Fear is, if I can paraphrase Roosevelt here, something to be feared. Fear kicks in when our body thinks we are in danger, resulting in a switch to a heightened mode in which small motor functions are decreased and bigger motor functions are increased. The implications?
1) If you are facing down a bear, you will probably not be able to tie your own shoelaces (even if you could normally), not fit the key in the lock or fold an origami crane. Why? That is fairly obvious. It’s not really important at that point.
2) If a guy is pointing an AK-47 at you, you will probably be able to run a lot faster than you normally would have, as well as react faster and lift heavier objects. Why? Because these actions are necessary for your survival.
3) When you’re in danger, your sense of pain will mostly disappear, meaning that you won’t notice if you injure yourself (as was the case with a man who clenched his jaw so hard that he broke a few teeth without realising it). Why? Because pain is the body’s way of telling you to stop what you’re doing, as it could be doing you harm. So in a situation where your life is in immediate danger, possibly doing you harm takes second boat.
Interestingly, the normal fight or flight reaction has recently gotten a new addition, freeze. This happens when the threat is not immediate, therefore your body hopes to avoid it entirely.