Up-to-date understanding of overtraining syndrome and overlap with related disorders
Abstract
Abstract:Introduction: The rigorous training regimens of elite athletes are, by nature, emotionally, physically, and mentally demanding. These regimens can result in marked improvements in athletic performance when they incorporate adequate intervals of rest and recovery. Continued training in the absence of adequate recovery progresses to a constellation of symptoms that affect medical and mental health, which is referred to as overtraining syndrome (OTS). Method: A literature review was conducted using PubMed, a database of reference on medical research maintained by the US national Library of Medicine and the National Institute of Health. Search terms included “overtraining syndrome”, “Unexplained Underperformance” (UUPS), “Burnout” in athletes. Results: The collection of the published work provides insight into the multidimensional composition and complexity of OTS in athletes. OTS progresses along a continuum without adequate recovery. Conclusion: Decreased athletic performance in the setting of increased training intensity without appropriate recovery, often accompanied by changes in mood, motivation, and general well-being are the distinctive features of OTS. OTS is preceded by stages of underperformance that are reversable and preventable, highlighting the importance of recognizing, managing, and preventing overtraining in athletes in order to prevent long-term negative outcomes is a maladaptive response to an imbalance between exercise and recovery.
Introduction
Elite athletes are no strangers to training regimens of high intensity and duration. The resulting injuries, fatigue, and mental strain are often accepted as part and parcel of the career of an athlete and perhaps even a necessary and expected sacrifice. However, over-emphasis on training without adequate rest and recovery can quickly tip the scales towards overtraining at the expense of performance. When the balance between training and recovery remains disrupted over a prolonged period of time, a constellation of adverse health impacts emerges that is often under-recognized. These manifestations, which include both general medical and mental health symptoms, fall on a spectrum of varying severity and duration, ultimately progressing towards a disorder known as overtraining syndrome if left unchecked.
Methods
A literature review was conducted with the intention of capturing the wide breadth of topics that are affiliated with overtraining syndrome. Due to the lack of consensus in defining OTS, a variety of keywords and topics were included in the literature search. The search began with evaluating studies that addressed mental health amongst athletes and identifying common themes and syndromes that arose. These included unexplained underperformance syndrome, relative energy deficiency in sport, the female athlete triad, overtraining syndrome, and literature that addressed primary psychiatric disorders in athletes, such as Major Depressive Disorder. The criteria outlined in the British Olympic Association’s 1999 expert panel statement that re-defined overtraining syndrome as Unexplained Underperformance Syndrome were further used to identify topics in the literature that relate to or explore OTS [1]. The shared themes that arose across these various, evidence- based sources were compiled, organized, and expanded upon to formulate the up-to-date understanding of OTS presented here.
Results
Overview of overtraining syndrome
overtraining syndrome (OTS) is a complex clinical disorder that occurs across occupations that demand intensive physical and emotional input in exchange for performance – most prominently amongst the elite athletic population. OTS manifests as a variety of outcomes that fall on a spectrum or ‘well-being continuum’, as described in the International Olympic Committee’s (IOC) consensus statement on load in sport and risk of injury and further illustrated in Figure 1 [2].
Physical and psychological load act as exertional forces that can move an athlete’s health along the continuum, progressing from the state of homeostasis through a series of adaptive and maladaptive changes that can ultimately lead to illness, injury, or even death on the other extreme of the spectrum. Load encompasses physiological, psychological, and mechanical stressors ranging from sub-cellular biological elements to general environmental factors that can originate from both within and outside of the sport [2].
External load refers to stressors that are independent of an athlete’s individual characteristics. These include the frequency, type, and duration of training or competition, distance traveled, number of repetitions, power output, and other measurements unique to the sport. External load also includes factors outside of training or competition, such as life events and the impact of the sport on personal domains like relationships and academics.
The external load, in conjunction with biological and environmental factors, contributes to physiological and psychological responses that are in turn referred to as the internal load. Some aspects of the internal load can be measured through objective and subjective tools. These include psychological inventories and measurements of sleep, laboratory data, and vital signs [2].
Meanwhile, recovery works as an opposite exertional force that shifts the athlete’s physical and psychological well-being back towards homeostasis. Recovery includes three essential components: sleep and rest, hydration and nutrition, and relaxation and mental recovery [3, 4]. The constant push and pull between load and recovery thus determines the athlete’s risk of developing any of the symptoms that characterize OTS. An athlete who is able to adequately incorporate recovery into their training lowers their risk of and is more likely to see their performance sustainably enhanced, rather than stagnate or even diminish.
Stages along the continuum
Overtraining – usually undertaken in a bid to enhance performance – occurs when increased training load is not counter-balanced by appropriate periods of rest and recovery. In the early stages of overtraining, an athlete may experience functional overreaching (FOR): a short period of decreased performance lasting up to two weeks that is followed by supercompensation – a fleeting, adaptive improvement in performance mediated by the body’s physiological compensatory response to training-related stress (5, 6]. If the initial period of decreased performance lasts three to four weeks and is not followed by supercompensation, then the athlete is beginning to experience the more severe stage of non-functional overreaching (NFOR) [5, 6].
As training load continues to increase without adequate rest and recovery, the athlete will enter a prolonged state of maladaptation with adverse changes in markers of performance and biological, neurochemical, and hormonal regulation mechanisms. This is known as overtraining syndrome.
The imbalance between training activity and recovery leads to chronic energy deprivation and inadequate mechanisms for repair and recovery. Given the deprived state and low energy availability, the central struggle will shift to balancing the continued desire to train hard with the reduced ability to do so. Overtraining can occur during all major forms of training, including resistance, anaerobic, aerobic, cognitive, and psychological training [7, 8].
Unexplained underperformance syndrome
‘Overtraining’ syndrome itself is a misnomer because it implies that overtraining is the sole cause of unexplained underperformance in athletes, which may in turn limit research and interventions to focusing only on training load. In reality, there are numerous factors outside of training that can cause diminished athletic performance. Such biopsychosocial and physiological factors include the presence of an underlying medical or mental health disorder, inadequate caloric intake, and poor sleep quality or quantity.
The term Unexplained Underperformance Syndrome (UUPS) has thus been suggested as more appropriate than overtraining syndrome, as it broadens the definition to include the different factors that can contribute to its development. This varying and overlapping terminology has unsurprisingly made research and the subsequent understanding of OTS difficult. It is unclear if athletes presenting with frequent medical or mental health issues and athletic underperformance are all overtrained, or if there are other more prominent etiologies to investigate [1].
Thus, in 1999 a panel of experts from the British Olympic Association held a round-table discussion at St. Catherine’s College in Oxford with the goal of re-defining overtraining syndrome as Unexplained Underperformance Syndrome (UUPS) [1]. The panel defined UUPS to as an unexplained performance deficit that is recognized by both the athlete and their coach and that persists despite two weeks of relative rest. Although relative rest could not be exactly defined, the panel agreed that this period should involve both a significant reduction in training and increase in recovery time. The panel further noted that after a period of heavy training and competition, athletes who develop UUPS typically report fatigue and an unexpected sense of effort during continued training. In addition, they may experience excessive sweating, frequent minor infections, unusually heavy, stiff, or sore muscles, disturbances in mood and sleep quality, as well as loss of energy, appetite, competitive drive, and libido [1].
Despite this push to redefine OTS using broader nomenclature, the term “overtraining syndrome” has persisted in the literature and become more immediately recognizable among clinicians, athletes, and trainers. For the purposes of clarity, this article will thus continue to use overtraining syndrome throughout this discussion with the caveat that training load should not be the sole focus of inquiry when evaluating and treating athletes for OTS.
Incidence and prevalence of OTS
The lack of standardized diagnostic criteria for identifying OTS makes it challenging to assess the exact prevalence and incidence of the syndrome. Current literature suggests that OTS may occur in 20% to 60% of elite athletes at some point in their careers [5, 7, 9, 10, 11]. Earlier studies have evaluated the more general concept of training with insufficient rest, which was estimated to occur in 7% to 20% of athletes per training cycle with a higher risk among elite runners (60%) compared to lower-level runners (38%) [11]. The lifetime risk of OTS amongst runners is estimated to be 64% and 60% for males and females, respectively [12].
What is more, athletes who have previously experienced OTS are more likely to relapse. For example, one study found that 91% of college swimmers who experienced OTS during their freshman year had a repeat episode the following year, compared to an incidence of 34% among swimmers who had not experienced an initial episode of OTS as a freshman [13].
Even higher rates of OTS and related syndromes have been reported in other studies, but these are likely inflated due to overlapping definitions of FOR, NFOR, and OTS/UUPS. Nevertheless, it is important to consider the widespread prevalence of these conditions, given the shared symptomatology and fluidity between these stages on the well-being continuum.
Theories related to the etiology of OTS
Most classic theories postulate that different levels of amino acids and neurotransmitters play a role in OTS development. For example, the neurotransmitter serotonin (5-HT) is a well-established regulator of mood, sleep, and behavior and is therefore implicated in the etiology of OTS as part of the central fatigue hypothesis. In the brain, 5-HT is derived from tryptophan. Prolonged exercise increases plasma levels of unbound or free tryptophan (F-TRP), leading to increased tryptophan uptake and 5-HT synthesis in the brain. Increased 5-HT, in turn, has been linked with feelings of lethargy and fatigue in humans [14, 15].
Meanwhile, the glycogen hypothesis theory proposes that the depletion of glycogen stores during exercise causes muscle fatigue and subsequent impairments in athletic performance – one of the hallmarks of OTS. Glycogen depletion also indirectly increases 5-HT levels and thus plays back into the central fatigue hypothesis. Decreased glycogen stimulates the oxidation of branched-chain amino acids (BCAAs) to glucose in order to increase immediately available energy sources. BCAAs and F-TRP use and compete for the same transport mechanism, so the resulting diminished BCAA levels lead to increased plasma F-TRP, tryptophan uptake, and 5-HT synthesis [7, 15, 16].
Other theories point to inflammation, cytokine release, and oxidative stress as sources of muscle damage, fatigue, and increased susceptibility to infections seen in OTS. A baseline level of oxidative stress is expected during exercise, as the reactive oxygen species released by damaged muscle cells facilitate cellular repair. The oxidative stress hypothesis, however, postulates that excessive oxidative stress can become pathologic. High levels of reactive oxygen species cause inflammation, muscle fatigue, and soreness resulting in decreased athletic performance [17, 18]. In a similar vein, the cytokine hypothesis proposes that damaged tissues elicit a local inflammatory response that involves the recruitment of cytokines. With adequate recovery, this inflammation can work to improve muscle strength and tissue healing. However, continued training in the absence of adequate recovery can amplify this response into a state of systemic inflammation. Persistent elevations in pro-inflammatory cytokines contribute to CNS and peripheral fatigue as well as negative changes in hormonal and metabolic function [19, 20, 21, 22].
Still, other theories illustrate the role of hormonal and nervous systems on dysregulation and the maladaptive responses implicated in OTS. The autonomic nervous system hypothesis proposes that an imbalance in the autonomic nervous system accounts for some of the psychological and physical symptoms of OTS, including fatigue, apathy, performance inhibitions, depression, and bradycardia [7, 23]. Various forms of negative feedback may cause this and sympathetic inhibition, resulting in surges of catecholamine release, increased metabolism, and elevated core temperatures during heavy exercise [24]. Similarly, the hypothalamic hypothesis emphasizes the role of the hypothalamic-pituitary-adrenal (HPA) axis in the hormonal dysregulation seen in OTS, as evidenced by alterations in key hormones such as cortisol, adrenocorticotropic hormone (ACTH), testosterone, growth hormone (GH), and prolactin in overtrained athletes [25, 26].
Evaluation and diagnosis of OTS
Athletes affected by OTS will most often initially present with vague reports of unexplained underperformance. A thorough history usually reveals descriptions of persistently decreased athletic performance and disturbances in mood despite weeks to months of recovery. The athlete may express that they feel the need to exert considerably more effort during training or competition. They may also report a wide range of mood-related symptoms including irritability, diminished motivation, depressed mood, and difficulty concentrating or cooperating with others. Athletes may notice changes in their sleep patterns with resulting feelings of persistent fatigue or daytime sleepiness. Finally, they may experience somatic complaints including persistent muscle soreness or stiffness, recurrent overuse injuries, unexplained weight loss, or frequent illnesses such as upper respiratory infections.
General medical and psychiatric evaluation
OTS is a diagnosis of exclusion. Athletes presenting with any of these symptoms should thus undergo a thorough general medical and mental health evaluation. The clinician should gain a nuanced understanding of the athlete’s training program including any recent changes in frequency, duration, or intensity of training and the presence or absence of dedicated time for adequate sleep and recovery.
Initial laboratory tests should include a complete blood count, basic metabolic panel, ferritin level, and thyroid stimulating hormone level. A monospot test may be indicated if the history suggests infectious mononucleosis whereas an elevated resting heart rate warrants an electrocardiogram. Longitudinal screening tests or logs that assess changes in sleep, nutrition, weight, or mood are also helpful in evaluating symptoms over time. Any additional data obtained through the athlete’s training program, such as training logs, reactive strength index, force plate data, counter jump data, or catapult data, could also provide more objective assessments.
Differential diagnosis
The signs and symptoms of OTS span a wide range of mental and other physical symptoms that can mimic disorders seen in the typical sport population. The most immediate causes of general fatigue should be considered first, including insomnias and other sleep disturbances, nutritional deficiencies, anemia, and primary mental health disorders such as depression, generalized anxiety, and eating disorders. From a systems-based perspective, the differential diagnosis should include endocrine disorders such as hypothyroidism, diabetes mellitus, adrenocortical insufficiency or excess, and cardiopulmonary conditions such as cardiomyopathies, congenital or acquired heart disease, and chronic pulmonary disease. Neurological conditions that should be considered include post-concussive syndromes as well as neuromuscular disorders. Infectious processes such as mononucleosis, respiratoria infections, hepatitis, and HIV should also be included in the differential. A thorough psychosocial history must be conducted and should screen for drug-related conditions such as substance misuse, medication side effects, and environmental and social stressors including interpersonal issues, major life events, and excessive cognitive demands like work or study hours.
Testing
At present, there is little evidence to support the use of biochemical, immunologic, or hormonal testing to diagnose OTS. Laboratory tests help rule out other disorders on the differential, but OTS remains primarily a clinical diagnosis of exclusion. There is better evidence to support the use of questionnaires or other scales, especially to monitor progression and recovery during OTS. The Borg Rating of Perceived Exertion (RPE) is a commonly used scale that measures an individual’s effort and exertion, breathlessness, and fatigue during a workout, indicating the intensity of a training activity [27]. An overtrained athlete may report higher RPE while simultaneously exhibiting decreased athletic performance.
Another more comprehensive tool to assess for early indicators of OTS is the Profile of Mood States (POMS) questionnaire, a 65-item survey with six subscales that assess mood disturbances, including tension, depression, anger, vigor, fatigue, and confusion [28]. The Recovery-Stress Questionnaire for Athletes (REST-Q) is another useful scale that measures the frequency of current stress along with the frequency of recovery-associated activities [29]. These various measures can help identify early OTS signs and monitor progression, but they are not specific for OTS. Nevertheless, they can be used in conjunction with one another and alongside a thorough history and diagnostic workup to gain a better understanding of OTS as well as an athlete’s projected recovery.
Mental health diagnoses associated with OTS
OTS and co-morbid mental health disorders are not mutually exclusive. They can co-exist, cause, exacerbate, or be mistaken for one another. Furthermore, many mental health disorders initially present during young adulthood, coinciding with the period when athletes are most likely to near the peak of their athletic training. These considerations further emphasize the importance of a thorough mental health evaluation in athletes in general and those suspected to be experiencing OTS in particular. The most common mental health diagnoses associated with OTS include mood disorders such as major depressive disorder (MDD), substance use disorders, disordered eating and sleep, anxiety, and trauma-related disorders.
Depressive spectrum in sport
Currie’s view of depression and sport presents depressive symptoms on a spectrum, with normal mood on one end and depressive illness on the other [30]. Normal mood can be sustained by sporting activities. Both the physical activity itself and its associated benefits – such as interconnectedness, social support, and regular routines – can improve mild depressive symptoms. As depressive symptoms worsen due to a primary depressive disorder or overtraining that can no longer be mitigated by the benefits of sport, the athlete may progress towards an overt depressive illness. Symptoms can manifest as Major Depressive Disorder (MDD) or severe under-recovery and overtraining that mimics MDD.
Briefly, signs and symptoms of MDD include depressed mood, anhedonia, changes in sleep, appetite, or weight, impaired concentration, fatigue or diminished energy, feelings of guilt, worthlessness, hopelessness, psychomotor changes, and suicidal thoughts. Most studies investigating depression among athletes have been conducted with collegiate level athletes and found the prevalence to range from 15.6% to 21%, with female athletes at higher risk [31].
Mood disturbances are a valuable early indicator of OTS. More than 70% of athletes with NFOR and OTS self-reported emotional disturbances, indicating that athletes often identify early signs of overreaching themselves. This makes it all the more important to listen to athletes when they begin to indicate changes in mood [32].
Other commonalities between OTS and depression include a lack of recovery from training, reduced performance, and chronic maladaptive responses. In addition to shared symptomatology, OTS and depression are characterized by remarkably similar changes in brain structures, neurotransmitters, endocrine pathways, and immune responses, suggesting similar origins [33]. Perhaps the only apparent difference between the disorders is the manner in which dysfunction manifests and the athlete recuperates. The overtrained athlete will mostly suffer from deficits in athletic performance, which may recover with prolonged rest. The depressed athlete, however, will experience social, cognitive, and work-related dysfunction and will often feel worse with rest [34].
Similar to depressive disorders, anxiety and trauma-related disorders also overlap with OTS and should be screened for on initial history. Like OTS, anxiety and trauma disorders interfere with sleep, disrupt the autonomic nervous system, and cause impairments in energy, endurance, reaction time, and coordination, resulting in impaired athletic performances.
Burnout syndrome in athletes
Another topic of recent interest and concern is the phenomenon of burnout, which can be seen across a wide spectrum of professions and occupations that demand high performance. It is therefore unsurprising that burnout has come to be a condition recognized specifically amongst athletes. Burnout Syndrome in Athletes (BSA) occurs when athletes lose energy and motivation to continue their training regimens. Athletes with BSA experience three central characteristics: emotional and physical exhaustion, a reduced sense of accomplishment, and sport devaluation [35]. Psychological symptoms include negative self-talk, deterioration of interpersonal relationships, behavioral issues, and decreased performance. Physical symptoms often manifest as headaches, insomnia, fatigue, diminished appetite, and an increased rate of infections. The psychological and physical impact of BSA shares many commonalities and often overlaps with OTS. From the literature it appears that OTS may have more disrupted and maladaptive functions related to psychophysiology, hypothalamus, pituitary adrenal axis hyporesponsiveness, hypometabolism, greater fatigue, and impaired immune and endocrine function compared to BSA. OTS is also more likely to have impaired fueling capacity from low protein and carbohydrate availability compared to BSA [36, 37, 38].
Disordered eating: Female athlete triad versus relative energy deficiency in sport
In 2005, the IOC released a Consensus Statement defining the Female Athlete Triad as the combination of disordered eating and irregular menstrual cycles among female athletes, leading to decreased endogenous estrogen and subsequent low bone mineral density [39]. In 2007, the American College of Sport’s Medicine updated the definition of the Triad to refer to the relationship between three inter-related components: bone health, menstrual function, and energy availability (EA) [40]. Since then, a growing body of literature has further expanded our understanding of the Triad, reframing it as a relative energy deficiency resulting from an imbalance between dietary energy intake (EI) and the energy expenditure required to support health and physical activities – a broader view that is not gender-exclusive. As such, the IOC released an updated Consensus Statement in 2014 that explores the newly coined Relative Energy Deficiency in Sport (RED-S). The statement defines RED-S as impaired physiological function including, but not limited to, metabolic rate, menstrual function, bone health, immunity, protein synthesis, and cardiovascular health caused by relative energy deficiency. The updated REDS consensus in 2018 was added to the literature [41]. Since the 2018 release there is emerging data demonstrating the role of low carbohydrate availability. The 2023 International Olympic Committee (IOC) consensus statement of Relative Energy Deficiency in Sports (REDS) was published and provides evidence that supports the association between mental health and REDs and more data elucidating the impact of LEA in males. As a part of this publication, his Physiological Model is proposed to demonstrate the complexity of LEA exposure, coupled with individual factors that can lead to changes in health and performance outcomes [42].
Athletes affected by RED-S may experience impaired self-body image, low self-esteem, decreased muscle strength or endurance, impaired coordination, and a depleted fuel source – symptoms that overlap with OTS.
Treatment and prevention of OTS
The most critical step in the treatment and management of OTS is to identify the syndrome itself as well the specific stressors that may be contributing to its etiology. A thorough history and diagnostic workup are critical to treating primary mental health and other underlying disorders. Treatment of FOR is relatively straightforward and involves balancing overload training with appropriate recovery, which typically requires a few days and minimal sacrifices in training. Rest and recovery are also the mainstays of treatment in NFO and OTS, though for prolonged periods that may require more disruptive sacrifices in training, such as missing competitions or delaying training cycles.
Given the significant role of mental health in the etiology, progression, and manifestation of OTS, referral to a mental health provider is often necessary. First, however, it is vital to build rapport with the athlete, listen to their concerns, and understand their perceptions. As previously mentioned, OTS and mental health disorders are commonly rejected diagnoses, so emphasis should be placed on reducing stigma and encouraging the acceptance of help in order to increase the likelihood of successfully incorporating mental health treatment into recovery.
Recovery and prevention of OTS rely heavily on monitoring and adjusting training load. Using scales such as RPE, POMS, or REST-Q helps to provide a more objective measure of progress and assessment of interventions. Effective use of performance measures, observation of training load, and utilization of mood questionnaires can interrupt the progression from FOR to NFO and OTS [7, 8, 43]. Education also plays a crucial role in the prevention of OTS. Athletes and trainers alike should be aware that the initial signs of overreaching include feeling the need to increase training to compensate for decreased performance. Education for the coaches may play a key role in the management of OTS. Another warning sign is an increased rating of perceived exertion for a given workload, which can be screened early using the RPE.
Conclusion
Overtraining Syndrome is a nebulous clinical diagnosis complicated by many overlapping symptoms, a multi-factorial etiology, and ever-changing terminology as the science and understanding continues to evolve. The hallmarks of OTS include decreased athletic performance in the setting of increased training intensity without appropriate recovery, often accompanied by changes in mood, motivation, and general medical health. OTS is a maladaptive response to an imbalance between exercise and recovery. It is mediated by complex neurohormonal processes and affects various domains of well-being, including both mental and physical health health. A thorough history is vital to identifying OTS, as it is ultimately a diagnosis of exclusion and there are emerging but limited laboratory tests that can aid in its diagnosis – though these are important in ruling out other illnesses. Validated scales, such as the RPE, POMS, and REST-Q, can be utilized to screen for early signs of OTS and to adjust training load and monitory recovery, which are the mainstays of treatment of OTS. Finally, it is crucial to listen to the athlete, who is often the first to realize warning signs of OTS. The growing incidence of OTS and mental health disorders in athletes, coupled with the potentially irreversible psychological, professional, and social consequences, make prevention, early identification, and effective management central components of ensuring the health and well-being of the athlete.
References
1 Redefining the overtraining syndrome as the unexplained underperformance syndrome. Br J Sports Med. 2000;34(1):67–8.
2 How much is too much? (Part 1) International Olympic Committee consensus statement on load in sport and risk of injury. Br J Sports Med. 2016;50(17):1030–41.
3 . Overtraining and recovery. Sports Med. 1998;26(1):1–16.
4 Mental health issues and psychological factors in athletes: detection, management, effect on performance and prevention: American Medical Society for Sports Medicine Position Statement – Executive Summary. British J Sports Med. 2020;54(4):216–20.
5 Prevention, diagnosis and treatment of the overtraining syndrome: Joint consensus statement of the European College of Sport Science (ECSS) and the American College of Sports Medicine (ACSM). Eur J Sport Sci. 2013;13(1):1–24.
6 Diagnosing overtraining syndrome: A scoping review. Sports health. 2022;14(5):665–73.
7 . Overtraining syndrome: a practical guide. Sports health. 2012;4(2):128–38.
8 . Prevention, diagnosis and treatment of the overtraining syndrome: ECSS position statement “task force”. Eur J Sport Sci. 2006;6(01):1–14.
9 . Prevalence of nonfunctional overreaching/overtraining in young English athletes. Med Sci Sports Exerc. 2011;43(7):1287–94.
10 . Overtraining syndrome symptoms and diagnosis in athletes: Where is the research? A systematic review. Int J Sports Physiol Perform. 2022;17(5):675–81.
11 . Psychological characterization of the elite female distance runner. Int J Sports Med. 1987;8(S2):S124–S31.
12 . Personality structure, mood states, and performance in elite male distance runners. Int J Sport Psychol. 1988.
13 . Overtraining and staleness – psychometric monitoring of endurance athletes. Handbook of research on sports psychology. Macmillan Pub Co.; 1993.
14 .
The clinical psychopharmacology of tryptophan . In: Wurtman RJWurtman JJ, Eds. Nutrition and the brain (USA). Raven Press; 1986.15 . Branched-chain amino acids prolong exercise during heat stress in men and women. Med Sci Sports Exerc. 1998;30(1):83–91.
16 . Dietary precursors and brain neurotransmitter formation. Annu Rev Med. 1981;32(1):413–25.
17 . Altered oxidative stress in overtrained athletes. J Sports Sci. 2010;28(3):309–17.
18 . Radical species in inflammation and overtraining. Can J Physiol Pharmacol. 1998;76(5):533–8.
19 . Overtraining syndrome in the athlete: current clinical practice. Current Sports Med Rep. 2014;13(1):45–51.
20 . Elucidating the unexplained underperformance syndrome in endurance athletes. Sports Med. 2003;33(10):771–81.
21 . Cytokine hypothesis of overtraining: a physiological adaptation to excessive stress? Med Sci Sports Exerc. 2000;32(2):317.
22 . Overtraining, excessive exercise, and altered immunity. Sports Med. 2003;33(5):347–64.
23 . Autonomic imbalance hypothesis and overtraining syndrome. Med Sci Sports Exerc. 1998;30(7):1140–5.
24 . Overtraining in endurance athletes: a brief review. Med Sci Sports Exerc. 1993.
25 . Hormonal response to a non-exercise stress test in athletes with overtraining syndrome: results from the endocrine and metabolic responses on Overtraining syndrome (EROS) – EROS-STRESS. J Sci Med Sport. 2018;21(7):648–53.
26 . Diagnosis of overtraining what tools do we have. Dtsch Z Sportmed. 2002.
27 . The Borg rating of perceived exertion (RPE) scale. Occup Med. 2017;67(5):404–5.
28 . Exercise and mood: A selective review and synthesis of research employing the profile of mood states. J Appl Sport Psychol. 2000;12(1):69–92.
29 . Recovery-stress questionnaire for athletes: User manual. Human Kinetics; 2001.
30 .
Eating Disorders . In: Currie AOwen B, Eds. Sports Psychiatry. Oxford: OUP; 2016.31 . Depression in athletes: prevalence and risk factors. Curr Sports Med Rep. 2015;14(1):56–60.
32 . Prevalence of non-functional overreaching and the overtraining syndrome in Swiss elite athletes. Schweiz Z Med Traumatol. 2013;61(4):23–9.
33 . Physical activity and mental health: the association between exercise and mood. Clinics (Sao Paulo). 2005;60(1):61–70.
34 . The stigmatisation and denial of mental illness in athletes. Br J Sports Med. 2000;34(1):4–5.
35 . Is athlete burnout more than just stress? A sport commitment perspective. J Sport Exerc Psychol. 1997;19:396–417.
36 Overtraining Syndrome (OTS) and Relative Energy Deficiency in Sport (RED-S): shared pathways, symptoms, and complexities. Sports Med. 2021;51(11):2251–80.
37 . Hormonal response to a non-exercise stress test in athletes with overtraining syndrome: results from the Endocrine and metabolic Responses on Overtraining Syndrome (EROS) – EROS-STRESS. J Sci Med Sport. 2018;21(7):648–53.
38 . Hormonal aspects of overtraining syndrome: a systematic review. BMC Sports Sci Med Rehabil. 2017;9:14.
39 . International Olympic Committee (IOC) consensus statement on the female athlete triad. 2005. Available from https://stillmed.olympics.com/media/Documents/Athletes/Medical-Scientific/Consensus-Statements/2005_position-stand-female-athletes-triad.pdf.
40 . American college of sports medicine joint position statement. nutrition and athletic performance. Med Sci Sports Exerc. 2016;48(3):543–68.
41 IOC consensus statement on relative energy deficiency in sport (RED-S):2018 update. Br J Sports Med. 2018;52:687–97.
42 2023 International Olympic Committee’s (IOC)consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57:1073–97.
43 . Psychological monitoring of overtraining and staleness. Br J Sports Med. 1987;21(3):107–14.
