Adults and mature children who voluntarily encounter a known risk are usually deemed to have consented to the injuries they receive as a result of that particular risk.
If you go to a no-leash dog park and you are injured by a dog, under circumstances other than a bite where the dog would not have injured you if it was leashed, then there is a very good argument that you assumed the risk. After all, you knew that leashes were optional at the park, but you went there anyway to take advantage of the same leash-optional law that resulted in your own injuries.
However, the assumption of risk doctrine will not be used against a responsible dog owner who goes to a dog park that is frequented by an irresponsible dog owner who fails to follow the rules of the dog park. The law states that every person can assume that others will abide by the law. Therefore, one does not assume the risk that another user of a dog park will violate the rules of the park. For that reason, courts will not permit this doctrine to be used as a shield for unreasonable or unlawful conduct.
Similarly, the assumption of the risk doctrine will not be used to permit a dog owner to evade responsibility when he brings a dangerous dog into a dog park. (See the next section.)