Center backs cover less ground on average than any other outfield player on the pitch, but they do need to be fit to play in the position. Opposition strikers will be looking to try and move them out of position with off-the-ball runs, and also get in behind the defense, particularly if they defend higher up the pitch. When defenses position themselves closer to their own goal, there is less space and less running for them to do but being able to stay alert for the full 90 minutes of the game is critical.