Perhaps not, but there are rules when arresting a citizen. I’m not sure if knees to the head are included.
The police are allowed to use whatever reasonable force is necessary to affect an arrest. Reasonable, as defined by the law, basically means whatever it takes as long as the force used was proportionate to the amount of resistance met.
So in this case, an officer applying a knee to the head could be considered reasonable if the arrested provided enough resistance that it was required.