With the growing long-term fear of concussion over the wellbeing of players, concussion substitutes have been introduced on a trial basis in the Premier League.Aston Villa’s home encounter with Arsenal was the first match to take place after they became part of the laws, but in what competitions can they be used and in which circumstances?What are concussion substitutions? Concussion substitutes have been introduced in a bid to stop clubs taking undue risks with players who have head injuries. There are concerns that footballers have played when concussed, and there are fears for their long-term health as a result.With concussion substitutes available, clubs are given another two potential changes to use over the course of a match if a player is suffering from suspected concussion.It is up to the discretion of club doctors to decide whether a player falls under this category. What is concussion? According to the NHS: “Concussion is a temporary injury to the brain caused by a bump, blow or jolt to the head.“It usually only lasts up to a few days or weeks, although it sometimes needs emergency treatment and some people can have longer-lasting problems.“It’s important to avoid head injuries as repeated concussions or blows to the head have been linked to serious problems, including a brain condition called chronic traumatic encephalopathy.”What happens when a concussion substitute is made? The process for a concussion substitute to be made is slightly different to the normal substitution procedure. Instead of the usual white form handed to the fourth official, clubs will use a green form, indicating the difference between the types of change.These players are permanently removed from the game.Additionally, it should be noted that if a concussion substitute is made, the opposition are given one additional change.For example, if Arsenal had used one concussion substitute against Aston Villa, the Birmingham side would have been given an additional change to make on top of their usual three.Do clubs have to make a concussion substitute when a player has a head knock? Although clubs are compelled to take players suffering from a head injury off, they do not necessarily have to use one of their concussion substitutes to do this.Why would they not? Simply because it would offer their opponents a chance to use that additional substitute.Let’s use the Aston Villa v Arsenal example again. If a Villa player suffered a suspected concussion after 85 minutes, they would be granted the opportunity to change them. Of course, if they had used their full complement of substitutes, they would have to dip into their concussion change pool, but if they had only used two of their regular changes, the player could be brought off in that manner.By using a regular substitution, Villa would be depriving their opponents of having the luxury of making an additional change.Who was the first to use a concussion substitute? No-one has used a concussion substitution in the Premier League as yet.How long does the concussion substitute trial last? The Premier League will trial these changes until at least the end of the 2020/21 season. It can be extended until the end of the 2021/22 campaign, however.Why are concussion substitutes only in the Premier League? It is important to stress that this is only a trial and not yet a permanent law of the game. The Premier League is the only major competition taking part in this trial, although less celebrated competitions are testing it.One of the reasons that Premier League clubs are doubtless so keen to implement it is because they are one of the few major competitions in which there are only three substitutes permitted this season.