Muscles and tendons
The muscles
Movement of an individual is possible thanks to muscular contraction, which is able to generate mechanical energy.
There are about 400 skeletal muscles designated for movement, each one is wrapped in a connective membrane, with the task of containing it during movement.
 The muscles, by means of terminal tendons, have 2 anchorage extremities with the bone: insertion, that is the anchorage area on the bone that, following contraction, is moved; and the origin, the anchorage area on the bone that remains fixed during contraction.
The central part of the muscle is called the muscular belly, and it is the fleshy part, made up of muscular fibers, blood vessels and connective tissue. In virtue of the shape we can subdivide muscles into:
- fusiform muscles (fig. 1): characterized by long fibers running parallel with regard to the traction line, which enable wide movements with a relatively small amount of force;
- semipennate muscles (fig. 3): characterized by short muscular fibers with little movement but a great force production potential;
- bipennate muscles (fig 4): similar to semipennate muscles but with the insertion of muscular fibers on both tendon sides;
- wide muscles (fig 2): with insertions connected to the tendons, which are called aponeuroses.
Considering the arrangement of the fibers, compared to the traction line, we can identify the muscles in: muscles with parallel fibers (fig. 1), arranged in a fusiform shape, able to perform contractions of a great extent and consequently a wide articular extent of the anatomical areas involved; and muscles with oblique fibers (figs. 3 and 4), arranged like a pinnate or semipennate shape, able to perform contractions of little extent but very powerful.
On average a muscle is able to shorten by 30-40% of its length, and express up to 3-4 kg of force for cm2 of surface.
The tendons
Tendons are anatomic structures that permit the transmission of movement to the bones. They are made up of connective tissue, collagen fibers and elastic fibers. The tendon structure owes its great ability to transmit force to the collagen fibers, which are able to orient themselves in virtue of the direction in which traction is exerted. A direct continuity between muscle and tendon structure has not been found.
Tendons can have different shapes, they can be fundamentally cylindrical, in the form of thin sheets or bands. The point where tendons and bones are joined is usually smooth, it tends to be rough where the connection occurs, at least partially, with the help of the fleshy parts of the muscle itself.
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