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Muscle and movement. Illustration of human muscle anatomy and movement systems. Movement depends on the interaction of multiple systems in the body, including muscle, bones and joints, nervous co- ordination and balance. The human body (at centre, posterior view) contains some 600 muscles, most of which are skeletal (voluntary) muscles under con- scious control. A skeletal muscle is seen at top right, with its internal fibres and nerve attach- ment (yellow). At top left, specialized regions of the brain co-ordinate movement, with the aid of the spinal cord and its nerve branches (bottom right). Joints, as at the knee (bottom left), give bones mobility as the muscles act upon them. (KEYSTONE/SCIENCE PHOTO LIBRARY/JOHN BAVOSI)