It is well known that athletes' performance improves when under considerable stress. Yet there remain unanswered questions as to why this occurs.
The answer to this question lies in one's response to stress-producing stimuli - commonly referred to as the "fight or flight" response.
This response is initiated by the amygdala section of the brain, which sends signals to both hypothalamus and adrenal glands. Subsequently, these glands secrete adrenaline - a hormone which positions your body to either fight or flee danger.
Adrenaline primarily acts to increase your heart rate and respiration, ensuring more oxygen reaches working muscles. Furthermore, adrenaline causes sugar breakdown in the liver which provides quick energy for short-term use.