VICTOR HABBICK VISIONS / SCIENCE PHOTO LIBRARY VICTOR HABBICK VISIONS / SCIENCE PHOTO LIBRARY
Muscular system. Computer artwork of a male runner demonstrating the human muscular system. These muscles, making up one of the outer layers of the body, are skeletal muscles, which are under the conscious control of the brain. They account for around 45% of the weight of the body. Muscles are mainly composed of proteins; in particular, the proteins actin and myosin are involved in making muscles contract. The muscles then pull on tendons that are attached to the bones of the skeleton, producing movements such as running. Skin and fat cover the muscles, while the internal organs and bones lie underneath.
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