How to Walk on Water and Climb up Walls by David Hu

Author:David Hu
Language: eng
Format: epub
ISBN: 9780691184081
Publisher: Princeton University Press

FIGURE 5.1. Human walking in an inverted pendulum gait. As the person’s weight is transferred over the supporting foot, the leg acts like a pendulum. The center of mass naturally rises as the leg becomes perpendicular to the ground. Redrawn from Matthis and Fajen 2013.

As I walk forward, my body stores kinetic energy associated with its being in motion. As I place body weight on my leading foot, making it my new supporting leg, my body momentarily slows down. Kinetic energy goes into raising my center of mass by a distance of 1.5 inches. My body’s center of mass is a point designated by the “crash dummy symbol” in Fig. 5.1. The change in position of my center of mass is so large because it greatly affected by the position of my heavy legs. This rising in center of mass is associated with storage of gravitational potential energy, which, like a battery, can store energy for brief periods of time. The amount of energy stored is the product of my body weight and the height increase. As my supporting leg pushes off, my center of mass falls downward again a distance of 1.5 inches. Gravitational energy is transferred smoothly into kinetic energy as my body picks up speed again. If I were to track my center of mass as I walk, it would be traveling up and down like a roller coaster. The amplitude of the peaks would be three inches and the wavelength equal to my stride length. Just like a roller coaster trackway, energy is stored at the top of each peak, and then expended as the “cars”—representing my center of mass—accelerate downhill. This continuous exchange of energy leads to walking’s low energy expenditure over multiple strides.

Although the body is very good at cycling energy, it is not perfect. Collisions with the ground cause energy to be lost, and in response, muscles have to inject energy at certain key moments. Greg’s mission was to pinpoint the specific locations in the leg where energy was being injected by the muscles. Once he found the timing and location of the energy expenditure, he could replace those motions with an exoskeleton to reduce energy consumption in human walking. When Greg and other locomotion scientists measure energy, they use units called joules. In everyday terms, a joule is the amount of energy needed to pick an apple off the floor and raise it to waist height. For every joule of mechanical work produced, human muscle needs four joules of food. In other words, human muscles, for all their versatility, are only 18 to 26 percent efficient. For the sake of argument, let’s call it 25 percent. Because of our muscles’ inefficiency, when we expend energy walking, we must eat four times the expected amount of food.

The low efficiency of the body converting food to usable work has a flip side: any device that saves the body from doing one joule of work will save the body four joules of food. This energy savings is strong motivation for designing an exoskeleton to reduce the work done by the body.

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