This paper will focus on Adenosine Triphosphate (ATP.) Its overall function, where it is stored, its structure, how it is produced, and how it works in anaerobic and aerobic exercise. Additionally, it will attempt to respond to the question of why a Sports Massage Therapist should know about ATP.
ATP is a chemical catabolized (i.e., broken down) from glucose and stored as energy in the mitochondria of cells throughout the body. It is the necessary fuel for all body cells; without it, cells, and therefore the body, cannot operate. The three main functions of ATP in cellular function are:
- Transporting organic substances—such as sodium, calcium, potassium—through the cell membrane.
- Synthesizing chemical compounds, such as protein and cholesterol.
- Supplying energy for mechanical work, such as muscle contraction
ATP is comprised of a substance called adenosine, plus 3 phosphate molecules. As fuel is needed by the cells, ATP releases energy by bonding with water (a process called hydrolysis.) This causes one of the phosphate molecules to be released, resulting in the formation of a new compound called adenosine diphosphate (ADP.) The release of the phosphate molecule through hydrolysis liberates approximately 12,000 calories of energy. This energy is used in a number of different ways, as illustrated above. In muscle cells, it is used to power the actin and myosin filaments that lead to muscle contraction.
Myosin and actin filaments are, respectively, thick and thin protein threads. While myosin is composed only of myosin molecules, actin is composed of three different proteins: actin, tropomyosin, and troponin. Both actin and myosin threads are found in muscle cells in contractile units called sarcomeres. Sarcomeres, aligned in a chain-like fashion, form myofibrils, which are fibers residing in the sarcoplasm. It is the actin and myosin filaments within the myofibrils of the muscle which are responsible for muscle contraction. When an action potential travels down the muscle fiber membrane, calcium ions are released into the sarcoplasm, producing a series of steps which result in contraction of the muscle. Although it is not known precisely how actin and myosin produce muscle contractions, the “sliding filament theory” is a possible explanation:
(The theory) suggests that stimulation of the fiber prompts the formation of tiny crossbridges that extend from the myosin filament and attach to active sites on the actin filament. The release of calcium ions within the muscle fiber exposes these active sites, facilitating the attachment of the two kinds of fiber to one another. Each crossbridge exerts a pull on the actin filament, causing the actin and myosin filaments to slide past one another. Under the influence of chemical substances released in the binding process, each crossbridge is then disconnected from its binding site on the actin filament and moves to a neighboring site. Since the process happens simultaneously in all of the cells of muscle, the entire muscle contracts. [i]
The actin and myosin interaction which leads to muscle contraction is fueled by ATP.
Only a small amount of ATP—about 3 ounces—is stored in the muscle cells at any given time. This is enough energy to support only a few seconds of muscle contraction during intense activity. As a result, ATP stores need to be constantly replenished in order to keep the muscle cells fueled. One of the ways this occurs is by transforming ADP back into ATP in the muscle fiber. As ATP is used up and ADP stores accumulate, the bonds of another phosphate molecule, creatine phosphate (CP), break. This releases energy that is used to rebond ADP and P to form ATP. However, there is very little CP in muscle cells. Therefore, the ADP/CP reaction supplies only enough energy to support an additional 3 or 4 seconds of intense activity. During periods of high intensity exercise, such as sprinting and weight lifting, when short bursts of maximum output are called for, the total energy released from the anaerobic ATP—ADP—CP—ATP cycle is only capable of sustaining the cells’ energy needs for about 8 seconds.. Beyond that, the body turns to other methods of generating ATP to keep the muscle fibers fueled.
During exercise periods lasting longer than 6 to 8 seconds, muscle fibers catabolize stored glucose—known as glycogen—into ATP for fueling contractions. This is done via two processes: Glycolysis, an anaerobic process, and oxidation, an aerobic process.
Glycolysis occurs during short, high energy bursts, after the ATP and CP in muscle cells have been consumed, and before the body has had sufficient time to meet its increased oxygen needs. Although glycolysis produces rapid release of ATP and doesn’t require oxygen to do so, it is not the most productive method of energy formation. Only a small amount of ATP is produced in this manner. Through a series of biochemical steps, this anaerobic process transforms one glycogen molecule into two molecules of pyruvic acid, resulting in a net gain of only two molecules of ATP, or 24,000 calories of energy. Additionally, an end product of the glycolytic reaction is lactic acid, which is formed from pyruvic acid when excess amounts of it accumulate in the muscle cells. Lactic acid diffuses out of the cell membrane and into the muscle and blood stream, slowing down the production of ATP, which results in decreased energy and exercise performance*. The decrease in energy is due to chemical changes brought on by the lactic acid which inhibits cells from responding to nerve stimulation.
If glycolysis were the only method available for the body’s production of ATP during maximum exertion, it could not sustain this type of intense activity for very long. When vigorous exercise exceeds 2 or 3 minutes, and as soon as the body has had time to meet its increased demands for oxygen, a third, more productive and efficient method of generating ATP—the aerobic process called oxidation—kicks in.
About 90% of ATP formation is accomplished through oxidation during prolonged exercise. During this aerobic process, the pyruvic acid catabolized from glycogen through glycolysis is transformed into ATP through a complex series of chemical reactions. The end result is that one molecule of glycogen produces 38 molecules of ATP, each ATP molecule trapping 7300 calories, resulting in a net gain of 277,400 calories of energy available for muscular contraction.
Skeletal muscles have two kinds of muscle fibers: Slow-twitch and fast-twitch fibers. Both types use ATP to function, but in different ways. How a muscle performs correlates directly to the type of fiber which predominates in that muscle.
Slow-twitch fibers have more mitochondria than do fast-twitch fibers, and can therefore produce more ATP. They also break down ATP at a slower pace, which means that during exercise, they tend to produce ATP at a faster rate than they use it up. Additionally, they have more myoglobin—which stores oxygen in muscle cells—and produce slower contractions than fast-twitch fibers. For all these reasons, slow-twitch fibers are better suited for aerobic exercise than fast-twitch fibers.
Fast-twitch fibers have less mitochondria than do slow-twitch fibers and therefore produce less ATP. They use the ATP at a quicker pace and are better suited for getting ATP from anaerobic exercise. Therefore, they are best for performing fast contractions during exercise which requires maximum output during a brief period.
As a healthcare professional working with the body’s soft tissue, it’s important for a Sports Massage Therapist to have a broad understanding of muscle physiology. Knowing how adenosine triphosphate works to fuel muscle contractions for producing movement is fundamental knowledge. Although this knowledge will probably rarely, if ever, be discussed with clients, the more educated a Sports Massage Therapist is, the better equipped she/he will be to work effectively with clients and communicate constructively with other healthcare professionals.
[i] I Feinberg, Brian. The Musculoskeletal System. New York: Chelsea House Publishers, 1991. P. 48.
* Normally, the blood transports lactic acid back to the liver, where it is transformed back into glucose through oxidation to be used for the body’s energy needs. However, during prolonged exercise, oxygen is in short supply because most of it is being used by the body to produce ATP for muscle contraction. As a result, lactic acid accumulates in the liver until sufficient oxygen is available, creating what is called an “oxygen debt”. This “debt” is generally “repaid” at the end of the exercise period when the athlete continues to breathe hard and take in large amounts of oxygen.
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