England coach Roy Hodgson is watching the fitness of Scott Parker and Steven Gerrard particularly carefully as he plots a path through Euro 2012.
Thirty-one-year-old Parker found it tough going against France in Donetsk on Monday and eventually had to be substituted.
Gerrard lasted the entire 90 minutes but, at 32, his stamina cannot be taken for granted either.
It is something Hodgson will have in his mind as he selects a team for Friday's Group D encounter with Sweden in Kiev.
"Scott Parker and Steven Gerrard are both over 30 and had to work really hard," said Hodgson, as he reflected on Monday night's 1-1 draw with France.
"But I'm sure I'm not going to be the only coach wondering 'can they do it every four days?' about their players. That is what tournament football is about.
"What we've got to make certain of, is that if they can't, or start to show signs of fatigue, that others are ready to go in and do exactly the same job."
Parker himself believes there should be problems with the fitness of the pair to face Sweden.
He said: "Myself and Steven (Gerrard) just have to keep going. We have to, don't we? If selected, I'm sure we can keep going.
"There's not a lot of cover in midfield, but I'm sure we'll be fine, and we have got other boys who can come in."
After praising his players for their resolute display in holding a France outfit now unbeaten in 22 games, Hodgson admits he may find it difficult to make changes for Friday.
"I will have to assess the freshness of my team and see whether they are able to do that again," said the England boss.
"My gut feeling, if we have a good recovery day on Tuesday, a sensible training day on Wednesday and sensible recovery day on Thursday, is that it will be very hard for me to leave people out.
"If we said 'we're going to give you a rest because it's too much to play two games in four days' some of them would have us up against the wall."
Only the non-playing members of England's squad did any serious work at their Krakow training base on Tuesday.
Amongst them was Wayne Rooney, who will complete his two-match ban when he sits out the Sweden game, with Hodgson expecting his team to still be in the thick of a qualification battle when he returns against Ukraine next Tuesday.
"He is our real ace in the hole because he is very fit and raring to go," said Hodgson.
"If he can play like we know Wayne Rooney can, we're going to be a bit more difficult to beat.
"You can only benefit from having someone of his quality in your team."