Walking

Walking is the main form of animal locomotion on land, distinguished from running and crawling. When executed in shallow water, it is usually described as wading and when executed vertically it becomes scrambling or climbing. The word walking is derived from the Old English walcan (to roll).

Walking is generally distinguished from running in that only one foot at a time leaves contact with the ground: for humans and other bipeds running begins when both feet are off the ground with each step. (This distinction has the status of a formal requirement in competitive walking events, often resulting in disqualification even at the Olympic level.) For horses and other quadrupedal species, the running gaits may be numerous, and walking keeps three feet at a time on the ground.

The average child achieves independent walking ability between 9 and 15 months old.

While not strictly bipedal, several primarily bipedal human gaits (where the long bones of the arms support at most a small fraction of the bodies' weight) are generally regarded as variants of walking. These include:
 * Hand walking; an unusual form of locomotion, in which the walker moves primarily using his hands.
 * walking on crutches (usually executed by alternating between standing on both legs, and rocking forward "on the crutches" (i.e., supported under the armpits by them);
 * walking with one or two walking stick(s) or trekking poles (reducing the load on one or both legs, or supplementing the body's normal balancing mechanisms by also pushing against the ground through at least one arm that holds a long object);
 * walking while holding on to a walker, a framework to aid with balance; and
 * scrambling, using the arms (and hands or some other extension to the arms) not just as a backup to normal balance, but, as when walking on talus, to achieve states of balance that would be impossible or unstable when supported solely by the legs.

For humans, walking is the main form of transportation without a vehicle or riding animal. An average walking speed is about 3 mph (5 km/h), although this depends heavily on factors such as height, weight, and age. A pedestrian is a walking person, in particular on a road (if available on the sidewalk/path/pavement).

Biomechanics
Human walking is accomplished with a strategy called the double pendulum. During forward motion, the leg that leaves the ground swings forward from the hip. This sweep is the first pendulum. Then the leg strikes the ground with the heel and rolls through to the toe in a motion described as an inverted pendulum. The motion of the two legs is coordinated so that one foot or the other is always in contact with the ground. The process of walking recovers approximately 60% of the energy used due to pendulum dynamics and ground reaction force. 

The biomechanist Gracovetsky argues that the spine is the major agent in human locomotion. He bases his conclusions on the tragic case of a man born without legs. The man was able to walk albeit slowly on his pelvis. Gracovetsky claims that however important to wellbeing, the function of legs is secondary in a strictly mechanical sense. Legs enable the spine to harvest the energy of gravity in an efficient manner. The legs act as long levers that transfer ground reaction force to the spine.  Lumbar motion during walking consists mostly of sideways rotation. Gracovetsky observes that fish use the same lateral motion to swim. He believes the mechanism first evolved in fish and was later adapted by amphibians, reptiles, mammals and humans to their respective modes of locomotion.