What Makes Meat Tough?
Collagen is a long, stiff protein that is the most prevalent protein in mammals. It's something like the way fibers are twisted around each other to form a rope. This structure is what makes the collagen so strong; this strength is also what makes it more difficult to break down. The more collagen there is in a piece of meat, the tougher it is to cut and to chew. For cuts that are high in collagen, cooking with methods that use moist heat are the best. Collagen is soluble in water and when it is cooked with moist heat, it becomes gelatin.
Weight-bearing muscles and muscles that are constantly used contain higher amounts of collagen than muscles that aren't used for support or aren't used as frequently. Cows and pigs have higher amounts of collagen in the legs, chest, and rump. Pork is generally more tender than meat because pigs are usually slaughtered at a younger age than cows, and so their muscles are less developed and have less collagen than do those of cows.