The Explanation
One unfortunate side effect of the serum, Steve noted, was the ‘zoomies’, as Bucky called them.
Erskine said that might be a side effect, that the serum might cause “periods of extreme hyperactivity”. And uh… extreme hyperactivity is correct.
Sometimes Steve will suddenly need to RUN, need to CLIMB STUFF, need to JUMP, need to HOIST STUFF, need to PUNCH THE SUN. Nobody else understands the intense urgency to let out the excess energy, but they seem to realize it’s out of his control, so the howlies don’t yell at him anymore when he’s running around at 2 am.
Just like his inability to get drunk, nobody told him about these “periods of extreme hyperactivity” until after his first one. Steve was just going about his day, when suddenly heat spread through his body, his muscles twitching, his body screaming at him to DO SOMETHING, RIGHT NOW. Steve just stood there awkwardly, trying to figure out what was going on, but the longer he waited, the worse the pain got. He took off running and must have run 30 miles around base camp before finally feeling like he was not about to explode with energy.
The pain, Steve decided, was the worst part. At first you only feel a deep need to get your energy out, but if you wait too long to do something about it, the need turns into pain, roaring like a fire through your veins. Which is extremely inconvenient if you get the zoomies in a situation where you can’t zoom. Luckily Colonel Phillips lets him quietly slip out of meetings if he needs to.
Bucky’s been great about it, helping him escape situations and cleaning him up afterwards, when he’s covered in sweat and dirt and exhausted. Bucky is used to having to help him out, and the rest of the howling commandos are learning.