I do think that 80% of people whining on injuries trained their boys on 3 bars. Hence, choose: less growth with 2 bars, or risky growth with 3. It's been 2 season i train with 2 bars and my players injured 2 times so far. Both the times were when i trained them right after a match.

So i tried with a ghost account squad at lv 1: surprisingly, every time i tried there to train (even on 2 bars) my players right after the match, i ended up with 1-2 injured each time, no matter what the type of training.

I concluded that the game engine takes into consideration also the stress endured playing matches. So AFTER having played and along for X hours you got a higher % of injury possible. Better train the peeps before the match calculating the recharge possible.