Mechanical energy oscillations of two brachiation gaits: Measurement and simulation

How do arm-swinging apes locomote effectively over a variety of speeds? One way to reduce the metabolic energy cost of locomotion is to transfer energ y between reversible mechanical modes. In terrestrial animals, at least two transfer mechanisms have been identified: 1) a pendulum-like mechanism for walking, with exchange between gravitational potential energy and translat ional kinetic energy, and 2) a spring-like mechanism for running, where the elastic strain energy of stretched muscle and tendon is largely returned t o reaccelerate the animal. At slower speeds, a brachiator will always have at least one limb in contact with the support, similar to the overlap of fo ot contact in bipedal walking. At faster speeds, brachiators exhibit an aer ial phase, similar to that seen in bipedal running. Are there two distinct brachiation gaits even though the animal appears to simply swing beneath it s overhead support? If so, are different exchange mechanisms employed? Our kinetic analysis of brachiation in a white-handed gibbon (Hylobates lar) in dicates that brachiation is indeed comprised of two mechanically distinct g aits. At slower speeds in "continuous contact" brachiation, the gibbon util izes a simple pendulum-like transfer of mechanical energy within each strid e. At faster speeds in "ricochetal" brachiation, translational and rotation al kinetic energy are exchanged in a novel "whip-like" transfer. We propose that brachiators utilize the transfer between translational and rotational kinetic energy to control the dynamics of their swing. This maneuver may a llow muscle action at the shoulder to control the transfer and adjust the b allistic portion of the step to meet the requirements for the next hand con tact. (C) 2001 Wiley-Liss, Inc.