Stepping lightly: new view of how human gaits conserve energy.

For 200,000 or more years, the fine-tuned mechanics of human motion have enabled our species to traverse enormous distances on foot with remarkably little energy expenditure. Scientists have long pondered which specific qualities of walking and running render those means of getting around so efficient.

Now, a pair of biomechanics theorists say that they've captured the essence of what makes human locomotion as thrifty as it is. Their model explains why any number of silly-looking gaits, such as the crouching strut of Groucho Marx, tire us out more than routine styles of perambulation do, says Manoj Srinivasan of Cornell University.

The analysis by Srinivasan and Andy Ruina, also of Cornell, offers a new "conceptual framework" for human locomotion, says Arthur D. Kuo of the University of Michigan in Ann Arbor. As such, it could prove useful in various biomechanics-associated fields, including treating disabled people and building better two-legged robots (SN: 8/6/05, p. 88).

Srinivasan and Ruina used a computer to create an exceptionally simple model of bipedal motion in which pistonlike legs propel the upper body, represented as a point-like mass, through space along perfect arcs.

The model, to be unveiled in an upcoming issue of Nature, assumes that a moving bipedal creature uses metabolic energy only when pushing against the ground at the start and finish of each of stride, Ruina explains.

In between, a walker's foot becomes a fixed point around which the leg swings like an upside-down pendulum. Just as a regular pendulum traces a circular arc with extraordinary energy efficiency, so, too, does this leg motion, the researchers propose. In running, the model shows that the energy efficiency derives mostly from the fact that the runner spends considerable time flying through the air, at no energy cost, along parabolic arcs.

When Ruina and Srinivasan simulated a wide range of possible gaits based on these fundamentals, they found that ordinary walking and running were two of the three most-efficient gaits.

The third efficient stride was a peculiar lope halfway between walking and running that resembles no gait known to biomechanics specialists. People with certain disabilities might use it without the gait having been previously recognized, the Cornell scientists suggest. On the other hand, the hybrid gait might be an artifact of the model's extreme simplicity, Srinivasan admits.

The researchers have practiced the newfound gait. "It's like the way I would run if I was really tired," say Ruina. Videos of Ruina and others demonstrating all three gaits are at http://ruina.tam.cor nell.edu/research/topics/locomotion_and _robotics/papers/WhyWalkandRun/ index.htm.

Antonie J. van den Bogert of the Cleveland Clinic Foundation faults the new model as too simple to be realistic. Yet he also notes that more-complex models haven't achieved such an "elegant proof" that our natural modes of motion are also the most efficient ones.