I've noticed it's not possible to be crouching, and then quickly sprint and return to crouch position - just by hitting the sprint key..
It's not game breaking as you can just Stand up -> Sprint -> and then Crouch again, but it's unnecessary and kinda mechanic to do all of that.
I really miss being able to crouch, and then if hell breaks loose - hit the sprint bind - sprint to safety or cover, and return to crouch position when releasing the sprint key.