They're tasked with organizing the defence, winning balls and shadowing the opponent's most dangerous players. They're responsible for keeping the shape of the team's formation, settling or quickening the pace of the game and barking out orders if necessary.
Centre backs, or central defenders are the heart of the backline.
For Canada's women's soccer team, these duties fall on the shoulders of veteran Candace Chapman and youngster Shannon Woeller. Along with their fellow defenders, they'll face their toughest test of the CONCACAF Women's Olympic Qualifying Tournament to date on Friday: A semifinal match with Mexico that will decide who gets to go to the 2012 London Olympics.
Lethal transition game
"It's going to be tough. We know the Mexican team will fight tooth and nail for every inch on the football field," Canadian coach John Herdman said on a conference call Thursday.
"And what history tells us is that these games are usually very, very close. There's only one goal in it, typically. We're going to have to fight all the way if we want to win this game.
"I think defensively on the transitions is the big area we need to be mindful of," Herdman added.
That's where Woeller and Chapman come in.
Canada has allowed just one goal in this tournament - in the waning minutes against Costa Rica - and they'd like to keep it that way, as it could mean all the difference in this zero-sum scenario they're facing.
On paper, the Canadians have a monumental advantage over their North American opponents. They're 16-1-1 all-time against Mexico and in the FIFA world rankings, Mexico is 21st to Canada's No. 7.
But that lone loss was a massive blow. Mexico denied Canada an Olympic berth in 2004.
Chapman, who will play her 100th cap Friday, has appeared in half of those 18 Mexican matches.
"They're going to battle," the Ajax, Ont., native told CBCSports.ca after the team's final training session Thursday night. "They're great on the ball and great on the counter attack."
Eye on Dominguez
"Dominguez is a real seasoned campaigner," Herdman said. "She's bright, she's sharp, even against the USA, she had two yards [advantage] on their back four with her movement. And unfortunately her teammates didn't find her very often, but she's a real deadly player. She's got 70-odd goals in 100-odd caps.
"When someone's that lethal, you've got to give them extra attention."
Despite their lack of distribution to Dominguez against the Americans, Herdman said the Mexicans are full of young, technically-gifted players. Woeller said that can't be overlooked.
"All their players are dangerous, so we're looking to make sure we're always staying a step ahead of them and making sure we don't get caught in a transition situation without enough numbers," said Woeller, who, if she plays Friday night, will earn her 13th cap for Canada.
"Just being ready for them to come at us and keep everything in front of us."
As for Dominguez?
"We'll take care of her as group," Woeller said.
Earning Herdman & team's trust
Buying into the team philosophy has helped the 21-year-old Vancouver native smoothly transition from the U-20 program to the senior side.
"It's been really great, essentially the entire back four and the keepers, Karina [LeBlanc] and Erin [McLeod] have been helping me and talking to me from behind. ... Just keeping me calm and confident and letting me know when I do things well and what else I should be doing," Woeller explained.
"And I've learned a lot playing beside Candace. Obviously she's got a ton of experience and she knows what she's doing."
Woeller had appeared in just four games for the senior team until Herdman assigned her the left centre back role at the Pan American Games in October. She started all five games at that tournament, and hasn't looked back.
The Rutgers University student, who turns 22 next week, is the only Canadian player to have played 90 minutes in the first three group games at the Olympic qualifiers.
"It's been great playing beside her and seeing such a young player really rise to the occasion," said the savvy veteran Chapman. "I'm really proud of her for that."
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