Define "unable". Somebody dislocates their shoulder and it's treated and healing. Their doctor tells them (I'm making this up: I have no idea if this is really an issue in that case, but suppose for the sake of this post that it is) "be careful not to lift your arm up too often over the next few weeks because if you do, you run the risk of another dislocation". This person is selected for WBI and says he can't lift his arm up. But it so happens that the checkpoint camera showed him lifting it up earlier to scratch his head. Did he refuse to lift his arm for the purpose of impeding screening or because he was concerned about his doctor's advice? How is a court to determine that?