I’ve never trained a soccer player before, but I do have a couple decades+ training high level military forces. Like them, soccer players need to be able to move quickly on their feet, but also have a very high level of endurance.
Squats are great for strength, but can also make one slow if done without a proper plan. I suggest along with your usual training protocols, adding in swings, get ups, cleans, carries (farmers, suitcase, single and double rack, and single overhead), windmills, and snatches if able. Most of the issues I work in regards to endurance and speed is where they feel sluggish and like they have weights in their legs. That acidity buildup is caused by an over abundance of carbon dioxide and nitrogen in their blood, which is slowing them down. They need to build their core muscles so their diaphragm can force them to take in more oxygen. They may not feel winded, but their slow or heavy legs say otherwise.
My methodology has been to teach them the hinge and ab contraction in whatever they’re doing, and it’s worked wonders.
As for their feet game though, that’s way beyond me. I teach quick upper body twitch, using the lower body as the stable base which needs to be able to rapidly move and pivot once weakened and smoked. -Precision combat shooting under extreme duress.
