The most important players for muscle mass:
1) Protein intake (at least 1.6g/kg body weight)
2) Resistance training
3) Omega-3s (2-4g/day, triglyceride form)
4) Heat stress (from a sauna or hot tub)
HEAT SHOCK PROTEINS
As the name implies, they're activated by heat stress
(sitting in a 163 ºF sauna for 30 mins. activates HSPs ~50% over baseline levels)
And one of their most important functions?
Help cellular proteins maintain their three-dimensional structure