Volenti or Voluntary Assumption of Risk

Volenti is a legal doctrine which states that if someone willingly places themselves in a position where harm might result, knowing that some degree of harm might result, they are not entitled to sue the person causing the harm. Volenti only applies to the risk which a reasonable person would consider them as having assumed by their actions; thus a boxer consents to being hit, and to the injuries that might be expected from being hit, but does not consent to (for example) his opponent striking him with an iron bar. 

The Court of Appeal for Ontario in Kemp v. Nguyen, 2015 ONCA 114 writes:

[49]     Volenti, however, is not a claim for declaratory relief: it is a full defence to a finding of negligence. In Dube v. Labar, [1986] 1 S.C.R. 649, at p. 658, writing for the majority, Estey J. endorsed the following description of the volentidefence from the court's earlier decision in Stein v. Lehnert, [1963] S.C.R. 38:

[W]here a driver of a motor vehicle invokes the maxim volenti non fit injuria as a defence to an action for damages for injuries caused by his negligence to a passenger, the burden lies upon the defendant of proving that the plaintiff, expressly or by necessary implication, agreed to exempt the defendant from liability for any damage suffered by the plaintiff occasioned by that negligence...

[50]     In Allen M. Linden and Bruce Feldthusen, Canadian Tort Law, 9th ed. (Markham, Ontario: LexisNexis Canada, 2011), the authors state, at p. 520, "[V]olenti is a question of fact normally decided by the jury".