Changes in awakening cortisol response and midnight salivary cortisol are sensitive markers of strenuous training-induced fatigue.

The aim of the study was to examine the effects of strenuous training on the hypothalamic- pituitary-adrenal axis activity. Exercise tests and saliva collections for analysis of awakening cortisol response (ACR) and midnight cortisol were performed before and after a 7-day period of intensified training in a group of 15 soccer players. Intensified training resulted in a performance decrement as shown by the pre-post-training changes in maximal values of counter movement jump (CMJ) height (p=0.008). Cortisol assessment during the first 30 min after awakening showed significant increases both before and after the 7-day period and post-training ACR higher than pre-training ACR (p<0.001). Midnight cortisol also significantly increased after training (mean+/-SD, before: 3.0+/-0.7 nmol/l vs after: 5.9+/-3.3 nmol/l; p=0.017). The analysis of individual data showed an important inter-individual variability in the pre-post-training changes: several subjects increased post-awakening peak of cortisol, rate of cortisol increase from awakening to peak, and area under the curve (AUC) values, whereas other subjects showed no training-related increases. Significant correlations were observed between pre-post-training change in CMJ and in the following variables: awakening cortisol (r=0.74), post-awakening peak of cortisol (r=0.81), rate of cortisol increase (r=0.75), and AUC (r=0.79). Briefly, the lower the performance decrease, the higher the training-associated ACR increase. These data could indicate that a dysregulated adaptation to exercise occurred in athletes who experienced a higher performance decrease after training and lower (or absent) hormonal changes. Future studies are needed to elucidate the physiological determinants which underlie the exercise-elicited changes in ACR and in midnight cortisol levels and their value in predicting impaired adaptations to exercise.