Movement of the vertebrate skeleton

Any voluntary movement is enabled by one or more muscles pulling on two bones across a joint. But, muscles can only contract, hence pull; they can’t push. Therefore, any back and forth movement, such as opening and closing the arm at the elbow, involves at lest two muscles pulling on opposite sides of the joint. The following site shows a video animation of the elbow. Note how/where muscles are connected to the bones. Note how closing the arm is done by contraction of the bicep muscle; opening arm is done by contraction of the tricep muscle. (The latter sections of the video get into technical terminology that is not necessary–omit.)

http://www.childrensuniversity.manchester.ac.uk/media/services/thechildrensuniversityofmanchester/flash/exercise_3_muscles.swf

(Please post questions/comments/ideas that kids have.)