Real Madrid's training session on Wednesday had a clear standout, and for once, it wasn’t a player.

It was the famous hypoxia masks, which make an appearance a couple of times each season at the club’s training center.

One of the day's highlights was seeing Kylian Mbappé wearing the mask. No player escaped using them—or suffering through them, depending on how you look at it.

These are typically the days when the first-team players remember Antonio Pintus, the team's fitness coach renowned for maximizing the physical performance of players.

But why are these masks used? Pintus himself explained:

"With these tests, I can then individualize the running work. The goal is to repeat it in March because it helps us understand if we need to continue with aerobic work or switch to sprinting and acceleration if the team is in good shape, which happened last season," he said in statements for the club's social media.

The images of players running with the masks are always striking, though they have become a routine part of the training regimen at the club.