriorated performance over a longer period. Several weeks of rest may be needed for sufficient recovery. In overtraining syndrome, the required rest is at least two months and there is an additional stress factor (in addition to training). Stress resilience also plays a role in the likelihood of overtraining syndrome. Physical and mental symptoms of overtraining can be divided into ‘uppers’ and ‘downers’ by adjusting the nervous system. Overtraining appears to occur more frequently in individual sports. Possibly due to higher training volume, but also due to increased mental stress. On the other hand, overtraining also occurs more frequently in sports that are physically less intense. I suspect because the mental stress factor is greater. A higher mental and social stress could also explain the higher rates of overtraining among women. There are several hypotheses pointing to processes that could cause overtraining. Although such processes play a role, they cannot be identified as a single cause. There is only one theory that could explain the role of all these processes reasoned from the immune system. However, this theory cannot yet be sufficiently substantiated. There are some tests that can be done to diagnose overtraining. However, in most cases, this is determined based on the symptoms and excluding other possible causes. Prevention and treatment are a matter of the right type of rest appropriate for the athlete and sport in question. The athlete’s mood can be an important indicator for any necessary adjustments in training intensity. References

1. Halson SL, Jeukendrup AE. Does overtraining exist? An analysis of overreaching and overtraining research. Sports Med. 2004;34(14):967-981
2. Meeusen R, Duclos M, Gleeson M, et al. Prevention, diagnosis and treatment of the overtraining syndrome: ECSS Position Statement Task Force. Eur J Sport Sci. 2006;6(1):1-14
3. Armstrong LE, VanHeest JL. The unknown mechanism of the overtraining syndrome: clues from depression and psychoneuroimmunology. Sports Med. 2002;32:185-209
4. Smith LL. Cytokine hypothesis of overtraining: a physiological adaptation to excessive stress? Med Sci Sports Exerc. 2000;32:317-331
5. Smith LL. Overtraining, excessive exercise, and altered immunity: is this a T helper-1 versus T helper-2 lymphocyte response? Sports Med. 2003;33(5):347-364
6. Overtraining and recovery. A conceptual model.Kenttä G, Hassmén P Sports Med. 1998 Jul; 26(1):1-16.
7. Meehan HL, Bull SJ, Wood DM, et al. The overtraining syndrome: a multicontextual assessment. Sports Psychol. 2004;18:154-171
8. Tenenbaum G, Jones CM, Kitsantas A, et al. Failure adaptation: psychological conceptualization of the stress response process in sport. Int J Sport Psychol. 2003;34:1-26
9. Psychological characterization of the elite female distance runner. Morgan WP, O’Connor PJ, Sparling PB, Pate RR Int J Sports Med. 1987 Nov; 8 Suppl 2():124-31.
10. Raglin J, Sawamura S, Alexiou S, et al. Training practice and staleness in 13-18-year-old swimmers: a cross-cultural study. Pediatr Exerc Sci. 2000;12:61-7
11. Hooper S, MacKinnon LT, Hanrahan S. Mood states as an indication of staleness and recovery. Int J Sport Psychol. 1997;28:1-12
12. Psychological monitoring of overtraining and staleness. Morgan WP, Brown DR, Raglin JS, O’Connor PJ, Ellickson KA Br J Sports Med. 1987 Sep; 21(3):107-14.
13. Mood state and salivary cortisol levels following overtraining in female swimmers. O’Connor PJ, Morgan WP, Raglin JS, Barksdale CM, Kalin NH Psychoneuroendocrinology. 1989; 14(4):303-10.
14. Seasonal variations of injury and overtraining in elite athletes. Koutedakis Y, Sharp NC Clin J Sport Med. 1998 Jan; 8(1):18-21.
15. Prevalence of nonfunctional overreaching/overtraining in young English athletes. Matos NF, Winsley RJ, Williams CA Med Sci Sports Exerc. 2011 Jul; 43(7):1287-94.
16. Kentta¨ G, Hassme´n P, Raglin JS. Training practices and overtraining syndrome in Swedish age-group athletes. Int J Sports Med. 2001;22:460–5
17. Cohn P. An exploratory study on sources of stress and athlete burnout in youth golf. Sport Psychol. 1990;4:95–106.
18. Kentta¨ G, Hassme´n P. Overtraining and recovery: a conceptual model. Sports Med. 1998;26:1–16.
19. Messner M. When bodies are weapons: masculinity and violence in sport. Int Rev Sociol Sport. 1990;25:203–18.
20. Effects of repeated days of intensified training on muscle glycogen and swimming performance. Costill DL, Flynn MG, Kirwan JP, Houmard JA, Mitchell JB, Thomas R, Park SH Med Sci Sports Exerc. 1988 Jun; 20(3):249-54.
21. Overtraining following intensified training with normal muscle glycogen. Snyder AC, Kuipers H, Cheng B, Servais R, Fransen E Med Sci Sports Exerc. 1995 Jul; 27(7):1063-70.
22. The unknown mechanism of the overtraining syndrome: clues from depression and psychoneuroimmunology. Armstrong LE, VanHeest JL Sports Med. 2002; 32(3):185-209.
23. The effects of the 5-HT2C agonist m-chlorophenylpiperazine on elite athletes with unexplained underperformance syndrome (overtraining). Budgett R, Hiscock N, Arida RM, Castell LM Br J Sports Med. 2010 Mar; 44(4):280-3.
24. Cytokine hypothesis of overtraining: a physiological adaptation to excessive stress? Smith LL Med Sci Sports Exerc. 2000 Feb; 32(2):317-31.
25. Budgett R. Fatigue and underperformance in athletes: the overtraining syndrome. Br J Sports Med. 1998;32:107-110.
26. Budgett R, Hiscock N, Arida R, et al. The effects of the 5-HT2C agonist m-chlorphenylpiperazine on elite athletes with unexplained underperformance syndrome (overtraining). Br J Sports Med. 2010;44:280-283.
27. Smith LL. Cytokine hypothesis of overtraining: a physiological adaptation to excessive stress? Med Sci Sports Exerc. 2000;32:317-331.
28. Budgett R, Newsholme E, Lehmann M, et al. Redefining the overtraining syndrome as the unexplained underperformance syndrome. Br J Sports Med. 2000;34:67-68.
29. Mackinnon LT, Hooper SL. Plasma glutamine and upper respiratory tract infection during intensified training in swimmers. Med Sci Sports Exerc. 1996;28(3):285-290.
30. Smith LL. Overtraining, excessive exercise, and altered immunity: is this a T helper-1 versus T helper-2 lymphocyte response? Sports Med. 2003;33(5):347-364.
31. Hiscock N, Pedersen BK. Exercise-induced immunosuppresion: plasma glutamine is not the link. J Appl Physiol. 2002;93:813-822.
32. Walsh NP, Blannin AK, Robson PJ, Gleeson M. Glutamine, exercise and immune function. Sports Med. 1998;28(3):177-191.
33. Castell LM, Poortmans JR, Leclercq R, et al. Some aspects of the acute phase response after a marathon race, and the effects of glutamine supplementation. Eur J Appl Physiol. 1997;75:47-53.
34. Tiidus PM. Radical species in inflammation and overtraining. Can J Physiol Pharmacol. 1998;76:533-538.
35. Tanskanen M, Atalay M, & Uusitalo A. Altered oxidative stress in overtrained athletes. J Sports Sci. 2010;28(3):309-317
36. Hohl R, Ferraresso RL, DeOliveira RB, et al. Development and characterization of an overtraining animal model. Med Sci Sports Exerc. 2009;41(5):1155-1163
37. Lehmann M, Foster C, Keul J. Overtraining in endurance athletes: a brief review. Med Sci Sports Exerc. 1993;25(7):854-862.
38. Gouarne C, Groussard C, Gratas-Delamarche A, et al. Overnight urinary cortisol and cortisone add new insights into adaptation to training. Med Sci Sports Exerc. 2005;37:1157-1167.
39. Hynynen A, Uusitalo A, Konttinen N, et al. Heart rate variability during night sleep and after awakening in overtrained athletes. Med Sci Sports Exerc. 2006;38(2):313-317.
40. Pichot V, Roche F, Gaspoz FE. Relation between heart rate variability and training load in middle-distance runners. Med Sci Sports Exerc. 2000;32:1729-1736
41. Urhausen A, Kindermann W. Diagnosis of overtraining: what tools do we have? Sports Med. 2002:32;95-102.
42. Robson PJ. Elucidating the unexplained underperformance syndrome in endurance athletes: the interleukin-6 hypothesis. Sports Med. 2003;33:771-781.
43. Edwards KM, Burns VE, Ring C, Carroll D. Individual differences in the interleukin-6 response to maximal and submaximal exercise takes. J Sports Sci. 2006;24(8):855-862.
44. Fry RW, Grove JR, Morton AR, et al. Psychological and immunological correlates of acute overtraining. Br J Sports Med. 1994;28(4):241-246.
45. Halson SL, Lancaster GI, Jeukendrup AE, et al. Immunological responses to overreaching in cyclists. Med Sci Sports Exerc. 2003;35(5):854-861.
46. Morgan WP, Brown Dr, Raglin JS, et al. Psychological monitoring of overtraining and staleness. Br J Sports Med. 1987;21:107-114.
47. Morgan WP, Costill DL, Flynn MG, et al. Mood disturbance following increased training in swimmers. Med Sci Sports Exerc. 1988;20(4):408-414.

Other sources:

For this article, I mainly used the work of Kreher and colleagues: Kreher JB, Schwartz JB. Overtraining syndrome: a practical guide. Sports Health. 2012;4(2):128–138. doi:10.1177/1941738111434406