Back in the day, early seventies, when I lifted weights and played football, I ate tons of protein.
Breakfast was a thick steak, half dozen eggs, pound of bacon, quart of milk, half loaf of bread with cheese. You get the picture.
Anyway, I was young and poor and needed an abundance of beef stored up in the fridge for breakfast and dinner. (I skipped lunch. Amazingly, that little breakfast held me over until dinner)
I bought the very cheapest cuts of beef available, T-bones, chuck, whatever was the absolute cheapest. If it was red meat and cheap, I stacked them up and literally cleaned out the grocers refrigerated bin. I remember paying sometimes 39 cents a pound.
So, the secret in making tough meat tender in those days was papaya (papain) and pineapple (bromelain) enzymes. Meat tenderizers had one or both of those enzymes in them, along with spices, salt, pepper.
I would marinate my tough, fibrous chuck in a mixture of papain and bromelain mixture and the action of the papain and bromelain was to break down the collagen and connective tissue of the tough rigid cheap meat and render it into a poor man’s filet mignon.