Backing the boys here, In red meat there is Fat and there is Connective tissue. Intramuscular fat, aka Marbling, contributes to flavor, moistness and tenderness. It is low density and will melt easily. Inter-muscular fat, between muscles, and Subcutaneous fat, aka Fat Cap, is a higher density fat that does melt and "Bastes" meat but requires a High temperature to render completely...Crisp Cooked Bacon. There are two main types of connective tissue that affects meat tenderness they are Collagen, that which holds individual muscle fibers together and also surrounds muscle groups and Elastin, that which connects muscle groups to Bone, aka Silver Skin and Tendon. Low and Slow cooking in the presence of Moisture will dissolve Collagen into Gelatin and the muscle fibers will fall apart, becoming tender and have that "rich mouth feel" we associate with great Pulled Pork.This collagen break down is mostly complete when the meat reaches 190* to 210*F. Elastin is virtually impossable to dissolve and has to be cut away, like the Membrane on the back of ribs! Notice I said, "in the presence of moisture", at 212*F internal temp, the moisture in meat evaporates, collagen breakdown stops and meat dries out and becomes tough as the Protein in the muscle fibers dehydrates and shrinks...becomes Jerky!
I hope this helps...JJ
