system
Given the following post, respond as if you're commenting on Reddit:
human
How does Tony Stark not get concussions all the time?

As we've seen in American Football, having hard head protection does little if anything to prevent the brain from impacting on the inside of the skull.  Tony Stark is getting hit in the head by massive hammers and Hulk fists, yet he doesn't seem to ever have any issues with this.
gpt
On an american footballer, the helmet sits freely on the head.  This means that if they get hit, the helmet and head either move laterally or rotate extremely quickly.  This has the effect of causing the brain inside to not keep up with the skull, causing internal bruising which is extremely dangerous.

The Iron Man suit from Mark II onward uses an articulated and powered helmet which is connected to the main body of the suit.  What this means is that when Tony gets hit on the chin, say, the head doesn't snap around the way it would if he had a free floating helmet.

Now if the Hulk were to hit him with a proper right hook, he'd still feel it.  But the rigid body in the Iron Man suit is going to translate a lot of that momentum into the chest, and limit the rate of head rotation to something safe.  The body will probably spin -- and Tony is going to go flying -- but flying is something he can recover from.