I'm sure the feeling of fear, as long as you can take advantage of it and not be rendered useless by it, can make you extend yourself beyond what you would regard as your capacity. If you're afraid, the blood seems to flow freely through the veins, and you really do feel a sense of stimulation.
Sir Edmund Percival Hillary KG, ONZ, KBE (20 July 1919 – 11 January 2008) was a New Zealand mountaineer, explorer and philanthropist. On 29 May 1953, Hillary and Nepalese Sherpa mountaineer Tenzing Norgay became the first climbers to reach the summit of Mount Everest. They were part of the ninth British expedition to Everest, led by John Hunt. Hillary was named by Time as one of the 100 most influential people of the 20th century. Wikipedia