When the fitness center is built, your players get additional 10% on their fitness training points (this works only when training fitness of course). The first upgrade raises this percentage to 15% and the second to 20%. The upgrades also improve the rehabilitation times for your injured players. The first upgrade gives your player 25% more chance every day to skip 2 days towards the full recovery, instead of just 1. The second upgrade raises that chance to 40%.