Gymnast About two-fifths of the body’s weight is made up by the skeletal muscles—more than 640 of them. These types of muscle keep us upright and provide the force that allows us to move our bones, and so our bodies, into different positions. They enable us to walk, stand, jump, pick things up and perform many other actions. In most parts of the body the skeletal muscles are in two main layers. There are superficial muscles just under the skin and deep muscles lying below them. Other types of muscle keep our hearts beating (cardiac muscles) and our other organs working (involuntary muscles).
What muscles do
Every movement that the body makes is powered by muscles. All types of muscle exert a pulling force by being able to contract, or shorten. Most skeletal muscles are attached to the bones of the skeleton and pull on them to make the body move. Skeletal muscles taper at each end into tough cords called tendons which attaches the muscle firmly to a bones of the skeleton. As a muscle contracts, it becomes thicker and pulls on the bone, moving that part of the body.
Combinations of muscles—sometimes hundreds at a time—bend, straighten, rotate and generally move bones in many directions, as when, for example, you straighten your arm then rotate your hand from palm-up to palm-down. One bone may have several muscles attached to it.
The tongue is made up of eight muscles. Four act to change its shape and are not attached to any bone. The other four act to change the tongue's position, and are anchored to bone.
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