Also known as overtraining syndrome, literature has suggested to rename this persistent decline in athletic performance to ´unexplained underperformance syndrome (UUPS)´ due to the multiple systems that may be involved and it´s diagnosis complexity.

What is unexplained underperformance?
The term is used to describe a significant episode of underperformance with persistent fatigue, that is, maladaptation to any training (1).
Recognised signs & symptoms of unexplained underperformance:
It is important to note that UUPS usually involves a combination of multiple signs & symptoms. Symptoms are often representative of underlying hormonal, immunologic, neurologic and psychologic disturbances (4). However, having one or more of these signs & symptoms does not necessarily mean UUPS. Currently, more research is needed to better understand UUPS.
Lack of adaptation to training
General constant fatigue
Low mood
Imbalanced hormone levels
Loss of libido
Increased rate of percieved exertion (RPE) for usually easier training sessions (long-term)
Loss of competitive drive
Loss of appetite
Unexplained weight loss or gain
Unexplained chronic stiff & sore muscles
Poor sleep
Possible contributors of unexplained underperformance:
An imbalance between training load and recovery may not be the primary cause for UUPS (1). It is difficult to pinpoint exact causes making diagnosis difficult.
Too much high volume & intense training with too little recovery
Lack of nutrients in the diet
Insufficient carbohydrates to suit training and lifestyle
Constant mental/ emotional stress
Poor quality/ insufficient sleep
Taking on more than you can handle
Monotonous training
Lack of education
Possible solutions for prevention of unexplained underperformance:
It is always advised to take steps for prevention. Aim to include a prevention strategy that addresses nutritional, physical & psychological methods.
Here are a few examples:
Daily contact with an experienced trainer
Individualised nutrition plans suited to the athlete´s needs
Education for athletes on UUPS
Ensure daily nutrient-rich meals
Avoid nutrient deficiencies
Regular blood & performance testing
Focus on everyday recovery & relaxation methods
Ensure sufficient rest days
Minimise mental stress
Minimise exposure to toxins
Make sure to have ´me time´
Listen to your body
Do you want personalised nutrition advice?
References:
1) Lewis, Nathan A; Collins, Dave; Pedlar, Charles R; Rogers, John P (2015). Can clinicians and scientists explain and prevent unexplained underperformance syndrome in elite athletes: an interdisciplinary perspective and 2016 update. BMJ Open Sport & Exercise Medicine, 1(1).
2) Budgett R, Newsholme E, Lehmann M, et al. (2000). Redefining the overtraining syndrome as the unexplained underperformance syndrome, British Journal of Sports Medicine. 34, 67-68.
3) Maughan, Ronald J. (2013). The Encyclopaedia of Sports Medicine (An IOC Medical Commission Publication) || Overreaching and Unexplained Underperformance Syndrome: Nutritional Interventions.10, 404–414.
4) Kreher J. B. (2016). Diagnosis and prevention of overtraining syndrome: an opinion on education strategies. Open access journal of sports medicine, 7, 115–122.