Meeting Abstract

S7-1.7  Friday, Jan. 6  On the speed of lever systems MCHENRY, MJ; Univ. of California, Irvine

The geometry of an animal’s skeleton affects its ability to move quickly. Many fast-moving species possess joints with a higher displacement advantage (the ratio of output to input displacement) than their slow-moving relatives. It has therefore been suggested that a high displacement advantage endows a species with a superior capacity for rapid motion. However, this idea has rarely been tested. The aim of the present research was to evaluate how the geometry of an animal’s skeleton affects their ability to move rapidly. We investigated the influence of geometry alone by mathematical modeling the lever mechanics of three different rapid lever systems: the locust leg, stomatopod raptorial appendage and frog jaw. As established by previous experimental studies, all three systems produce explosive motion with stored elastic energy. We examined how the maximum speed of this motion varies among simulations that were provided with an equal amount of stored elastic energy, but differed in displacement advantage. Assuming conservation of energy, the locust leg was found to kick at the same maximum speed, regardless of differences in the displacement advantage of its knee joint. In contrast, a higher displacement advantage in the stomatopod raptorial appendage acts to reduce maximum speed. This is because the long excursion created by a high displacement advantage causes a loss of kinetic energy through the generation of drag. In the frog jaw, energy is lost within the muscle on the input and of the lever. As a consequence, a large displacement advantage helps to minimize energetic losses and thereby act to enhance high-speed motion. Therefore, the energetics of an elastically-driven lever system mediates the relationship between the displacement advantage and speed of a lever system.