Burnout syndrome in the occupational context has been a topic of concern for many years, affecting predominantly employed populations. The expression to be "burned-out" describes that a person is no longer able to "burn" or to be productive concerning their responsibilities, thus often their paid work. This requires that the affected persons assigned their work a high value. This may give people the impression that the person suffering from burnout has a high and therefore positive evaluation of his or her work effort and thus perceives his/her absence from work due to burnout not as a personal weakness. This "positive" use of the term burnout, in comparison to the concept of depressive disorder, which appears to be viewed significantly more negative, allows affected persons to expect less stigmatisation. The term burnout is therefore used more often than the term depression (Bahlmann et al., 2013; Berger et al., 2012). The International Classification of Diseases published by the World Health Organization has not taken up burnout as an independent disease (Krollner and Krollner, 2018). It lists, in an appendix, factors which can affect the wellbeing of individuals. In section Z 73, "problems associated with difficulties in coping with life" are listed. Burnout is considered to be a state of physical and mental exhaustion and is encoded in a diagnosis by the number Z 73.0 (Berger et al., 2012). Burnout is described by the German Society of Psychiatry and Psychotherapy, Psychosomatics and Neurology as an exhaustion and overload syndrome, which is a risk condition for a later psychiatric or physical illness (DGPPN, 2017). The burnout syndrome is mainly work related and occurs very often after a long period of high workload in a combination with perceived stressful working conditions (Naczenski et al., 2017). According to Maslach (Maslach, 2003), job burnout is defined by three dimensions: exhaustion, cynicism and a sense of inefficacy. Current reports show an increase of the prevalence of the burnout syndrome. For example, the AOK--one of Germany's largest health insurance companies --published a report in 2014 focusing on work absenteeism (AOK, 2014). This report, written by the scientific Department (WidO) of the AOK, reported that between 2004 and 2013 there was an increase from 8.1 days to 87.6 days taken off work per 1000 insured persons of the AOK per year due to a medical inability to work classified under the diagnosis group Z 73. Associated with this increase in work absenteeism is of course the economic damage caused by losing time at work. The study by the Association of Psychotherapists in Germany (Bundespsychotherapeutenkammer, BPtK) on medically excused days off work from 2015 is based on data from nearly 85% of the statutorily insured individuals in Germany (BPtK, 2015). This study demonstrates that one in every 5 individuals who were medically excused from work for more than six weeks were diagnosed with a mental disease, such as burnout.
As burnout syndrome is not listed as an independent disease in the international classification of diseases, no clear diagnosis can be made based on explicit symptoms (ICD-10, 2017). In the literature, up to 100 single symptoms are mentioned as an indication of burnout (Ahola et al., 2005; Ekstedt et al., 2006; Kakiashvili et al., 2013; Kissling et al., 2014; Mohren et al., 2003; Peterson et al., 2008; Soares et al., 2007; Toker et al., 2012). The most frequently mentioned symptom in people with burnout is an excessive, persistent or even chronic physical as well as mental fatigue (Ahola et al., 2005; Appels and Mulder, 1989; Ekstedt et al., 2006; Kakiashvili et al., 2013; Mohren et al., 2003; Peterson et al., 2008; Soares et al., 2007; Toker et al., 2012). The condition of fatigue is considered a risk factor for a number of other diseases, like acute myocardial infarction, heart disease, common colds, flu, gastrointestinal disorders, infections (Honkonen et al., 2006; Mohren et al., 2003) and many more (Corrigan et al., 1995; Kakiashvili et al., 2013; Peterson et al., 2008; Toker et al., 2012). Some studies mention that burnout increases the occurrence of mental and physical exhaustion and of diseases such as cardiovascular disease, metabolic syndrome and depression (Ahola et al., 2005; Appels and Mulder, 1989; Bianchi et al., 2015; Kakiashvili et al., 2013; Kitaoka-Higashiguchi et al., 2009; Soares et al., 2007; Toker et al., 2012). Studies conclude that the effects of burnout and cardiovascular disease incidence are interdependent (Appels and Mulder, 1989; Honkonen et al., 2006; Kakiashvili et al., 2013; Shirom, 2005; Soares et al., 2007; Toker et al., 2012).
This wide range of possible comorbidities weakens affected persons, thereby intensifies the state of exhaustion, which further encourages the above-described relationships. Medicine is confronted with the problem that for burnout there is no consensus in neither diagnosis and treatment (Kakiashvili et al., 2013). Therapy programs are highly varied and often unclear. Treatments for burnout include encouraging a healthy diet, taking vitamin supplements, reducing alcohol consumption, naturopathic remedies, relaxation and mindfulness therapies, gymnastics, sports, massages, creative activities, spiritual and/or religious practices, adopting an ancient or far-eastern philosophy, animal therapy, spending time with friends etc. (Hillert, 2012a). Often, individuals with burnout are treated with regimens designed to treat depression (Ahola et al., 2005; Kakiashvili et al., 2013). In fact, the most common drugs administered in the treatment of burnout are antidepressants (Kakiashvili et al., 2013). However, the general treatment is also aimed at mitigating the personal or environmental causes of the syndrome like developing coping skills or creating organizational changes at work (Kakiashvili et al., 2013). The contents of the various therapies are often comprising three parts: general-strengthening, relaxation and occupational support (Hillert, 2012b) in form of decreasing job demands, increasing job control or the level of participation in decision making (Awa et al., 2010). The content, however, is usually not selected on the basis of empirical findings, but rather on subjective experiences of the clinic and the possibilities of the respective treatment location (Hillert, 2012b).
A series of studies suggest that physical activity based interventions are indicated in the treatment of burnout in an occupational setting. The positive impact of physical activity is often mentioned, but the reasons for that remain unexplained. The effect of physical activity on burnout may be ascribed to psychological changes. Physical activity can be seen as a behavioural distraction from stressful situations and hereby degrade the psychological impact of a situation (Altshuler and Ruble, 1989). Physical activity fosters the development of perceptions like mastery and self efficacy (Salmon, 2001) and has therefore the ability to diminish sensitivity to negative stimuli. Physiological changes by physical activity could reduce the persons' physiological sensitivity to chronic stress (i.e. burnout) (Forcier et al., 2006). Employees could deal with stress at work without being physiologically overwhelmed by it. This could lead to faster physical recovery after a stressful experienced situation and therefore to a reduction of the risk of burnout (Altshuler and Ruble, 1989; Jackson and Dishman, 2006; Klaperski et al., 2014; Salmon, 2001). Fundamentally, it is well established that regular physical activity has a preventive effect on various diseases (Pedersen and Saltin, 2015). For example it has a demonstrated positive effect on coronary heart disease, cerebrovascular diseases, arterial hypertension, diabetes mellitus, stress anxiety disorders and depression (Pedersen and Saltin, 2015; Reimers and Brooks, 2003). In addition, aerobic physical activity reduces anxiety which appears in an occupational context (Salmon, 2001). There is a positive relation between physical activity and emotional well-being (Galper et al., 2006). Studies show that the preventive effect of physical exertion increases with the progression in exercise intensity and duration (Kushi et al, 1997; Lee and Skerrett, 2001; Thompson et al., 2003).
In one study, more than half of the participants with mild to moderately severe depression, experienced a reduction in depression symptoms after an exercise intervention based on running (Greist et al., 1979). Another study shows that exercise (running) has positive effects on work-related fatigue and employee well-being (de Vries et al., 2017). It has been shown that aerobic exercise groups and/or strength training groups, compared to a control group, lead to a clear reduction in the depression score (Pappas et al., 1990). Furthermore, McCann & Holmes show in their study that a group, which conducted exercise achieved significantly better treatment results than the group which performed relaxation exercises as well as the control group (McCann and Holmes, 1984). The authors report a marked antidepressant effect after the endurance training protocol, while the relaxation exercises remained ineffective. This finding is supported by Blumenthal, who assigns an effect to exercise that is comparable to that of adequate psychopharmacological treatment (Blumenthal et al, 1999). The sport or exercise type does not seem to be decisive for the effectiveness of the exercise-based therapy.
The existing literature describes the effects of physical activity on symptoms and comorbidities of burnout, however, the listed therapeutic content does not have a high-quality, empirical evidence-base. A rigorous evaluation of high-quality studies with regard to the efficacy of a treatment of burnout through physical activity is still lacking. A first attempt was made by the systematic review of Naczenski et al. (2017)...