CONCERNS about the declining number of contested marks being taken this year will see AFL umpires crack down on defenders blocking or checking forwards off the ball in 2013.

The umpires will be asked to slightly adjust where they position themselves so they can pick up more infringements inside the 50 metre arcs.

They will also be tighter when adjudicating on infringements that happen when the ball is more than five metres away from the contest.

The average number of contested marks per game declined from 25.4 per game in 2011 to 23.2 in 2012, as players got smarter at stopping a forward's run without giving away a free kick.  

This statistic was a reversal of the upward trend in the average number of contested marks per game that had been happening since at least 2008.

Feedback from clubs and players indicated that increased congestion and the emphasis in recent years on stopping holding at stoppages had drawn umpires closer to ball-ups around the ground and made it more difficult for those officiating to detect defenders blocking or hampering forwards before the ball arrived.

AFL football operations manager Adrian Anderson said the move to have umpires throw the ball up around the ground will also help decision makers achieve a better balance between having eyes on the stoppage and eyes on what is happening at either end of the ground.

He said the rationale for the change was simple enough once the problem was identified.

"We want to see players taking big grabs and not having their run impeded," Anderson said.

"We love seeing [players such as Melbourne's] Jeremy Howe jump and be able to jump at the ball and compete. We really want to protect the contested mark as a great feature of the game."

Howe, the Melbourne high-flier and mark of the year winner, told AFL.com.au at the end of the season that opponents became much better as the year went on at impeding his run. He said he had to adjust his game late in the season to continue to fly for marks.

"[You've] just got to be on the move," Howe said. "[I'm] obviously not going to get that perfect run and jump at it any more [so we're] just working out ways around it."

Click here to see some of Jeremy Howe's 2012 screamers

Howe won eight nominations for mark of the round during 2012