Skeletal muscles are the mechanism for powering human movement. While individual muscles are typically regarded as distinct organic structures, the skeletal muscles are the largest organ grouping in the body (the skin is the largest contiguous organ). Virtually all joints are moved by pairs of muscles working in contrasting but complimentary ways, one set providing the extension of the joint (extensors), the opposing, or antagonist, set countering with flexion, or bending capability.
All muscles are composed of specialized muscle fibers. While the fibers are made up of the similar kinds of cells, fluids, blood vessels, and nervous system components, muscle fibers have certain key physical distinctions that create two distinct kinds of fibers, fast-twitch (type II fibers) and slow-twitch
The fast-twitch and slow-twitch fibers are distributed throughout the muscles of the body. The percentage distribution is determined genetically and it is unique to every human being. There is considerable scientific authority for the proposition that, through specific training such as intense endurance training, the composition of muscle fibers in an athlete's legs can be altered, converting fast-twitch fibers for use as slow-twitch fibers to better support the endurance activity.
Whether a muscle fiber functions as a fast-twitch or slow-twitch fiber is subject to a number of physical and neurological factors. Slow-twitch fibers are governed by slow conduction neurons, the relay switch of the nervous system that governs a group of muscle fibers ranging in size from as few as 10 to as many as 2,000 fibers. Fast-twitch fibers are governed by fast-acting neurons, which are capable of transmitting or firing the nerve impulses that command movements by the muscle 10 times more frequently than the slow-twitch neurons will fire.
The energy metabolism characteristics of each type of fiber also contribute to the function of each type. Fast-twitch fibers store glycogen within the cells of the muscle fiber. Glycogen, the storage form of the carbohydrate product glucose, is then utilized at the muscle in the cycle of electrochemical reactions that produce adenosine triphosphate (ATP), the source of energy within the muscle. To utilize this store of energy, a process known as glycogenosis occurs within the cell, where glycogen is converted to a compound, glucose-1-phosphate, which then participates in the energy production cycle within the cell involving creatine phosphate and ATP. The muscles store glycogen in quantities that total approximately 1% of the muscle mass, a reserve that is quickly depleted through intense exercise; the muscles can only produce ATP through glycogenosis for an approximate maximum of 90 seconds. As the ability of the fast-twitch cells to produce ATP and energy is limited, the fast-twitch cells quickly become fatigued. Examples of this energy metabolism frequently occur in sports such as sprinting. Races such as the 400 m and 800 m events are often described as the toughest of the running distances, because optimum performance is demanded of the body as it is running out of the energy stores capable of being generated in its fast-twitch fibers.
Slow-twitch fibers require glycogen, broken into its constituent glucose, before energy can be produced in its cells. The requisite glycogen is transported to the cells through the circulatory network that ends in the capillaries that service the cells of each muscle fiber. Because the energy production process in slow-twitch fibers is much slower, it can be sustained for much longer periods, and the slow-twitch fibers do not fatigue as readily as the fast-twitch fibers.
The mitochondria is the portion of the muscle cell where the energy production occurs. The mitochondria in the anaerobic fast-twitch fibers are significantly larger than that of the slow-twitch aerobic fibers. It is within the mitochondrial membrane that, depending on the energy sources available, either fatty acids are reduced for energy production, or glucose is ultimately converted to lactate as a part of the ATP energy cycle.
The skeletal muscles also perform important roles relative to the body's energy system while the body is fasting or otherwise not ingesting foods to be converted into useful energy sources. The skeletal muscles release amino acids during periods of fasting, particularly alanine and glutamine. These acids work in the bloodstream to maintain the body's blood glucose levels, stimulating the conversion of glycogen stored in the liver into glucose.