PHYSIOLOGICAL RESEARCH • ISSN 0862-8408 (print) • ISSN 1802-9973 (online)  2016 Institute of Physiology of the Czech Academy of Sciences, Prague, Czech Republic Fax +420 241 062 164, e-mail: physres@biomed.cas.cz, www.biomed.cas.cz/physiolres Physiol. Res. 65: 109-120, 2016 The Importance of the Training-Induced Decrease in Basal Cortisol Concentration in the Improvement in Muscular Performance in Humans M. GRANDYS 1 , J. MAJERCZAK 1 , J. KULPA 2 , K. DUDA 3 , U. RYCHLIK 2 , J. A. ZOLADZ 1 1 Department of Muscle Physiology, Chair of Physiology and Biochemistry, Faculty of Rehabilitation, University School of Physical Education, Krakow, Poland, 2 Department of Clinical Biochemistry, Cancer Institute, Krakow, Poland, 3 Institute of Health Care, State Higher Vocational School in Tarnow, Tarnow, Poland Received March 16, 2015 Accepted August 28, 2015 On-line November 24, 2015 Summary Acute exercise-induced changes in cortisol concentration (C) and training related adaptation within hypothalamic-pituitary-adrenal (HPA) axis has been widely examined, but their influence on muscle strength performance is at best uncertain. Twenty four young healthy men were randomly assigned to an endurance training group (ET, n=12) or to a non-exercising controls (CON, n=12). ET performed supervised endurance training on cycle ergometer for 20 weeks. Endurance training program improved exercise capacity (14 % increase in power output generated at peak oxygen uptake – VO2peak), muscle strength performance (increase in MVC – maximal voluntary contraction – by 9 % and in TTF 50 % MVC – time to fatigue at 50 % MVC – by 21 %) and led to a decrease in basal serum C concentration (P=0.006) and an increase in basal testosterone to cortisol (T/C) and free testosterone to cortisol (fT/C) ratios (P=0.01 and P=0.02, respectively). It was found that the decrease in C concentration (ΔC) was positively correlated to the increase in local muscular endurance (ΔTTF 50 % MVC). No significant hormonal changes were seen in CON group. It is concluded that greater decrease in cortisol concentration after the endurance training is accompanied by poorer improvement in skeletal muscle performance in previously untrained subjects. Key words Glucocorticoids • Testosterone • Strength • Local muscular endurance Corresponding author J. A. Zoladz, Department of Muscle Physiology, Chair of Physiology and Biochemistry, Faculty of Rehabilitation, University School of Physical Education, Al. Jana Pawla II 78, 31-571 Krakow, Poland. Phone/Fax: 48 12 683 13 16. E-mail: jerzy.zoladz@awf.krakow.pl Introduction Activation of the hypothalamic-pituitary-adrenal (HPA) axis is indispensable for the appropriate regulation of the energetic, metabolic and immunological processes in response to exercise stress (Galbo 1986). Acute and training-induced changes in cortisol (C) concentration and its physiological role has been widely studied (O'Leary and Hackney 2014), and it is known that exercise training leads to exposition to high glucocorticoids concentration, both in animal (de Araujo et al. 2015) and human studies (Hill et al. 2008). It is of great importance, especially in endurance type of training, because prolonged hypercortisolemia may have also adverse effects on different cell functions in humans, e.g. in leading to muscle proteolysis and protein degradation (Schakman et al. 2013), impairment of insulin secretion of beta-cells (van Raalte et al. 2010) and suppression of the nitric oxide system in the endothelium (Liu et al. 2009). These can occur as a consequence of elevated circulating glucocorticoids, but also due to enhanced activity of the glucocorticoid receptor or the https://doi.org/10.33549/physiolres.933039