I don't see the Cop/Doctor analogy being Apples/Oranges. Just as a Cop is trained to Assess the Speed of a Vehicle a Doctor is trained to Assess the amount of damage to the Human body. He can certainly tell if enough force was used to break an arm, or if an excessive amount of force was used to SHATTER an arm bone, without being there, eh?
"Excessive Force" is a legal concept and has a specific meaning. If someone gets mouthy with me and I punch them in the stomach for it, even though I cause no injury at all other than pain, it is still excessive force because their level of resistance did not match with the level of control I chose to use. Yes, there are some cases when that is the appropriate use of force to control a violent suspect trying to hurt you.
As to a broken arm, the courts have ruled that if force is justified and applied in "good faith" and an injury occurs than the officer is not at fault. For example (true court case out of Dearborn, MI), a woman is unrulely in a public place, and the police attempt to calm her down and remove her. The woman refuses and keeps escalated her verbal abuse and is not complying with directives to leave. The officer puts his hands on her arm to escort her out of the movie theater. The woman attempts to resist and tries to pull away. To maintain control, the officer goes for a straight armbar takedown (which is the technique taught and used for that resistance in his department). As the officer is taking the woman down, the woman further tries to resist and takes a step forward in an attempt to resist the takedown. Due to the momentum of the officer, the position of her straightening out her arm as she pulled away and then bracing herself during the takedown, her arm is broke.
Now, you can look at it two ways:
1) The officer broke her arm for being disorderly (which everyone can agree is excessive).
OR (as the courts ruled)
2) The officer WAS using the appropriate force in that situation and the suspect further resisting caused an unforseen circumstance to change that outcome and if had not been further resisting no injury would have occured.
You see, in that example, the doctor can
only tell you that the arm is broken, and how many pounds of pressure it would take to cause that type of injury. The doctor
can't tell you if 1) the officer was legally using force in the first place or 2) if the injury was intentional or accidental or 3) if the officer was using the appropriate amount of force for the situation.
That is why it is apples and oranges.