Walking

Stand still and your center of mass is poised above your feet. Begin to walk and it leaves them. At every point in the gait cycle that follows, your center of mass is somewhere it would not be at rest — a fraction of a meter ahead of the planted foot, headed for the ground. If the swinging leg failed to land in time, you would topple forward onto your face.

In 1978, Mary Leakey’s team in Tanzania uncovered a 27-meter trail of about seventy hominin footprints, preserved in volcanic ash that had set after a brief rainfall like cement. The gait was, in its essentials, modern — arched foot, big toe aligned with the others, heel-strike then toe-off. The species that made them — almost certainly Australopithecus afarensis, Lucy’s people — had a brain about a third the size of yours.

In 1911, in Sherrington’s laboratory in Liverpool, Thomas Graham Brown severed the spinal cords of cats just above the hindlimbs. The textbooks said walking was a chain of reflexes; break the chain and the walking should stop. It did not. The isolated lumbar cord, disconnected from any brain, produced rhythmic alternating leg movements indistinguishable from stepping. He had discovered the central pattern generator — a self-oscillating circuit that produces walking with no input required. Your cortex modifies walking. Your cortex does not produce it.

A patient with advanced Parkinson’s disease provides the clinical complement. He approaches a doorway and stops. His feet are glued to the floor. Mind willing, body unresponsive. Now paint horizontal stripes on the floor, spaced one stride apart. Or invert his walking cane, so the handle creates a small visual obstacle at floor level. The patient resumes walking — instantly, no drug, no surgery. He steps over the line. The next. The next.

You walk by vaulting your body, like an upside-down pendulum, over a stiff planted leg. At midstance, the center of mass is at its highest — peak potential energy, minimum kinetic. As you fall forward into the next step, potential converts back to kinetic. Walking lets the planet move you.

The Weber brothers — Wilhelm, Gauss’s collaborator on the electric telegraph, and Eduard, a physician — published Mechanik der menschlichen Gehwerkzeuge in 1836: the first treatment of walking as classical mechanics rather than animal spirits. Walking did not change in 1836. The way humans were allowed to think about it did.

A century and a half later, Tad McGeer built a machine. Two legs, a hip joint, no motors, no sensors, no controller. He set it onto a shallow slope. It walked down with a gait so human that videos of it have been mistaken for footage of a person from behind. Walking is not, fundamentally, a control problem. It is a geometry problem with a stable solution.

In 1872 Leland Stanford asked a question: does a galloping horse ever have all four hooves off the ground simultaneously? Classical paintings showed horses in rocking-horse poses, and Stanford suspected the artists were wrong. He hired Eadweard Muybridge to settle it.

On June 19, 1878, at Stanford’s Palo Alto Stock Farm, twelve cameras with electrically-tripped shutters captured a single galloping stride of the mare Sallie Gardner at intervals of about a thousandth of a second. The photographs answered the question: yes, the horse becomes briefly airborne — but not when the legs are splayed. The unsupported moment is when the legs are gathered together under the body. The artists had been wrong by exactly one geometry.

Four years before the photographs, in 1874, Muybridge had murdered his wife’s lover. He drove to the man’s house, identified himself politely, and shot him in the chest. The Napa County jury acquitted him on grounds of “justifiable homicide.” He returned to photography within months. By 1887 he had published Animal Locomotion — 781 plates, 20,000 images of humans and animals in motion. The atlas became reference work for Francis Bacon, for Disney, for every subsequent physiologist of movement. The Zoopraxiscope he built in 1879 was the immediate predecessor of cinema. The technology to see walking and the technology to show walking were the same invention.

In May 2023, Nature published a paper from Grégoire Courtine and Jocelyne Bloch’s team at EPFL in Lausanne. The patient was Gert-Jan Oskam, a Dutchman paralyzed below the hips since a 2011 motorcycle accident in China. The team installed two electrode arrays: one over the motor cortex, decoding the intent to walk; one over the lumbar spinal cord, delivering stimulation to the dormant walking circuit that Graham Brown had discovered in 1911.

Between them, a computer Oskam wore in a backpack — the “digital bridge.” Thought lands on the cortical array, is decoded, is transmitted wirelessly, is shaped into a stimulation pattern, and arrives at the spinal walker as a command. Oskam can walk over uneven terrain. He can climb stairs. He can stop, start, and turn at will. After a year, he could walk with crutches even when the implant was switched off. The bridge had taught the bridge to become unnecessary.

Walking is what the body did before there was a thinker to claim it. Every step you take while reading the next sentence is a successful negotiation of a physical instability your ancestors solved before the cortex existed, by anatomy your great-great-grandparents could not have explained, broadcasting your state into the world at exactly the rate the species walks. The cadence is your signature. The horizon is your guide. The rest is what the body does while you are paying attention to something else.