MILITARY MEDICINE, 184, 5/6:e192, The Overtraining Syndrome in Soldiers: Insights from the Sports Domain Susan Vrijkotte*†‡; Bart Roelands*; Maj Nathalie Pattyn, *†§; Romain Meeusen* ABSTRACT Introduction:: Soldiers are exposed to extreme training regimens in order to optimally prepare for real battle. High attrition rates are a known issue during training courses, especially for elite troops. An underlying factor might be the disbalance between stress/strain and recovery. The aim of this review is to give insight in the current knowledge about functional overreaching (FOR), non-FOR (NFOR), and the overtraining syndrome (OTS) in the mili- tary. Materials and Methods: A systematic literature review was conducted. PubMed, IngentaConnect, Science Direct, and Web of Science were screened for the following keywords and combinations of search terms; military, personnel, OT, soldier. Results: Seven studies investigating the effects of OT during training courses were selected. The defini- tions used for OT varied widely and there is no systematic use of markers to determine FOR, NFOR, or the OTS in the military. Conclusions: Much research on NFOR/OTS has been conducted in the sports domain and the military could make use of these insights to promote a more efficient balance between training load and recovery. It is suggested to regularly test soldiers on physical performance, psychomotor speed and mood using ideally a military-specific test or the 1.5-mile run, psychomotor vigilance test and Profile of Mood States. The two-bout exercise test can be used as a specific test if previous testing indicates the development of NFOR/OTS and can be combined with metabolic and immunological testing to exclude pathological causes. INTRODUCTION Ideally, training soldiers could be described as the fine art of pushing boundaries to enhance their capabilities without increasing drop-out rates. However, the number of drop-outs during military training courses is high. Percentages of 50% are not uncommon 1 while Walker et al 2 reported an attrition rate of over 70% during a 24-month military training pro- gram. Reduction of these high drop-out rates can significantly improve the return on investment from training courses. In case of the 24-month military training program, the average training course costs were $250,000 per graduate, with an average of nine graduates per class. 2 Adjustments to the train- ing program resulted into 16 graduates per class and thus lowering the average costs per graduate to approximately $136,500. 2 Reducing drop-out rates, and thus increasing completion rates, will have large economic consequences, allowing to focus resources on candidates who will be effec- tive unit members. The thin line between peak performance and impaired performance has been studied extensively in (elite) ath- letes. 3,4 Apart from regular training, overtraining (OT) is used in training programs to improve performance. It com- prises at least one intensified period of heavy training and minimal recovery. 5 OT can lead to different severities of overreaching, namely functional recovery are optimally bao- verreaching (FOR) and non-FOR (NFOR). In FOR, the ath- lete strives for supercompensation to occur, 6 which is favorable for performance and follows a recovery period of days up to week(s). When an imbalance between training and recovery is experienced for too long, NFOR develops with performance being negatively affected. Recovery from NFOR will take weeks up to months. In the most extreme situation, OT can lead to the overtraining syndrome (OTS), during which performance is severely affected and recovery takes months or even longer. 7 It is difficult to differentiate between NFOR and OTS without having an objective test available, which is why the actual diagnosis can currently only be made retrospectively. Figure 1 depicts the clinical continuum of the differences between regular training, FOR, NFOR, and the OTS. In applied settings like the military, soldiers are being prepared to being resistant to a high load of mental and physical stress and thus are they striving to be as fit and resilient as possible. Soldiers can develop NFOR and OTS during either training courses or military operations. 8 The circumstances are challenging for optimal performance, since extreme conditions have to be endured until the task is fin- ished, after which soldiers can start to recover. Recruits can experience high physical and cognitive stress levels com- bined with lack of sleep and limited food. 9 The physical and mental load in military circumstances approaches or even goes beyond human capability limits, which is different in athletes where food, sleep, and recovery are optimally *Human Physiology Research Group, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium. †LIFE Department, Vital Signs and Performance Monitoring (VIPER), Royal Military Academy, Avenue de la Renaissancelaan 30, B-1000 Brussels, Belgium. ‡Zorggroep Solis, Hermelijn 2, 7423 EJ Deventer, The Netherlands. §Department of Experimental and Applied Psychology, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium. doi: 10.1093/milmed/usy274 © Association of Military Surgeons of the United States 2018. All rights reserved. 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