As a general rule, persons have a duty to use due care to avoid injury to others, and may be held liable if their careless conduct injures another person. (See Civ. Code, 1714; Knight v. Jewett (1992) 3 Cal.4th 296, 315.) But "assumption of risk" can be a complete defense to a claim of negligence. Assumption of risk may be express or implied. Express assumption of risk concerns "instances in which, as the result of an express agreement, the defendant owes no duty to protect the plaintiff from an injury-causing risk." (Knight, at p. 308, fn. 4.) It is analogous to implied primary assumption of risk. (Ibid.) As we explain in greater detail in the next part, implied primary assumption of risk is founded not on an express agreement, but on the nature of the activity and the relationship of the parties to that activity. Accordingly, in Moser v. Ratinoff (2003) 105 Cal.App.4th 1211 at pages 1217-1218, the court found that the plaintiff's written release of " 'event holders, sponsors and organizers' " was inapplicable to the coparticipant in the activity that caused the plaintiff's injuries because the coparticipant was not a party to the agreement. Nevertheless, the court found that the implied primary assumption of risk doctrine barred the plaintiff's claims against the coparticipant who was not a party to the written agreement. (See also Levinson v. Owens (2009) 176 Cal.App.4th 1534 applying primary assumption of risk to a party guest injured when she fell off a horse owned by the host.) In Record v. Reason (1999) 73 Cal.App.4th 472, the court chronicled the activities to which courts had to that date applied primary assumption of risk (water skiing, sport fishing, white water rafting) and those to which courts had found the doctrine inapplicable (recreational dancing) and articulated the following test for application of the doctrine: "An activity falls within the meaning of 'sport' if the activity is done for enjoyment or thrill, requires physical exertion as well as elements of skill, and involves a challenge containing a potential risk of injury." (Id. at p. 482 riding an inner tube towed by a motorboat is an activity subject to primary assumption of risk.) In Shannon v. Rhodes (2001) 92 Cal.App.4th 792, 797 112 Cal. Rptr. 2d 217, the court agreed with the basic criteria set forth in Record but added that its "review of Knight and subsequent cases leads us to conclude, 'sport' as intended by the Knight court necessarily entails some pitting of physical prowess (be it strength based i.e., weight lifting, or skill based, i.e., golf) against another competitor or against some venue." The Shannon court concluded that being a passenger in a boat is "too benign" to be subject to primary assumption of risk. (Shannon, at p. 798.) The court in Shannon reasoned that the Ford court "focused on the physical skill and risk involved in the waterskiing itself to conclude that the activity of waterskiing was a sport, and the boat driver a co participant in that sport. ... The same certainly cannot be said of a mere passenger in a boat, particularly when the boat is simply a pleasurable means of transportation. To conclude otherwise would mean that because a car can be used in a race, that riding in a car is participation in a sport. We perceive the categorization as a sport to turn not just on the thing used (in this case a boat) but also on the manner of use." (Id. at p. 798; see also Bush v. Parents Without Partners (1993) 17 Cal.App.4th 322, 328 reversing summary judgment granted against plaintiff who slipped and fell while dancing and in favor of defendant who sponsored the event and public agency that owned the dance hall, reasoning that recreational dancing does not fall within the ambit of Knight because it is not a potentially dangerous sport or activity.) In Moser v. Ratinoff (2003) 105 Cal.App.4th 1211, the primary assumption of risk doctrine was applied to an organized, noncompetitive, bicycle ride in which one rider collided with another rider. Applying Record, the court reasoned that, although bicycle riding, like driving an automobile, can be a means of transportation, "organized, long-distance bicycle rides on public highways with large numbers of riders involve physical exertion and athletic risks not generally associated with automobile driving or individual bicycle riding on public streets or on bicycle lanes or paths. Bicycle rides of the nature engaged in by the parties here are activities done for enjoyment and a physical challenge. ... In view of these considerations, the organized, long-distance, group bicycle ride qualifies as a 'sport' for purposes of the application of the primary assumption of risk doctrine." (Id. at p. 1221.) In Truong v. Nguyen (2007) 156 Cal.App.4th 865, two personal watercrafts (strangers to one another) collided, resulting in injury to one of the drivers and the death of that driver's passenger. The court concluded that "the primary assumption of risk doctrine applies to the activity of riding personal watercraft, regardless of whether the rider is operating the vessel." (Id. at p. 887.) "Riding as a passenger on a personal watercraft, meets the test from Record, because it 'is done for enjoyment or thrill, requires physical exertion as well as elements of skill, and involves a challenge containing a potential risk of injury.' " (Id. at p. 888.) The Truong court distinguished Shannon, observing, "Unlike the benign activity of riding in a boat, riding a personal watercraft requires physical exertion and balance by the passenger to hold on to the operator or grips or handles on the vessel to avoid being thrown off or rolling off the craft. " (Id. at p. 889.) It rejected the contention that primary assumption of risk should be applied only to "extreme" operation of personal watercraft and not to "casual" or "ordinary" operation. (Id. at pp. 890-891.) Applying the doctrine to the sport of motorcycle "off-roading," the court in Distefano v. Forester (2001) 85 Cal.App.4th 1249, held that a participant in that activity may not sue a coparticipant for negligence because the activity involved an inherent risk that participants may be involved in collisions causing death or serious injury. And in Rosencrans v. Dover Images, Ltd. (2011) 192 Cal.App.4th 1072, 1083, the court held that, although falling down and being struck by other riders is an inherent risk of motocross, the owner of a motocross track (as opposed to a coparticipant) has a duty to minimize that risk by providing an adequate system to warn other riders of a fallen rider. Some courts have expanded the application of primary assumption of risk beyond "sports" to activities that might be accurately described as "recreational."