Exhaustive endurance exercise such as cycling has been shown to decrease testosterone levels for up to 72 hours after exercise [
1]. Cortisol (a glucocorticoid hormone) increases during endurance exercise, while testosterone (an anabolic-androgenic steroid hormone) decreases and this may attenuate muscle-building after an exhaustive endurance session[
1].
Other research has suggested a link between endurance-trained athletes and low resting testosterone levels [
2].