In the Demons' 94-point loss to West Coast on Saturday at the MCG, they played well for a large part of the first half but even then fell off late in both quarters.
The Demons hit the front at the 16-minute mark of the first quarter and held the lead for 10 minutes before the Eagles put together three unanswered goals to lead at quarter time.
The same thing happened in the second quarter – the Dees hit the lead 13 minutes in and held on for seven minutes before the Eagles kicked three goals to one to round out the half.
The visitors then slammed on 11 goals to one in the third quarter to open up a commanding 72-point lead at the final change.
While the Demons were encouraged by the first half in Saturday's performance, they were left searching for the reason they lapsed so dramatically after half time.
"I wouldn't say it's a fitness thing, because we've shown throughout the pre-season with the way we train and everything the boys are running the best times they've ever run," Trengove told AFL.com.au.
"You hear [elite performance manager] Dave Misson comparing us to sides he's worked for previously, saying we're right up there.
"We've got full confidence we can run out games but it makes it a bit harder when you're chasing tail in that third quarter because you feel like you're running that extra bit and it takes it out of you even more.
"It's hard to put it down to one thing. Their A-grade midfielders really stepped up and we weren't quite up for it."
Coach Mark Neeld said the players had hit internal indicators with their first half and emphasised the last thing to change in any rebuild was the scoreboard.
The Demons' ability in the first half didn't go unnoticed by the opposition, which was also gunning for its first win of the season.
"I can sympathise with them now. I couldn't sympathise with them before the game," West Coast midfielder Daniel Kerr said.
"I can see where they're going. I can see they're building a list. We went through a similar thing and it can turn around really quickly.
"They are playing good patches of football and there are some talented guys out there and I'm sure they'll turn it around."
Trengove said the key to moving forward was to make sure the group stayed "tight", to keep training hard and to develop a resilience that would stand up to an onslaught like the one the Eagles launched at them.
He said the first half, off the back of last Saturday's 148-point loss to Essendon, had given them optimism to keep going.
"It was a great response, and even at the end of the game, the West Coast guys were saying in that first half, they felt the pressure," he said.
"That's the way we want to play. We felt like an AFL team.
"West Coast are aiming to be a top four side this year so it was a great effort by us to show that we could do it but for some reason it just didn't happen in that third quarter.
"They just got on top of us and we couldn't prevent them from that run on.
"That's something as a development group we've got to learn to be able to withstand, whether it's just lock it down and slow the game down and take the punch out of it, or just to be able to get the ball back in our hands and get the control back our way."
Jennifer Phelan is a reporter for AFL Media. Follow her on Twitter @AFL_JenPhelan.
