Increased neuroendocrine response to a repeated bout of endurance exercise

This study was designed to compare a first bout of high-intensity endurance exercise with a second bout of similar exercise on the same day, and there by test the hypothesis that the endocrine response elicited by a second bou t is more pronounced compared with a single bout of exercise. Nine male, el ite endurance athletes participated in three trials of 24-h duration: 1) co mplete bed rest (REST), 2) one bout of exercise (ONE), and 3) two bouts of exercise separated by a 3-h rest period (TWO). Each exercise bout consisted of a 10-min warm-up at 50% of ((V) over dot)O-2max followed by 65 min at 7 5% of ((V) over dot)O-2max on a cycle ergometer. Exercise was performed bet ween II:OO am. and 12:15 a.m. (only in TWO) and 3:15 and 4:30 p.m. (both ON E and TWO). The subjects rested in bed at all hours except when exercising. Blood was sampled ii times at identical time-points until 7:30 a.m. the ne xt morning. We observed significantly increased levels of epinephrine, nore pinephrine, ACTH, cortisol, and growth hormone, and decreased levels of tes tosterone during and/or after the second bout of exercise compared with the first bout. No difference was observed for insulin, follicle stimulating h ormone, luteinizing hormone, thyroid stimulating hormone, free fraction of thyroxin or insulin-like growth factor I. Thus, this study demonstrates a m ore pronounced neuroendocrine response to a second bout of exercise on the same day compared with a first/single bout, involving both the sympatho-adr enal system and the hypothalamopituitary-adrenal axes.