Atlético had a pretty long period of time where the team wasn't on top of their physical ability after the new year until the end of February which also resulted in quite a few injuries (7 injuries in the same amount of games, one after another in February) on important players. The heavy load of games between January and up until the game against Osasuna at the end of February was a major issue for the training staff and physios for several reasons. There was simply no time to recuperate the players physically which finally resulted in a really tough month during February where the losses kept piling up. However, the training staff with Profe Ortega have finally had time to help the players recuperate their physical level during March because there was 8 days between the games against Osasuna and Real Madrid and 6 days between Real Madrid and Celta Vigo. Those 14 days have been crucial for the new arrivals to bed in better tactically but most importantly, finally use some well needed time to work on the conditioning of each player. That being said, there were another 8 days between the Espanyol-game and Real Betis yesterday. So, in total, they've had some 19-22 days to work with the players so they'd recuperate their level physically. Major factor. While we're at it; I think Atlético are lasting 15-20 minutes too short in comparsion to the first 4 months of the season. I'm pretty sure they'd love to have a 10 day period off before the really big games the next month to use it as a mini pre-season.
Also, when you rotate, you have to be smart. It's not just a matter of putting a few players into the starting line-up because you need to keep a structure of how you play. Simeone's actually used 17 players in 5 different games as starters in March. He's rotating but the players who usually get into the side that aren't starters have to show something as well. I'd say the maximum to keep a stability is possibly 4, anything over 4 new players from one game to another becomes an issue for the structure. What's even more important is that Simeone and the training staff have done Europa League as well with the players which gives them important experience. Thursday-Sunday back in 2011-2012 was super hard to endure, they've used that knowledge of how to prepare the team this season and the season before as well. Very important factors.
The reason why he brought in players like Sosa and Diego was because they would be able to play in different positions in the attacking line which then itself meant Atlético could rest the starters against some of the smaller sides or at least rotate a bit. Diego was also brought in to control possession better against these smaller sides so the team wouldn't lose enormous amounts of energy like they did between August-December. The best example of this are the games against Valladolid and Real Sociedad where he just controlled things really well. He was also brought in to lock up really defensive sides but never got the chance against Almería where he was good but Simeone made a mistake in subbing him off. Sosa brings hard and disciplined work when the team needs to rotate and his set-pieces are really good so the team doesn't lose anything in that department at times. Where we do lose is in terms of fluidity because he's not as good as Koke in open play. Alderweireld is very good and the difference is minimal between him, Miranda and Godín when one of the last two are missing.
Profe Ortega usually says a top physically lasts from 6-8 weeks before the players need to build it up again. The good part about Atlético is that they're a team that can rely on different sorts of things to score goals when they need it, especially set-pieces which is a huge part of the success. What's missing right now is the long range efforts to open up defences, even though Gabi is providing them since Real Madrid. The difference between Madrid, Barcelona and Atlético is that the two first mentioned have a much higher individual quality. That's why they're rightly favourites for the title.