The principle Children’s bodies are dynamic systems means that:

Development in one component of the body may affect development in another component. Children’s growth reflects a continuing pattern of speeding up, slowing down, speeding up, slowing down again, and so on. Development proceeds in a top-down fashion, from the head to the feet. Development proceeds in an in-out fashion, beginning with the head and torso and proceeding to the arms and legs and then to the fingers and toes.