The AFL have resisted the temptation to introduce an interchange cap but have tweaked the dynamics of the bench, claiming players would have been put in a "dangerous" environment without the change.

From next season, one of the four players on the interchange bench will be designated a substitute.

He can be put into the game at any time and for any reason, but the man he replaces cannot return to the field.

The AFL had seriously considered placing a cap on the number of interchanges to 80 per game.

But they hope the substitute will have a similar effect in lowering rotations which, in turn, they claim will reduce congestion, the number of severe impact injuries and also increase the competitiveness of a side who loses a player to injury early.

"One of the key issues was we were advised that there's a risk of more severe collision-type injuries if we don't act to stop the interchange taking player speed to a new level," AFL football operations manager and chairman of the laws of the game committee Adrian Anderson said on Monday.

"This will prevent the clubs from being able to take the interchange and player speed to a new level which we're advised is ... a potentially dangerous level.

"The advice we have includes advice that severe collision injuries, including head and neck injuries, may get worse if we don't take precautionary action.

"We want to act before it becomes a real problem.

"On the subject of fairness, we want to see less games decided by injury."

From 2005 to this season, average interchanges have increased from 36 a game to 117, while average player speed has increased from 6.76 km/h to 7.48 km/h.

"We expect the effect of this change (in introducing a substitute) is that it's predicted to take the league average (of rotations) no further back than what it was in 2009," Anderson said.

"There is less chance of an injured player staying on the field than if we went with the option of a cap."

The AFL also tightened rules regarding the bump after several controversial incidents this season.

Like the head-over-the-ball rule, any player who now chooses to bump will be liable for any forceful contact to the head unless there was no other way to contest the ball or the contact was beyond the control of the bumping player.

That means the bruising shepherd delivered by West Coast forward Josh Kennedy on Colin Sylvia earlier this year, which left the Demons midfielder with a broken jaw, would be penalised.

The AFL also amended the advantage rule, with the player and not the umpire now to decide whether to continue the play after receiving a free kick.

Anderson said the AFL decided against changing the scoring system or length of the match.

But two other amendments were made.

Emergency field umpires will now be able to award obvious prohibited contact free kicks from the bench and there will be more strict interpretation of deliberate out of bounds when players walk the ball over the line.

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