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These contractile organs -650 in total- are made up of muscle tissue. Their main function is to perform all of the body’s movements, like walking, running, talking, making gestures, moving objects, etc. They also carry out other tasks, such as maintaining the balance or posture of the skeleton, protecting and supporting our internal organs and participating in bodily functions, such as generating heat.

The basic building block of the inner structure of a muscle is muscle fiber, a multinucleate (several nuclei) cell that is elastic and cylinder-shaped and which groups up with others of its kind to form hundreds of fascicles. Each muscle fiber is surrounded by a sheath called endomysium, and each fascicle is surrounded by the perimysium.

Muscle fibers stretch out and pull back and then return to their original shape, thus allowing the body to move and stay in a certain position.

In addition, each fiber is surrounded by a thin plasma membrane called sarcolemma. A muscle fiber contains thousands of myofibrils, which are smaller fibers bunched up into groups.

Myofibrils have two protein myofilaments: myosin or thick myofilament, and actin or thin myofilament, which are arranged in repeating patterns (matrixes), called sarcomeres. Between these myofilaments there are thin membranes called Z bands, which act as a separation. Myosin is located at the center of the sarcomere, and actin, which surrounds myosin, is bound to the Z band.

Types of fibers

Muscles are made up of two kinds of fibers:
– Fast twitch fibers (white muscle, they fatigue quickly): they correspond to the white flesh of the muscle.
They provide strength in bursts because they contract quickly. For example, kicking a ball, running short distances, swinging a racket, etc.
– Slow twitch fibers (red muscle, resistant to fatigue): they correspond to the deep, red colored flesh. They provide continuous tension, only stopping if they run out of energy. For example, swimming, pedaling long distances, etc.

It’s important to note that a single muscle can contain both types of fibers and the ratio of said mix will depend on the person.