I did a test, first I tried training where all my coaches with 1 in Discipline. Then with 20. I also edit my players professionalism to 1 and 20. The training session was balanced for everone - everything on medium.
The result ?
1. When the coach have 20 in "level of dicipline" and players on 20 professionalism; they trained hard and the stats kept a high value during the whole season.
2. When the coach have 1 in "level of dicipline" and players on 1 professionalism;
there level of training was low all season, noone stats was going up.
3. When the coach have 1 in "level of dicipline" and players on 20 professionalism;
Players level of training was good all season, some gained a few points here and there.
4. When the coach have 20 in "level of dicipline" and players on 1 professionalism;
alot of unhappy players about there training level, and there attributes were none consistant.
Conclusion: Professional players is what to look for if you want them to train hard