Putting one foot in front of the other is such a big accomplishment for little ones, as it is the beginning of their ability to explore their world. As they grow up, their legs get stronger, and they learn to run, to sprint, and to jump! The legs are the foundation of the body. Fun, dynamic exercise can be a great way for kids to build strong bodies, and to help kids learn to understand more about the way their bodies work. There are 13 primary muscles that help the legs go, that work together in groups.