The R's are currently getting ready for a campaign in the Championship after failing to avoid relegation from the Premier League.
"You need fit players to survive in the Championship or to get out of the league," Redknapp told the club's official website.
"Pre-season is important because that's where you get the fitness in. I think there's some players that we can get fitter than last year."
QPR start the season on August 3 when they welcome Sheffield Wednesday to Loftus Road.