As mentioned in other headcanon posts, Jay did some extensive testing for some experimental military drugs starting at age 16 in exchange for a certain sum of money. These drugs had three long-lasting effects on him.

First, they stunted his growth. He was supposed to grow a few inches taller, but stalled at 5′6.5′’ because of them. Second, they gave him ridiculous strength for his height and build. Quickly put, he can yeet a fridge at someone. (I think I had a more extensive headcanon on that somewhere else.)

The last trait, however, is probably the one that can be the most beneficial but also the most dangerous for him. Below a certain threshold, Jay does not register pain. At all. 

Now some people would be like ‘no that’s actually cool,’ but the truth of it is pain is a defense mechanism. It’s a sensation that tells you your body could be in danger. For example, pain can be a sign of some underlying illness. It can be a symptom of something else. Without that pain response, it makes it hard for people to diagnose health problems with Jay. 

When it comes to fighting and injuries, Jay has to closely monitor himself to know what kind of hits he receives and where, since he won’t feel the pain of constant bruising, along with most cuts and stabs. Gunshots barely make it past the threshold of pain he would be able to feel, but on the moment, the pain response is swallowed by the other hormones that lessen the sensations even more. Which means he could get gravely injured and not notice, then keel over from the consequences of injuries he didn’t feel. 

Pain from bone fractures and cracks will instantly register, and so will grave injuries (gunshots, heavy damage to limbs). A stab, however? Hell, the knife could still be in there and he’d notice discomfort after a while. Which can lead to all sorts of infections. So it’s really important he keeps an eye out for whatever damage he’s suffered. 

On the regular side of things, muscle soreness manifests more as stiffness when he feels it. It’s mild discomfort and reduced mobility/flexibility, and he’ll have to stretch to lighten it up a little.