The most straightforward way is cutting, pounding, and grinding which physically break down the structure of the muscle bundles.
Another method is using plant enzymes, mainly papain found in Meat Tenderizer . Enzymes break down muscle and connective proteins into amino acids. The problem is that only the surface of the meat is exposed to the enzyme, so that the piece of meat gets mushy on the outside while the inside stays the same.
Wine and vinegar marinades tenderize the meat surface, denaturating surface proteins.
are no really satisfactory ways to tenderize meat chemically.