There is a viewpoint that changes to the rules and scheduling of modern cricket need the blessing of the Board of Control for Cricket in India (BCCI), such is the financial dominance that India has in this sport. Twenty-20 cricket did not become a prominent form of the game until India won the 2007 T20 World Cup in South Africa; suddenly, it was the potential saviour of the sport just as the Indian Premier League was born. Although an injury prevention argument has been made for the introduction of substitutes in Test cricket,1 the traditionalist view is that cricket should remain a unique sport where substitutes are not permitted for the specialist positions in the game (batting, bowling and wicketkeeping). If this traditionalist view is ever reversed, historians may look back on a hamstring injury to Zaheer Khan, India's fast bowler, on the opening day of the 2011 first Test against England as one of the major catalysts. The much anticipated series between England and India turned out to be a major disappointment for those who love a close contest. The issue of whether cricket will finally join every other team sport on the planet in allowing substitutes or interchanges (for injury or performance reasons) now finally has some traction.2 Not only did Zaheer's injury handicap the Indians in the first Test, the fact that the remainder of India's first-choice bowling attack needed to be ‘overbowled’ in this Test to make up for their missing strike bowler possibly affected their performance for the remainder of the summer. We already know that excessive bowling loads increase the risk for future injury.3 It is easy to presume from these data that fatigue is probably a risk factor for injury and that, therefore, substitution in cricket might be protective against injury by reducing fatigue in the other players. Because it would change the nature of the contest so much, it is not necessarily inevitable that cricket will embrace substitutes as other team sports have done. With the ongoing and seemingly irreversible expansion of the international cricket calendar, the debate is now a worthy one (with medicos and injury prevention experts deserving a seat at the table). The traditionalist argument is also a strong one but many other cricket traditions are changing (…in 2013 we'll have ‘back to back’ Ashes series, which will hopefully both be decided by the best players, rather than the ‘B’ choice bowling attacks if the ‘A’ players are primarily exhausted or injured by the end of the second instalment).

An interesting research question is to determine, for each sport, the point on the curve at which substitution or interchange laws are most protective with respect to injury. If it was simply a matter of fatigue being the only risk factor, then the safest sport should be American football. In the National Football League (NFL), players can be interchanged an unlimited amount of times with regular stops in play and an enormous bench available, so fatigue is minimised. However, the minimisation of fatigue means that this sport evolves bodies that are bigger, stronger and faster (but less aerobically fit) than the other varieties of football. Although comparisons in injury rates are not easy to make, it is widely assumed that of all of the professional sporting competitions in the world, the NFL gives rise to the most severe profile of collision injuries.4 Fatigue is therefore only one of many risk factors for injury, along with speed of the game and size of the average player.

The Australian Football League (AFL) has undergone radical changes in the use of the interchange bench in recent years. From 1998 until 2010, Australian football was played with 18 players on the field at each time with an unlimited interchange bench of 4 additional players. Teams averaged fewer than 20 interchanges per game in the early years of this system, but over 100 interchanges per game in the latter years, as team coaches realised that rotation of players reduced fatigue and improved performance. An unforeseen side effect was an increasing impact of an injury that occurred early in the game, as it meant one team would be restricted in rotations compared with their opponents. In 2011, the AFL changed the make-up of the four-man bench to become three interchange players and one substitute, for reasons including fairness of minimising the competitive disadvantage on a team unfortunate to have a player suffer an injury early in the game. This rule change also created a precedent of a hybrid bench divided between interchange players and substitutes. Other than cricket (which allows neither, except for fieldsmen), most sports are divided in those that permit non-reversible substitutes (eg, soccer, baseball, rugby union) and those that permit interchanges or reversible substitutes (eg, American football, basketball, rugby league, ice and field hockey, water polo). These groups can be subdivided further based on whether the rules limit the number of substitutes or interchanges and/or number of available players on the bench.

Rugby union is a sport that basically uses a substitute system (rather than interchange) but makes an exception for bleeding players, who can be interchanged to allow medical staff to stitch a wound and then return to the game. Although this would seem to be a sensible exception, the advantage to a team of being able to bypass the substitute system and achieve a player interchange led to abuse and the Bloodgate scandal.5 Authorities need to consider all implications of interchange and substitute rules and how players, coaches and medical staff will react to specific situations under the rules of the day. ‘Diving’ (including the removal of a player on a stretcher who miraculously comes back to life once the stretcher leaves the pitch) is considered a plague of football (soccer). Again authorities need to consider that a system which does not allow player interchange will evolve player behaviour that includes stalling to stop the game when temporarily injured.

Another ‘hot’ issue in sports medicine is the risk of allowing a concussed player to continue playing, with current consensus guidelines erring on the side of caution and recommending that concussed players do not return to play.6 Team physicians are more likely to comply with such recommendations if the decision to rule out a player for the remainder of the game does not affect a team's chance of winning, taking into account the number of remaining substitute/interchange players. Again, there is a new imperative for rule-makers to consider the optimum management of concussion when determining rules on substitutes/interchanges.

Basically, there are both benefits and risks of all substitute and interchange rules with respect to injuries and their causation. Allowing substitutes means that injured and/or fatigued players are not under team pressure to push through the pain of an injury and stay on the field. Allowing players to interchange freely means that an injured player can be temporarily examined in the isolation of the dressing room to determine fitness to return to the field. However, unlimited interchange (meaning that fatigued players can rest as often as required) will almost certainly increase the speed of the game, potentially increasing the risk of injury. Trying to determine the relative effects of fatigue and speed of the game on injury risk is a new area of sports injury research.7

All team sports now have a strong interest in getting the balance right on interchange and substitution rules. Sports medicine research (particularly injury epidemiology) has an important role to play in helping the administrators choose the most appropriate rules for each sport.