An actor is subject to liability for a battery if he acts intending to cause a harmful or offensive contact to a person, or an apprehension that such a contact will imminently occur, and that person or another person suffers such a contact as a result.  Restatement (Second) of Torts §13

Case on point: Vosburg v. Putney

Boy kicks, or lightly touches, another boy’s leg just below the knee while they are at school. The kicked boy’s leg eventually had to be amputated. The kick was found to be the cause of the harm.

Also, note that consent is a defense to Battery.