Sports physiotherapist Ben Warbuton explained that these masks are traditionally used in controlled environments, such as on a treadmill or exercise bike, to assess fitness levels. However, Real Madrid have adapted the technology so it can be used during normal training sessions.

“They’ve got hold of high-tech equipment to complete it during training,” Waarbuton said. “This benefits the coaches because they can test the players and train at the same time.”

He added that traditional VO2 max testing can be impractical during the season. “If all players did a treadmill or bike test, it would be very time-consuming. They would lose a training session, and a treadmill is a harder and different surface to grass,” he explained.

By integrating the masks into football-specific drills, Warbuton says there is also a reduced injury risk.

“By wearing these masks in training, there’s a slight decrease in the risk of soft-tissue injuries, such as tight calves. They don’t waste a session and still get valuable fitness information.”

The data gathered allows staff to assess each player’s condition and tailor workloads accordingly.

“The results will tell the coaches how fit the players are,” Warbuton added. “They’ll compare them to their own baselines, or to other players in the same position. Players with a lower VO2 max may need extra fitness and conditioning work.

“It’s just one way of looking at a player’s conditioning levels. I imagine it’s very expensive, which is why not all clubs adopt it.”

This article is the latest from BBC Sport’s Ask Me Anything team.