Gordon Freeman

The closest analogue to the Combine soldier's responses is the Earth martial art of krav maga. It assumes all combat is final combat, that no quarter will be given, and that the objective is attacker neutralization. It is not, however, a set of coded responses that has seen much practice. The Combine generally rely heavily on their guns and armor, as that's been most of what's necessary to date.

And having to repeatedly defend one's unexpectedly vulnerable head puts a real crimp in the drive to move from defense to offense.