Excessive exercise without sufficient recovery can lead to 'overtraining syndrome', with impairment of muscle function, the immune system, hormonal balance and mental well-being. The balance between training load and recovery is therefore just as important as the training itself.
Regular physical exercise is good for mental health: mood improves and symptoms of anxiety and depression decrease. But there is a downside. People who train structurally too hard without sufficient recovery can actually sustain damage to both body and mind. This phenomenon is called 'overtraining syndrome' (OTS).
OTS arises from a prolonged imbalance between heavy training and too little rest. Muscle function and athletic performance then deteriorate over an extended period. Free radicals (reactive oxygen and nitrogen species) and inflammatory processes in the body are likely the main causes of this damage. Physical signals include measurable declines in maximum oxygen uptake (VO2max) and blood lactate levels, as well as complaints such as persistent fatigue, dizziness and shortness of breath. However, there are no official diagnostic criteria for OTS yet, which makes recognising it difficult.
Hormone balance also becomes disrupted with excessive training. Levels of hormones such as cortisol (the stress hormone) and testosterone change in ways that provide insight into an athlete's stress load. Regular measurement of these hormones can help detect OTS early and adjust the training schedule accordingly. In addition, chronic overload also raises inflammatory markers and oxidative stress; adipose tissue may play a role in this through certain signalling molecules (adipokines), but that evidence is still limited.
The immune system also suffers under prolonged intensive training. White blood cells (neutrophils), antibodies (immunoglobulins) and natural killer cells function less effectively, and the risk of respiratory infections increases. This appears to be linked to the training load itself, regardless of whether full OTS is present. Furthermore, researchers describe OTS as a neuroendocrine disorder: the secretion of stress hormones such as adrenaline decreases, and the autonomic nervous system (which regulates the heart, blood pressure and digestion) can become disrupted. No single measurement can confirm OTS; a combination of markers is needed.
On the mental level, non-functional overreaching (a precursor to OTS) and OTS itself lead in elite athletes to disturbances in mood, increased stress, burnout symptoms and fatigue. OTS also appears to impair cognitive inhibition, meaning it becomes harder to ignore distractions or suppress impulsive behaviour. In strength sports the situation is more nuanced: short-term, controlled overload can serve as a training stimulus, but chronically high training volumes also lead to problems here. Fully developed OTS is poorly studied in strength sports.
The claims are based on multiple studies (PMIDs 15838583, 32179050, 40628368, 39595594, 38612899, 32602418, 41580212, 11050533). The evidence is largely associational; randomised experiments are ethically difficult to conduct. Evidence for some components (adipokines, strength sports, cognition) is limited and heterogeneous. There are no official diagnostic criteria for OTS yet.