MEAT CONTENTS
Meat that we eat is essentially made of muscle tissue, but it also has various amounts of extra fat and connective tissues. Muscle fibers are often interspersed in between thinner layers of connective tissue and fat (the so-called marbling that we see in meat). Of course, around the bulk of muscle fibers are even bigger layers of connective and fat tissue, although we don't usually eat these. In any case, the relative amount of all the three tissue types will determine the flavor of the meat, as well as its color and texture. That's why it is important to look for the marbling in meat before we buy it.
The muscle fibers, the most precious part of meat, can come in different variations - colors, texture, flavor - it all depends not only on other tissues but also on the animal, how it was living, what it ate, and how old it was at the time of slaughter. As you might know from your own life, the muscles get bigger with age. A young kid don't have the muscle mass of an adult. The less muscle mass, the more tender the meat, because the muscle fibers didn't have chance to enlarge themselves and become thicker (which is why some meat is tougher). An adult's muscle mass comes from exercise and hormonal stimulance. So is the case with animals. That's why it was popular to treat animals with hormones that would make the production of meat quicker.
The age of the animal also determines the structure of connective tissue, which directly transforms into our eating experience. One major protein of connective tissue, collagen, is very abundant but loose in younger animals. This directly contributes to the more tender texture of the meat - as we cook it, the protein becomes more jelly-like and blends nicely with the thin muscle fibers. The result is tender meat. When it comes to older animals, their collagen is less abundant although it becomes more tougher due to chemical reactions, partially imposed by exercise. So, tougher collagen and thicker muscle fibers give us sensation of a tougher meat.
Fat tissue can also be found in between the bundles of muscles. This is what we call marbling. Fat is not solely deposited in sequestered spaces of the body, but can lie blended together with the muscles. Contrary to what some people believe, the fat tissue is never transformed into muscles after exercise. It is used as an energy depot, which can be used by the working muscle if its primary fuel (glycogen, creatine) ends, or it can be the primary fuel of long-lasting work muscles. When it comes to cooking, fat (marbling) gives the meat more tenderness as it melts in the heat of the preparation, and enters and loosens up the muscle fiber network. It can also contribute some additional flavor.
Whether the meat is tough or tender depends on the above mentioned factors. How do they all come into play? Besides age, the tissue content and quality is largely determined by where in the animal the particular cut of meat resides. Its location will determine how much stress and exercise it will get - and so, what kind of texture we can expect from it. If you are not familiar with the relative tenderness of the different cuts of meat, we suggest that you get down on all fours and walk around the room for a while, like an animal does. Notice which parts of the body (which muscles) receive the most workout. Naturally, the animal has the same experience. Now, you should have a fair idea about what to expect when buying certain cuts of meat! If you don't know the names of them, learn them from some charts. Some cuts are named after their texture, e.g. tenderloin (it is tender, because it doesn't receive much strain and the muscle is...well, tender). And, as mentioned above, the younger animals have thinner, therefore more tender, meat. If that's what you're looking for, you will like veal, lamb, pork, or chicken, especially the tender cuts.