NORTH Melbourne youngster Jack Ziebell is doing the extra miles at training to ensure he can run out games, despite being the fittest he has ever been.

For the third week in a row, Ziebell was subbed out of the game with Ben Speight replacing him in the final quarter.

It has been a frustrating season for the 20-year-old as he tries to build upon his aerobic base after injury setbacks in his first two years of AFL football.

Ziebell broke his leg in his first season, and made a solid return last year only to suffer the same injury in round 14 against Geelong.

While his aerobic capacity has improved immensely in his third season, Ziebell knows there is room for improvement, but building match fitness is even more difficult when you're constantly substituted out of a game.

"It is a little frustrating [being subbed off] because you want to stay out there," Ziebell said.

"I've got a really good understanding of what I can do and the coaches do as well and we make the best decision on the day of the game.

"I still feel like I'm the fittest I've ever been but I've still got a long way to go, and I know that I've just got to work really hard."

And work hard he does.

Ziebell is part of a young group of players who spend extra time on the track working on their aerobic fitness.

"Away from the main training group we probably put in [an extra] four or five hours a week building aerobic base," Ziebell said.

"In the long term hopefully that will improve us all and we'll be able to stay out there for four quarters and have a really good impact over four quarters."

Ziebell may not have been able to play a full game on Saturday, but his effort and work ethic was first rate.

The 20-year-old was given the important job of tagging Melbourne midfielder Brent Moloney who was in devastating form last week with 31 possessions and 19 clearances in the Demons' win over Adelaide.

Ziebell kept Moloney to 17 disposals and six clearances for the day and the Demon was unable to influence the game while managing 19 touches and eight tackles himself.

Coach Brad Scott was full of praise for his young midfielder.

"He's going to be a terrific player," Scott said after the game.

"He had a really important role on Moloney today who was absolutely outstanding last week against Adelaide.

"He kept Moloney quiet but had a fair bit of the footy himself and showed glimpses of the player he's going to be."

Scott also said he had every intention of playing Ziebell for the full game, unlike previous weeks where his substitution was pre-planned, but the 20-year-old cramped late in the match and was unable to continue.

"He's a really powerful 20-year-old, he's going to be a real player of the future for us," he said.

"We're just going to ask him to keep improving, keep working on his fitness and we've got a long term program in place for that and he's certainly doing what we're asking him to at the moment."