When North Melbourne's medical team rushed out to a flattened Nathan Thompson after he crashed through Sydney's Martin Mattner on Sunday, a collective gasp could be heard around Telstra Dome.

There were concerns that Thompson, 30, had reinjured the knee that required a reconstruction last year, ruining his 2007 season.

But after being helped up by `Roos trainers, Thompson went for a light canter around the forward line to the relief of the North faithful.

Thompson revealed it was actually his lack of conditioning and not his knee which was to blame for him being unable to get up.

"I've got no worries with the knee at all, Thompson said.

"It was purely my legs cramping up under me, just through fatigue.

"I physically couldn't straighten my legs so I had to get the doctors out to help me stand up and once I got onto my feet and started moving, the cramps subsided a little bit."

Despite being well-hydrated before the game, Thompson said he had suffered cramps for much of the second half.

The loss of his forward partner Aaron Edwards (broken leg) just before halftime had caused a large increase in his workload.

"My fitness just didn't hold up," Thompson said.

"Aaron went down and I wasn't able to have a break so I was pretty much playing centre half-forward, full-forward for the whole game.

"Whereas over the past few weeks I was able to get off and have a bit of a spell and a rest."

With the Swans clearly missing Barry Hall, Thompson looked like he was going to be the difference for the Roos in attack.

The North spearhead was on top of Sydney's Leo Barry in the second quarter but failed to convert all his chances in front of goal.

Thompson also said his team-mates needed to improve their delivery into attack, which had been clogged up by Sydney's renowned flood.

"What we've got to do better is create out own space and a bit more movement in there.

"Once the other team's getting numbers in there we're becoming a bit stagnant whereas we've got to help each out and move out of each other's space a bit better.

Thompson admitted the task would not be any easier against Adelaide on Sunday at AAMI Stadium, when he will be opposed to either Ben Rutten or Nathan Bock.

"I certainly won't be getting any cheap kicks from those guys," Thompson said.

Thompson has earmarked youngsters Lachlan Hansen and Josh Smith to replace the injured Edwards.

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