THE AFL expects interchange numbers will remain high this year, despite its radical decision to reduce the numbers on the bench to three.

But the AFL will not measure the success of its new rule - in which a substitute has replaced a fourth interchange player - by the numbers of rotations, but by four factors, including the speed of the game, congestion and injury rates.

The AFL's head of football operations Adrian Anderson yesterday said that there was no specific number that the league expected for interchanges, which have increased dramatically over the past few seasons, to a peak of about 160 last year. That increase prompted a change in the rules in a bid to slow the pace of the game and prevent injuries.

''I don't think there's any magic number and I suspect that interchange numbers themselves may stay high,'' said Anderson, the architect of the decision to reduce the bench from four to three plus a sub.

''But what's relevant from our point of view is that the rest that they're getting, so you can't physically get as much rest for the players and keep them going at the same pace with three rotating instead of four.

''I'm expecting that some teams will still have high interchange numbers. But what we would be measuring is not the numbers of the interchange but the effect of that overall on the speed of the players and the congestion of the game and also the injury rates … and the fairness factor.''

Clubs that have successfully used a high interchange system - notably premier Collingwood and the Western Bulldogs - were against the rule change, on the grounds that high interchanges led to fewer injuries (because of less fatigue).

The AFL, however, was concerned that the increased speed was making collisions more dangerous and also enabling more congestion. Anderson said there was a trend towards more congestion in 2010, leading to a less attractive spectacle.

Anderson outlined the four factors that it would measure the success of the interchange rule. They were speed of the game - and the AFL was hoping it would slow down - congestion, injury rates and the question of whether it was a ''fairer'' system in which clubs were not as disadvantaged if they lost a player mid-match.

''We'll measure congestion … We will look [at] injury rates, that have concerned us and we'll measure that. And fairness, which the RMIT study showed that teams were getting more and more disadvantaged when they got injuries because they couldn't rotate. We will measure that factor as well.

''We will look at all the things that measured in introducing a restriction on the interchange and then compare those at the end of the year as to where each of those factors has got to …''

Anderson said the AFL could not rule out a further change to the interchange rules if it was not satisfied its objectives had been met. ''I'm not going to rule anything in our out at this stage.

''That's something we'll need to get the data discussed with the laws committee, see whether it's heading in the direction we expected.''

Anderson anticipated hostility to the new rule. ''I am certain about that there will be the usual sort of cries that it's the end of the game .