Sam Allardyce believes his side are now "15-20 percent" fitter now thanks to the training methods that he has installed.
The Sunderland manager has long been a believer in sports science, and Allardyce believes his side can now "physically produce a performance which they weren't capable of" when he first arrived.
Intensity increased by Allardyce
Allardyce was quick to make changes when he arrived at the club earlier in the season, increasing the intensity of training, as well as appointing Mark Taylor as performance director, whom he worked with at Blackpool, Bolton and Newcastle.
With levels of fitness improving, Allardyce believes it is now down to "technique and quality" as they battle against relegation.
"We've had to get the players to a different level of fitness, so they've got more physical capability of achieving a result," said Allardyce.
This could be seen in Sunderland previous game away to Liverpool, with the Black Cats scoring two late goals to rescue a point at Anfield.
Allardyce worried about new players
Despite paying tribute to his squad's new found fitness, Big Sam has some concerns about the "fatigue level" of his new signings.
Allardyce brought in Jan Kirchhoff, Dame N'Doye, Lamine Kone and Wahbi Khazri in January, and the Sunderland boss is worried they may "hit the wall" in the upcoming games.
"In your first game, there's so much adrenaline pumping through your body, that you get through it," said Allardyce.
Crashing out of the FA Cup could be a blessing for Sunderland, as it will give the players a chance to "get a breather and recover".
Sunderland's January arrivals are expected to feature in Saturday's home game against Manchester United, with Allardyce determined to "get them through" it and "push on".
Sunderland remain in the bottom three, four points from safety with 13 games to play.