A large grasshopper such as a locust can jump about twenty body lengths without using its wings; the acceleration peaks at about 20g. Grasshoppers jump by extending their large back legs and pushing against the substrate (the ground, a twig, a blade of grass or whatever else they are standing on); the reaction force propels them into the air. They jump for several reasons; to escape from a predator, to launch themselves for flight, or simply to move from place to place. For the escape jump in particular there is strong selective pressure to maximize take-off velocity, since this determines the range. This means that the legs must thrust against the ground with both high force and a high velocity of movement. However, a fundamental property of muscle is that it cannot contract with both high force and high velocity, which seems like a problem. Grasshoppers overcome this apparent contradiction by using a catapult mechanism to amplify the mechanical power ^] produced by their muscles.