Cortisol responses to exercise stress and training adaptation in human sport science

Abstract

Exercise induces a coordinated neuroendocrine response that is essential for maintaining homeostasis and supporting adaptation in humans. This mini-review examines the role of the hypothalamic-pituitary-adrenal (HPA) axis and cortisol in the response to exercise stress, with particular emphasis on the non-linear relationship between training load, endocrine responses, recovery, and performance. Cortisol promotes metabolic substrate availability during exercise and contributes to short-term physiological adaptation, but excessive or insufficiently recovered training may alter HPA-axis responsiveness and contribute to maladaptive outcomes. Evidence indicates that acute cortisol responses are strongly dependent on exercise intensity and duration, with responses becoming more pronounced above approximately 60% of maximal oxygen uptake (VO₂max). However, greater endocrine stress does not necessarily translate into greater adaptation, as moderate training doses may provide a more favorable balance between physiological challenge and recovery. Repeated training also modifies HPA-axis behavior, with trained individuals showing altered cortisol responses to standardized exercise and other stressors. Particular attention is given to the distinction between functional overreaching, non-functional overreaching, and overtraining syndrome. Current evidence suggests that resting cortisol concentrations alone have limited diagnostic value because of substantial biological and contextual variability. Dynamic responses to standardized exercise or endocrine stimulation may provide more informative indicators of altered HPA-axis function, although these responses remain heterogeneous. Overall, cortisol should be interpreted as a component of a broader physiological framework rather than as an isolated biomarker of training status or overtraining.