Why human walking is an inverted pendulum
Walking vaults the body over a stiff limb each step, an inverted-pendulum gait used even by many-legged arthropods. Crosswalk pace averages about 5 kilometres per hour, while champion racewalkers clear more than 14 over a full 20 kilometres.
Among bipeds, walking keeps at least one foot grounded with a double-support phase; running begins when both feet leave the ground—an official racewalking rule. Centre-of-mass height peaks at mid-stance in level walking and bottoms out in running, though grades above about 10 percent blur that split for some people. Contact lasting over half a stride usually marks inverted-pendulum mechanics across limb counts. Average crosswalk speed sits near 5.0 km/h (1.4 m/s); older pedestrians often fall between 4.51 and 4.75 km/h, younger between 5.32 and 5.43, with brisk walks near 6.5. Japan standardizes 80 metres per minute (4.8 km/h). Children typically walk independently around eleven months.
Fossil footprints in Kenya suggest humanlike bipedal walking as far back as three million years, and bipedalism appears among the earliest hominin hallmarks. Human gait is metabolically thrifty: upright posture lines ground forces through hip and knee so muscles work less. A 2007 comparison found moving a human body costs about 75 percent less energy than chimpanzee locomotion of similar scale; chimp bipedalism costs roughly 10 percent more than their quadrupedal travel, and true quadrupeds spend far less than crouched chimps. Double-pendulum mechanics help: the swing leg is one pendulum, the stance vault another, trading potential and kinetic energy and reclaiming roughly 65 percent of the energy that pure muscular pushing would demand.
Evolution of walking among tetrapods is thought to begin with underwater "walking" air-breathing fish, possibly kin to Tiktaalik, while arthropod lineages invented walking separately many times. Little skates still shove along the seafloor with pelvic fins using neural circuitry dated near 420 million years—before vertebrates came ashore. Absolute walking speed hits a ceiling when upward centre-of-mass acceleration exceeds gravity and the walker goes airborne; animals usually switch to running earlier for efficiency. Word roots reach Old English wealcan, "to roll," and in Britain and Ireland "walking" often simply means hiking.
Source: Walking
