A thing about The Flash...

[The AstroForce]

The law's physics Action-Reaction applies to him?
He breaks walls easily if he spins very fast towards them...
But if he breaks a wall with a punch in super-speed it will hurt him?

[Fanatix]

I'm hardly a flash expert, in fact I only started reading recently. But my guess would be that he can vibrate the air around him creating a cusion between him and the wall, so that its actually sped up air breaking the wall and not his bone hitting it straight on. So no, it wouldn't hurt him.