Causes of extreme fatigue in underperforming athletes - a synthesis of recent hypotheses and reviews

The underperformance syndrome (UPS), previously known as the overtraining syndrome (OTS), has been defined as a persistent decrement in athletic performance capacity despite 2 weeks of relative rest. Clinical research has suggested that cytokines play a key role in fatigue in disease and chronic fatigue syndrome. Furthermore, it has recently been demonstrated that exogenous administration of interleukin-6 (IL-6) increases the sensation of fatigue during exercise. In light of current cytokine and chronic fatigue syndrome research, this article reviews and updates the cytokine theories that attempt to explain the aetiology of the debilitating fatigue experienced in OTS/UPS. Initially, it was proposed that UPS may be caused by excessive cytokine release during and following exercise, causing a chronic inflammatory state and ‘cytokine sickness'. More recently, the hypothesis was extended and it was proposed that time-dependent sensitisation could provide a model through which the aetiology of UPS may be explained. According to this model, the principal abnormal factor in UPS is an intolerance/heightened sensitivity to IL-6 during exercise. South African Journal of Sports Medicine Vol. 18 (4) 2006: pp. 108-114