How do humans walk? To find out, Giovanni Cavagna, a physiologist from the University of Milan, has been studying the concept of human motion for nearly forty years. From his experiments, he has determined that human motion is similar to that of an inverted pendulum. A pendulum is a device that transforms kinetic energy into potential energy, and back in equalizing ratios. Cavagna describes the inverted pendulum motion as,

You pivot around the foot that's on the ground, as if you were using that leg to pole-vault, and your center of mass, somewhere in the belly, describes an arc. As you plant a foot on the ground in front of you, the ground exerts a force back up your leg that slows you down, and you continue slowing as you rise up on that foot to the top of your arc. At that point your kinetic energy is at a minimum--but your potential energy is at a maximum. As you fall forward into the next step, that stored potential energy is converted back into kinetic energy, and you accelerate again. (Kunzig)

A perfect pendulum has a conversion from kinetic to potential energies of almost 100 percent. However, from his experiments, Cavagna has determined that humans are only about 65 percent of a perfect pendulum. Therefore, calories are burned while walking. The difference of these values corresponds to the idea that for each step that is taken 35 percent of the energy required for the step is taken from food energies. So where does the loss of energy come from? Cavagna determined that what keeps humans from reaching 100% efficiency is the resistance in leg muscles which creates heat. Due to the Theory of Conservation of Energy, the energy required from walking now must share some energy with thermal energy loss rather than the perfect direct ratio between only kinetic and potential energies.

What's the purpose of Cavagna's research? According to the 67 year old man, "I'm not doing this because it's useful, I'm doing it because it's amusing." (Kunzig)

Kunzig, Robert, "Falling Forward" Discover, July 2001, pg.24-25 (follow the link to the article above)