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Muscle Structure

The human body possesses more than 300 clearly defined muscles, comprising about 40% of the total body weight. Each muscle has an upper origin (head) and a lower insertion, with the bulky part between them
(the belly) forming the actively contracting portion. The muscle is attached to bone by tendons. A skeletal muscle is composed of thousands of long, narrow muscle cells or fibers containing contractile elements, and
is surrounded by a membrane or sheath. The muscle fibers are bound together in bundles (fasculi), which are combined to form the muscle belly. In some muscles, the belly is divided into several parts. Each belly has its own origin or head, a muscle with two heads being known as a biceps, three heads a triceps, and four heads a quadriceps. There are two basic types of muscle fiber—slow (type I or red) and fast (type II or white). Slow fibers obtain their energy from glucose derived from glycogen in the presence of oxygen via the blood circulation (aerobically), while fast fibers obtain theirs from glucose stored in muscular glycogen and converted into
energy without the use of oxygen (anaerobically). Slow fibers, in comparison with fast fibers, are smaller, have a lower anaerobic glycolytic capacity, and a slower speed of contraction; they are supplied with a greater network of capillaries, have fewer nerves and have a lower level of endurance. The slow fibers respond very well to static and low dynamic exercise, while the fast fibers respond better to dynamic exercise, especially of high intensity. The fast fibers are subdivided into type IIa, characterized by great strength lasting over a long period, and type IIb, having similar strength over a short period. When a muscle is contracted, the different fibers are activated sequentially-type I, followed by type IIa, and finally type IIb.There are considerable variations in the composition of individual muscles. The most usual combination is that of equal numbers of fast and slow fibers, but in an athlete who excels in endurance sports (e.g. marathon running) there will be a preponderance of slow fibers, while in a sprinter there will be more fast
fibers. A knowledge of fiber distribution in muscles is important when training for specific sports. Muscle tissue is invested with an extensive network of small blood vessels (the capillaries) averaging about 3000 per mm2 on cross-section. When the muscle is at rest, 95% of the capillaries are closed, but when physical activity is undertaken they open progressively to ensure an ample blood flow to the working tissue.
Training can result in the following effects on muscle:
– muscle enzyme levels increase;
– the number of subcellular units in which energy conversion takes place (mitochondria) increases with
aerobic training;
– storage of fuel for the production of energy increases;
– the capillary network increases;
– muscular volume increases (hypertrophy) with strength training. This combination of effects increases muscle strength, stability, stamina, and a capacity for rapid contraction.

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