ONE OF the biggest selection decisions Fremantle coach Justin Longmuir had to make at the start of this season was how many tall defenders he could fit in the Dockers' backline.

The availability of captain Alex Pearce and Brennan Cox and the emergence of Josh Draper last year meant there were already three key defenders deserving of selection alongside Heath Chapman and Luke Ryan, who routinely play tall.

Not all could fit in the first two rounds as Chapman and Draper were selected as substitutes against Geelong and Sydney, but the Dockers embraced their height against West Coast and produced an imposing defensive performance.

The 193cm Chapman played one of the better games of his career, according to Longmuir, while 197cm Draper took on the challenge of playing small on premiership Eagle Liam Ryan and held him to one goal.

With smaller defenders Brandon Walker (shoulder) and Corey Wagner (calf) missing, Chapman said the versatility in the Dockers' backline was proving important. 

"We have a few boys that can play tall or small, so that makes it really comforting and we're able to swap pretty seamlessly and trust that the bloke next to you is going to get the job done," Chapman told AFL.com.au.

"We have a lot of trust in each other, and it can be very different week to week in terms of personnel.

"But we watch a lot of vision going into games and put the work into our prep and trust that game day we can get the job done."

Fremantle's backline against West Coast conceded a total of two contested defensive losses, with the coaches thrilled with the defenders' ability to support each other with spoils and intercepts.

On his own polished performance, which included several intercepts as he pressed up the ground aggressively, Chapman said he had learned from his early selection as the substitute.

"That speaks back to the versatility that our backline group has. We've got nine or ten blokes that are probably knocking the door down to a play senior footy, so it's a great spot to be," the 23-year-old said.  

"It was great learning for me. I went back and worked on my game and I'm better for the run."

While some clubs turn to smaller forward lines, Longmuir said the added benefit of playing a tall and versatile backline was the ability to give the key defenders an extra break during games to keep them fresh.

Pearce, for example, played 88 per cent game time against the Eagles while shutting down opposition skipper Oscar Allen. He regularly stayed on the field for 95 to 100 per cent of matches last year.

"We are a tall back line, and having those guys being able to play tall and small gives us great flexibility," Longmuir said.

"It also allows us to get a rotation into Coxy and Pearcy because they (Draper and Chapman) can just slide over to a tall if they're playing on a small. 

"It's been a new transition for [Draper]. He needed to be key back last year, but we've thrown the challenge to him to play on the smalls, and there's no bigger challenge really in the AFL than Liam Ryan.

"So I thought he handled that pretty well and it was one of Chappy's better games I would have thought.

"He was really strong in the contest, decisive in all areas of his game, and I was really proud of the way he played."