system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 why your legs/arms feel like jelly when you are nervous or have adrenaline. Isn't that counterintuitive for a "fight or flight" response?

When you have a fight/flight response, it's supposed to be to get stronger and have more endurance for a period of time so that you can have peak physical performance. But how come when you get adrenaline, like before a sports game or a running thing, why does it make your legs feel like jelly, which feels worse/slower when running?
gpt
Adrenaline basically forces your body into maximum energy mode. 

Your liver starts producing more glucose which fuels muscles. Blood flow is increased to your muscles to deliver that energy. You start breathing faster so your muscles can use that energy. Lots of changes happen that have the net result of giving your muscles a major boost of energy so you can respond to danger (real or not). 

That comes at a cost. After it subsides, your body has basically used up it's available energy. So your muscles feel weak for a short period of time while your energy rebuilds.