There are three types of muscle:

Smooth muscle or "involuntary muscle" is a spindle shaped muscle found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, ureters, bladder, and blood vessels. Smooth muscle contains only one nucleus, and no striations.
Cardiac muscle is also an "involuntary muscle" but it is striated in structure. Like Smooth muscle, cardiac muscle contains only one nucleus. Cardiac muscle is found only within the heart.
Skeletal muscle or "voluntary muscle" is anchored by tendons to the bone and is used to affect skeletal movement such as locomotion.

Skeletal muscle is further divided into several subtypes:

Type I, slow oxidative, slow twitch, or "red" muscle is dense with capillaries and is rich in mitochondria and myoglobin, giving the muscle tissue its characteristic red color. It can carry more oxygen and sustain aerobic activity.
Type II, fast twitch, muscle has three major kinds that are, in order of increasing contractile speed:
a) Type IIa, which, like slow muscle, is aerobic, rich in mitochondria and capillaries and appears red.
b) Type IIx (also known as type IId), which is less dense in mitochondria and myoglobin. This is the fastest muscle type in humans. It can contract more quickly and with a greater amount of force than oxidative muscle, but can sustain only short, anaerobic bursts of activity before muscle contraction becomes painful (often attributed to a build-up of lactic acid). N.B. in some books and articles this muscle in humans was, confusingly, called type IIB
c) Type IIb, which is anaerobic, glycolytic, "white" muscle that is even less dense in mitochondria and myoglobin. In small animals like rodents or rabbits this is the major fast muscle type, explaining the pale color of their meat.

ATP

Adenosine 5'-triphosphate (ATP) is a multifunctional nucleotide that is most important as a "molecular currency" of intracellular energy transfer. In this role ATP transports chemical energy within cells for metabolism. It is produced as an energy source during the processes of photosynthesis and cellular respiration and consumed by many enzymes and a multitude of cellular processes including biosynthetic reactions, motility and cell division. ATP is also incorporated into nucleic acids by polymerases in the processes of DNA replication and transcription. In signal transduction pathways, ATP is used as a substrate by kinases that phosphorylate proteins and lipids, as well as by adenylate cyclase, which uses ATP to produce the second messenger molecule cyclic AMP.

ATP in the Human Body
Muscles cells, like all cells, use ATP as an energy source. The total quantity of ATP in the human body at any one time is about 0.1 Mole. The energy used by human cells requires the hydrolysis of 200 to 300 moles of ATP daily. This means that each ATP molecule is recycled 2000 to 3000 times during a single day. ATP cannot be stored, hence its consumption must closely follow its synthesis. On a per-hour basis, 1 kilogram of ATP is created, processed and then recycled in the body. Looking at it another way, a single cell uses about 10 million ATP molecules per second to meet its metabolic needs, and recycles all of its ATP molecules about every 20-30 seconds.

Lactic Acid
Catabolized carbohydrates is known as glycolysis. The end product of glycolysis, pyruvate can go into different directions depending on aerobic or anaerobic conditions. In aerobic it goes through the Krebs cycle and in anaerobic it goes through the Cori cycle. In the Cori cycle pyruvate is converted to lactate, this forms lactic acid, lactic acid causes muscle fatigue. In the aerobic conditions pyruvate goes through the Krebs cycle. For more about Krebs cycle refer to chapter 2 Cell Physiology.

Krebs Cycle

Nitrogen

Cellular Action of Skeletal Muscles

During cellular respiration the mitochondria, within skeletal muscle cells, convert glucose from the blood to carbon dioxide and water in the process of producing ATP (see cell physiology). ATP is needed for all muscular movement. When the need of ATP in the muscle is higher than the cells can produce with aerobic respiration, the cells will produce extra ATP in a process called anaerobic respiration. The first step of aerobic respiration(glycolysis) produces two ATP per glucose molecule. When the rest of the aerobic respiration pathway is occupied the pyruvate molecule can be converted to lactic acid. This method produces much less ATP than the aerobic method, but it does it faster and allows the muscles to do a bit more than if they relied solely on ATP production from aerobic respiration. The drawback to this method is that lactic acid accumulates and causes the muscles to fatigue. They will eventually stop contracting until the breakdown of lactic acid is sufficient to allow for movement once again. People experience this most noticeably when they repeatedly lift heavy things such as weights or sprint for a long distance. Muscle soreness sometimes occurs after vigorous activity, and is often misunderstood by the general public to be the result of lactic acid buildup. This is a misconception because the muscle does fatigue from lactic acid buildup, but it does not stay in the muscle tissue long enough to cause tissue breakdown or soreness. During heavy breathing, following exercise, the cells are converting the lactic acid either back into glucose or converting it to pyruvate and sending it through the additional steps of aerobic respiration. By the time a person is breathing normally again the lactic acid has been removed. The soreness is actually from small tears in the fibers themselves. After the fibers heal they will increase in size. The number of mitochondria will also increase if there is continued demand for additional ATP. Hence, through exercise the muscles can increase in both strength and endurance.

Another misconception is that as the muscle increases in size it also gains more fibers. This is not true. The fibers themselves increase in size rather than in quantity. The same holds true for adipose tissue--fat cells do not increase in number, but rather the amount of lipids (oil) in the cells increase.

Muscle fibers are also genetically programmed to reach a certain size and stop growing from there, so after awhile even the hardest working weightlifter will only reach a certain level of strength and endurance. Some people will get around this by taking steroids. The artificial steroids cause all sorts of trouble for the person. They can cause the adrenal glands to stop producing corticosteroids and glucosteroids. This leads to the atrophy of the gland's medulla and causes permanent loss of the production of these hormones. The testicles may also atrophy in response to steroids. Eventually the testes will stop making testosterone and sperm, rendering the male infertile.

One of the more serious problems associated with abnormal gain of muscle mass is heart failure. While for most people gaining muscle and losing fat is desirable, a body builder is at risk of producing more muscle mass than the heart can handle. One pound of fat contains about 3.5 miles of blood vessels, but one pound of muscle has about 6.5 miles. Hence, additional muscle causes the heart to pump more blood. Some people that have too much muscle will be very strong but will not have a healthy aerobic endurance, in part because of the difficulty of providing oxygenated blood to so much tissue.