An apparent record number of interchanges last Friday night has underlined the AFL's concerns about the increased use of the bench.

The AFL says there were an average 91 interchanges per team last season - in Friday night's round-one NAB Cup match, Collingwood used the bench 145 times and St Kilda had 137.

Increased interchanges have become a noticeable trend over the last few seasons as teams use rotations to keep players, especially midfielders, fresher during matches.

There are concerns that extra rotations might lead to more injuries and more stoppages.

Friday night was unusual because of the hot conditions at Etihad Stadium, but umpires director Jeff Gieschen said the interchange numbers were still noticeably big.

"I think we saw a record number of interchanges last Friday night in a shortened game," Gieschen said.

"The conditions were hot, though, and you can fully understand why the clubs were wanting to rotate on a heavy basis.

"They also have a couple of extra interchange players as well ... but that's an extraordinary amount."

Gieschen also said there was a big drop in field bounces during round one of the NAB Cup compared to the same matches last year - from 22 down to 13.

He has pointed to the league's crackdown this season on the holding the ball rule, where a player is tackled with no prior opportunity, but tries to lock the ball in rather than dispose of it.

"The early indications are that the stronger awareness about when players take the ball and haven't had a prior opportunity, trying to keep the ball alive by making an attempt, seems to have had an impact," Gieschen said.

"We're only one round in, but we'll keep monitoring that situation."

Gieschen also suspects there is less congestion around field bounces because the "no-go" zone behind the umpire at the centre bounce now applies around the ground.

"The awareness of that no-go zone area has led to players to stay out of it," he said.

The league is also cracking down on 50m penalties for encroaching on the mark and not giving the ball to an opponent properly.

There was an unusually-high number of 50s during Sunday's North Melbourne-Geelong game at Etihad Stadium.

"Most of the clubs have been really good in that area, they've stayed out of the protected area, they've marked the mark really well, they haven't given the ball back in a sloppy manner," Gieschen said.

"But old habits die hard, just occasionally we see a player slip in concentration.

"We want our umpires to keep penalising that so the players will soon work out what they can and can't do."

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