Movement of the body depends on the interaction of muscles, bones, and joints in response to signals from the brain and nerves. A muscle typically connects two bones and crosses the joint between them. When a muscle contracts, it pulls on the bones to which it is attached and produces movement. Muscles can only pull, not push. Therefore, many muscles are arranged in pairs, one on each side of a joint, so that they produce opposing movements. An example is the pairing of the triceps and biceps muscles in the upper arm.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.