METHODS

The definition of an injury was “any physical or medical condition that prevents a player from participating in a regular season (home and away) match.” The rationale for this definition was that club “senior lists” (player rosters) were kept relatively constant throughout the season and the central AFL administration kept a “player movement” record for every “senior list” player during every round of the season (table 1⇓). Every week (round) a player was either playing a match, not playing because of injury, or not playing for other reasons. Other reasons included suspension, personal reasons, being taken as a travelling emergency player and not used, and not being selected in the AFL team when the player's affiliated lower grade team did not play during that week. The AFL standard player contract required that clubs paid a minimum payment to any player missing a match because of injury, and that these payments were registered with the AFL to ensure that all teams stayed under a “salary cap”.

The team medical recorders were required to provide details of all injuries, specifically diagnoses and times of onset, rather than determine which injuries met the survey definition. The team medical recorder was one of the team doctors in 12 teams and one of the team physiotherapists in the other four teams.

If a player was missing through injury as a result of more than one diagnosis, only the primary diagnosis (as assessed by the recorder) was considered an injury for survey purposes. An injury was considered to have recovered when the player returned to match playing. An injury was determined a “recurrence” if the player had suffered an injury of the same type to the same body part, on the same side earlier in the same season.

The players included in the injury survey were all AFL senior listed players, which were specified at the start of the season. Team senior lists varied in size from 38 to 44 players. AFL regulations dictated that no players could be removed entirely from a team's senior list during the regular season. The only additions to the senior list were from a team's “rookie list” of up to six junior players. Rookie list players could only be added to a team's senior list in the event of a long term injury to one of the senior list players. If a rookie was added to the senior list during the season (which was an uncommon event), he became part of the survey population from the round after being added. If a player retired during the middle of the season, he was considered missing for reasons other than injury (if his retirement was for personal reasons) or missing because of injury (if his retirement was forced by injury). When a case of retirement was questionable, the cause was determined by whether the player received injury payments after his retirement.

The injuries were all coded and categorised according to the Orchard sports injury classification system,²⁰ which was created specifically for this survey.

RESULTS

During seasons 1997–2000, every AFL team participated in the injury survey, and the weekly status of every senior listed player (whether playing, injured, or not playing for non-injury reasons) was obtained from AFL records.

Table 2⇓ lists the scope of the survey and the average number of players playing and missing through injury each round of each season. The number playing AFL level football per club per week increased from 21 to 22 in 1998, as the AFL changed the number of players per team on the interchange bench from three to four (the number of players per team on the field stayed constant at 18).

Table 3⇓ documents injury incidence. The most commonly injured body region is the thigh, and the most common type of injury is the muscle strain. Hamstring injuries are a common member of both of these categories, being muscle stress injuries of the thigh and totalling about 6 injuries per club per season. This represents 15% of all injuries. Hamstring injuries have the highest recurrence rate of all injuries, 34% of the incidence of new hamstring strains. Overall, the rate of recurrent injuries is 17% of the rate of new injuries.

Table 4⇓ compares the match injury incidence for AFL and other (lower grade) matches. The injury incidence rates are generally very similar. It is possible that there is a greater willingness to play with minor injury if the player is playing at AFL level (compared with lower grade level). Given that injuries that do not cause games to be missed are not considered in this survey, this is a bias that would affect any analytical comparison between the match injury rates for the different levels of play. Table 4⇓ shows that the major injuries that occurred outside matches were medical illnesses and muscle strains. There were an average of 5.5 preseason injuries per club per season.

Table 5⇓ details injury prevalence. In an average season, a club of 40 players suffers 142 missed matches through injury over the 22 rounds, which equates to an injury prevalence of 16% (that is, an average of 16% of senior listed players being unavailable to play through injury in any given round).

Average injury severity is not reported as a specific table. It can easily be calculated for each category by dividing injury prevalence (table 5⇑, units of missed games per club per season) by injury incidence (table 3⇑, units of injuries per club per season), yielding a unit of missed games per injury.

DISCUSSION

The injury definition of this survey is somewhat controversial, because by taking into account only injuries that cause matches to be missed, it is argued that not all important injuries are reported.^{16, 21} The counter argument in favour of this injury definition, however, is very strong. Because it is consistent and is—in the context of this survey—assessed reliably and independently, the injury rates reported by the survey are consistent, reliable, and meaningful.

If this survey had used one of the more comprehensive definitions that relied on factors such as missed training sessions or presentation to team medical staff, then the rates of injury incidence would be biased by team specific factors. For example, if team doctor A attends every training session of his or her team, whereas team doctor B only attends one training session per week, then there would be a bias towards more injuries at team A. Therefore, comparison of injury incidence between teams would not be meaningful, as it would be impossible to determine whether apparent differences were due to bias. In addition, comparison from season to season would also be meaningless, because bias (caused by changes in observation, participation, recall, and/or enthusiasm) may be responsible for any observed differences. This would make it very difficult for such injury surveillance to be used as the backbone of a comprehensive injury prevention paradigm, such as that suggested by van Mechelen et al.¹⁴

Other studies that have been based on the AFL injury surveillance system have already benefited from this lack of bias. For example, it has been observed that the incidence of non-contact ACL injuries is higher at more northern (warmer climate) venues, and is higher in early (warmer) months of the season.^{22, 23} The warm season (northern) bias is also apparent for some other lower limb non-contact injuries.²⁴ These differences are statistically significant at very low levels and therefore highly unlikely to be due to chance. Because of the methods used in this survey to determine injuries, these differences are far less likely to be due to an observational bias—for example, that team doctors in northern cities are more likely to attend training and therefore more likely to report injuries—than if a subjective definition is used.

The fall in ACL injury incidence documented in this paper (table 3⇑) and analysed elsewhere²⁵ is also unlikely to be due to observational bias, and perhaps may be related to changes in ground preparation to attempt to reduce shoe-surface traction.²⁵

Future analysis of risk factors for AFL injuries will involve multivariate analysis of both intrinsic and extrinsic variables.^{22, 26} Unfortunately, it is unlikely that randomised control trials will be permitted within the AFL as a professional competition, so the reversibility of risk factors will need to be tested in lower level competitions.

There are potential ways in which the AFL injury definition could be expanded in the future while remaining independent of the recorders of team medical data. If a central register was kept for all injuries that required the player to be assisted from the field using a stretcher or motorised vehicle, then an instance of this occurrence could be included within the injury definition without compromising its objectivity. Similarly, if the AFL required all occurrences of players being injected with local anaesthetic during a game to be registered as part of doping procedures, then this could also be included within the injury definition.

Currently AFL injury surveillance does not report rates for preseason or finals matches. This is because clubs are not required to complete a “player movement” sheet during these weeks, as match payments are not made. In the preseason period it would be more difficult to determine whether a player who does not play is missing through injury or other reasons, as many players are “rested” for preseason games. During the finals it would be more difficult to determine status for those players not selected at AFL level than during the regular season. In the regular season, fit players who are not selected for their AFL team are eligible to play a match each week with an affiliated lower grade team, but their affiliated team may not be participating during the finals. In the off season (when no matches are scheduled but players sometimes train), injury incidence would be very difficult to calculate as exposure would be very difficult to assess, and the previously mentioned dilemmas in defining an injury would also apply.