See how they run: motion symmetry.

See how they run: Motion symmetry

Robots are starting to move. In recent years, a number of legged machines have taken their first hops, steps and jumps (SN: 7/6/85, p.9). These efforts to design ealking and running robots are also leading to a better understanding of how two- and four-legged animals move.

Researchers have discovered that human runners and animals such as cats sometimes adopt a simple, symmetric gait that is more often associated with legged robots than with animals. In this type of motion, reversing both the direction of forward travel and the direction of time (equivalent to running a movie backward) would not affect the pattern of footfalls and of body movement.

Running is a series of bouncing and ballistic motions that accelerate the body within each stride, says robot designer Marc H. Raibert of Carnegie-Mellon University in Pittsburgh. The bouncing motions result from the rebound of the body when the legs push on the ground.

For a legged system running at a constant speed with a stable upright posture, the net acceleration of the body over an entire stride must be zero. Many patterns of body movement satisfy this constraint. People and many animals generally move their legs in complex, asymmetric modes and still travel at constant speed. Symmetric leg motions provide especially simple solutions that have been applied in the design of one-legged hopping machines, four-legged trotting machines and other mobile robots.

"The importance of symmetry in the control of legged robots," says Raibert, "raises the question of what role symmetry may play in the behavior of running animals." Raibert's study of symmetry in running appears in the March 14 SCIENCE.

About 20 years ago, zoologist Milton Hildebrand of the University of California at Davis observed that the left half of a horse often uses the same pattern of foot-falls as the right half, but 180[degrees] out of phase. Raibert's study of a human running on a cinder track and a cat trotting and galloping on a treadmill looked at a body's path through space and the trajectories of its feet with respect to the body. These data revealed another, robotlike symmetry in the way some animals run.

Why animals sometimes choose this symmetric type of motion isn't clear, says Raibert. As in robots, such patterns may simplify the controls necessary for steady movement. Instead of controlling the detailed motion of each leg joint, all an animal need to do is provide the initial conditions that automatically lead to steadh-state forward travel. That's the way robots run.