Why do your legs go wobbly when you're scared?

On twitter, @pejw7 passed on this question from their son, who asked it after getting scared by a sudden ambulance siren. Sam Virtue is here to tackle it...

Sam - So this is quite a nice question to follow on from the one Olivia just answered, because what we're talking about here is the sensible physiological responses that we have to something that frightens us. And so you may have heard of the fight-or-flight response. Well actually there's a third component to that, which seems to have got lost in general usage, which is the fight, flight or freeze response. And so we can think about this, when you're presented with a threat your body can respond in different ways. So one of the ways it can do, and this will happen subconsciously, you're probably not going to be making this decision, is you could decide whilst this is a threat, I reckon I can defeat it and your body pumps out hormones to make you better able to fight and you will attack it.

Equally you may look at the threat and think whilst this is a big scary threat, I can probably run away from it and you will run away. But the third form is that you may freeze, because there is a chance if you freeze and it's better than moving, that the predator or the threat may not see you and you might get away from it. And interestingly the wobbly legs is probably a manifestation of this freezing response, because you don't actually want to give yourself away, so your body can just freeze entirely and you don't even then have conscious control to move. And it's interesting that size is a very important part of how you perceive threat, and children are small so they're much more likely to have the freezing response to a threat which would manifest in a slightly less extreme form than wobbly legs. But the truth is there's nothing actually wrong with your legs. They're perfectly able to work, it's your brain telling them how to behave.

Adam - Why is freezing useful? What's going on there, that would be beneficial?

Sam - So if we think about something really small like a mouse, and it's being threatened by something like a bird of prey, it has absolutely no chance of fighting it because it's about 50 times smaller and it's essentially going to lose against a load of talons and beaks. It can't run away because the bird can dive over 100 miles an hour and catch it. But if it stays really still when it hears the bird there is a chance the bird might not see it. So that's how it can evolve