There need be no injuries, even though that is the way it is worded; the complainant must only have suffered pain which is legally equated with injury. If you punch someone in the arm, even if it leaves no marks or bruises, but the person says it hurts (and a jury believes that beyond a reasonable doubt), you can be convicted of assault.
Hire a good lawyer.
I agree with Ms. Henley. Though I might add if there are no visible injuries, sometimes a prosecutor may not treat the case as severely as with visible injuries. A prosecutor would still have the discretion to dismiss, even if there was a complaint of pain.