Energy speed

How do you increase the rate of your players healing rate. I have some players that have 1.3energy per hour
Build an infirmary, this is your only option.
Are energy recovery speeds constant, for example if a player has 3 energy recovery per hour will they have that until they retire or does it change naturally? ( the external effect of an infirmary not withstanding)
Thankfully it does not change with age imagine tho poor old buggers needing 2 days recovery after every game. The only way to change is the infirmary which you unlock with the lvl 8 stadium