What Makes a Quarter Horse Different From a Thoroughbred

Each muscle consists of thousands of cells that are bundled together to form one functional unit. Skeletal muscles are covered with a protective sheath that eventually comes together to form tendons and ligament. Muscles have a plentiful blood supply, because they require constant delivery of oxygen and nutrients. The blood supply also takes away the toxic substances that muscles build up because of their high activity level.

Muscle Energy Sources

All cells are reliant upon high-energy phosphate bonds for energy. When these powerful phosphate chemical bonds break down, they release energy which is used by the body to power things, like pumps, enzymes, and revolving doors for proteins to come and go from cells.

The fastest method of producing energy comes from a molecule that stores phosphate, known as creatine phosphate. However, this hot fuel called in creatine phosphate lasts only 5-10 seconds.

That's a quick burnout. For more continuous function, e.g. to cross a river, the muscles will be required to make and store a more reliable energy source.

Most of this reliable energy is found in a molecule called ATP. It's so important it's often called "energy currency." The P in ATP stands for phosphate – and, once again, it is in the P bonds that energy is stored. There are three of these high-energy bonds in ATP, which makes it a particularly valuable energy source.

ATP itself must come from the breakdown of nutrients – sugar (glucose, or glycogen in its stored form), fats, and protein. ATP is continually used, remade, and reused. It's such important stuff, only a very small amount is actually stored in the muscle cells. But there has to be this stuff around, because if a muscle cell runs out of ATP, it can no longer function. Rigor mortis, the stiffness seen in death is due to a final lack of ATP.

Type I muscles break down sugars by a process that requires oxygen, much like the gas burning engire that requires oxygen from air. The form of sugar breakdown that occurs in the presence of oxygen is known as aerobic glycolysis. Aerobic glycolysis is well suited to endurance type muscles, because it produces a very large amount of ATP (36 in total) for every molecule of glucose that is broken down.

Type I muscles also use fats for production of ATP, producing an astounding 460 ATP for every molecule that it broken down. Therefore, fat is an extremely dense source of energy production. Both aerobic glycolysis and the breakdown of fats are relatively slow processes. Anaerobic glycolysis is the process of breaking down glucose without the presence of oxygen. The net production of ATP from one sugar is only two ATP. So it we only get two dollars (ATPs) versus 460 from a molecule of fat, what is the advantage of anaerobic glycolysis? Actually it's great because it is extremely fast, and speed is the name of the game with muscles.

The down side to anaerobic glycolysis is a greater production of by-products, especially lactate. Lactate accumulation blocks anaerobic glycolysis itself, and thus muscle function.