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Walking (also known as ambulation) is one of the main gaits of locomotion among legged animals, and is typically slower than running and other gaits. Walking is defined by an 'inverted pendulum' gait in which the body vaults over the stiff limb or limbs with each step. This applies regardless of the number of limbs - even arthropods with six, eight or more limbs.
The word walk is descended from the Old English wealcan "to roll". In humans and other bipeds, walking is generally distinguished from running in that only one foot at a time leaves contact with the ground and there is a period of double-support. In contrast, 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. For quadrupedal species, there are numerous gaits which may be termed walking or running, and distinctions based upon the presence or absence of a suspended phase or the number of feet in contact any time do not yield mechanically correct classification. The most effective method to distinguish walking from running is to measure the height of a person's centre of mass using motion capture or a force plate at midstance. During walking, the centre of mass reaches a maximum height at midstance while during running, it is at a minimum. This distinction, however, only holds true for locomotion over level or approximately level walking. For walking up grades larger than nine percent this distinction no longer holds for some individuals. Definitions based on the percentage of the stride during which a foot is in contact with the ground (averaged across all feet) of greater than 50% contact corresponds well with identification of 'inverted pendulum' mechanics and are indicative of walking for animals with any number of limbs, although this definition is incomplete. Running humans and animals may have contact periods greater than 50% of a gait cycle when rounding corners, running uphill or carrying loads.
Although walking speeds can vary greatly depending on a multitude of factors such as height, weight, age, terrain, surface, load, culture, effort, and fitness, the average human walking speed is about 5.0 kilometres per hour (km/h), or about 3.1 miles per hour (mph). Specific studies have found pedestrian walking speeds ranging from 4.51 kilometres per hour (2.80 mph) to 4.75 kilometres per hour (2.95 mph) for older individuals and from 5.32 kilometres per hour (3.31 mph) to 5.43 kilometres per hour (3.37 mph) for younger individuals; a brisk walking speed can be around 6.5 kilometres per hour (4.0 mph). Champion racewalkers can average more than 14 kilometres per hour (8.7 mph) over a distance of 20 kilometres (12 mi). An average human child achieves independent walking ability at around 11 months old.
Sustained walking sessions for a minimum period of thirty to sixty minutes a day, five days a week, with the correct walking posture, reduce health risks and have various overall health benefits, such as reducing the chances of cancer, type 2 diabetes, heart disease, anxiety and depression. Life expectancy is also increased even for individuals suffering from obesity or high blood pressure. Walking also increases bone health, especially strengthening the hip bone, and lowering the more harmful low-density lipoprotein (LDL) cholesterol, and raising the more useful good high-density lipoprotein (HDL) cholesterol.
The CDC's fact sheet on the Relationship of Walking to Mortality Among U.S. Adults with Diabetes states that those with diabetes who walked for 2 or more hours a week lowered their mortality rate from all causes by 39 per cent. "Walking lengthened the life of people with diabetes regardless of age, sex, race, body mass index, length of time since diagnosis, and presence of complications or functional limitations." 
Judging from footprints discovered on a former shore in Kenya, it is thought possible that ancestors of modern humans were walking in ways very similar to the present activity as many as 1.5 million years ago.
It is theorized that "walking" among tetrapods originated underwater with air-breathing fish that could "walk" underwater, giving rise to the plethora of land-dwelling life that walk on four or two limbs. While terrestrial tetrapods are theorised to have a single origin, arthropods and their relatives are thought to have independently evolved walking several times, specifically in insects, myriapods, chelicerates, tardigrades, onychophorans, and crustaceans.
While not strictly bipedal, several primarily bipedal human gaits (where the long bones of the arms support at most a small fraction of the body's weight) are generally regarded as variants of walking. These include the following:
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 sixty per cent of the energy used due to pendulum dynamics and ground reaction force.
Walking differs from a running gait in a number of ways. The most obvious is that during walking one leg always stays on the ground while the other is swinging. In running there is typically a ballistic phase where the runner is airborne with both feet in the air (for bipedals).
Another difference concerns the movement of the centre of mass of the body. In walking the body "vaults" over the leg on the ground, raising the centre of mass to its highest point as the leg passes the vertical, and dropping it to the lowest as the legs are spread apart. Essentially kinetic energy of forward motion is constantly being traded for a rise in potential energy. This is reversed in running where the centre of mass is at its lowest as the leg is vertical. This is because the impact of landing from the ballistic phase is absorbed by bending the leg and consequently storing energy in muscles and tendons. In running there is a conversion between kinetic, potential, and elastic energy.
There is an absolute limit on an individual's speed of walking (without special techniques such as those employed in speed walking) due to the upwards acceleration of the centre of mass during a stride - if it's greater than the acceleration due to gravity the person will become airborne as they vault over the leg on the ground. Typically however, animals switch to a run at a lower speed than this due to energy efficiencies.
Fitness walkers and others may use a pedometer to count their number of steps. The types of walking include bushwalking, racewalking, weight-walking, hillwalking, volksmarching, Nordic walking and hiking on long-distance paths. Sometimes people prefer to walk indoors using a treadmill. In some countries walking as a hobby is known as hiking (the typical North American term), rambling (a somewhat dated British expression, but remaining in use because it is enshrined in the title of the important Ramblers), or tramping. Hiking is a subtype of walking, generally used to mean walking in nature areas on specially designated routes or trails, as opposed to in urban environments; however, hiking can also refer to any long-distance walk. More obscure terms for walking include "to go by Marrow-bone stage", "to take one's daily constitutional", "to ride Shanks' pony", "to ride Shanks' mare", or "to go by Walker's bus". Among search and rescue responders, those responders who walk (rather than ride, drive, fly, climb, or sit in a communications trailer) often are known as "ground pounders".
The Walking the Way to Health Initiative is the largest volunteer led walking scheme in the United Kingdom. Volunteers are trained to lead free Health Walks from community venues such as libraries and GP surgeries. The scheme has trained over 35,000 volunteers and have over 500 schemes operating across the UK, with thousands of people walking every week.
Professionals working to increase the number of people walking more usually come from six sectors: health, transport, environment, schools, sport and recreation, and urban design. A new organization called Walk England launched a web site on 18 June 2008 to provide these professionals with evidence, advice and examples of success stories of how to encourage communities to walk more. The site has a social networking aspect to allow professionals and the public to ask questions, discuss, post news and events and communicate with others in their area about walking, as well as a "walk now" option to find out what walks are available in each region.
The world's largest-registration walking event is the International Four Days Marches Nijmegen. The annual Labor Day walk on Mackinac Bridge draws over sixty thousand participants. The Chesapeake Bay Bridge walk annually draws over fifty thousand participants. Walks are often organised as charity events with walkers seeking sponsors to raise money for a specific cause. Charity walks range in length from two mile (3 km) or five km walks to as far as fifty miles (eighty km). The MS Challenge Walk is an example of a fifty mile walk which raises money to fight multiple sclerosis. The Oxfam Trailwalker is a one hundred km event.
In Britain, the Ramblers is the biggest organisation that looks after the interests of walkers. A registered charity, it has 139,000 members. The Ramblers’ run Get Walking Keep Walking project provides free routes, led walks and information specifically designed for people new to walking.
Regular, brisk cycling or walking can improve confidence, stamina, energy, weight control and life expectancy and reduce stress. It can also reduce the risk of coronary heart disease, strokes, diabetes, high blood pressure, bowel cancer and osteoporosis. Modern scientific studies have shown that walking, besides its physical benefits, is also beneficial for the mind — improving memory skills, learning ability, concentration and abstract reasoning, as well as reducing stress and uplifting one's spirits.
As a form of tourism there are many options for walking. The most famous one would be "walking tours" normally offered in different cities by paid guide tours. However, there are some volunteers that can conduct walking tours for tourists and do not charge for it, but just ask for a small tip at the end of the walk.
Walking is the most basic and common mode of transportation and is recommended for a healthy lifestyle, and has numerous environmental benefits. However, people are walking less in the UK; a Department of Transport report found that between 1995/97 and 2005 the average number of walk trips per person fell by 16%, from 292 to 245 per year. Many professionals in local authorities and the NHS are employed to halt this decline by ensuring that the built environment allows people to walk and that there are walking opportunities available to them.
There has been a recent focus among urban planners in some communities to create pedestrian-friendly areas and roads, allowing commuting, shopping and recreation to be done on foot. The concept of walkability has arisen as a measure of the degree to which an area is friendly to walking. Some communities are at least partially car-free, making them particularly supportive of walking and other modes of transportation. In the United States, the active living network is an example of a concerted effort to develop communities more friendly to walking and other physical activities.
Walking is also considered to be a clear example of a sustainable mode of transport, especially suited for urban use and/or relatively shorter distances. Non-motorised transport modes such as walking, but also cycling, small-wheeled transport (skates, skateboards, push scooters and hand carts) or wheelchair travel are often key elements of successfully encouraging clean urban transport. A large variety of case studies and good practices (from European cities and some worldwide examples) that promote and stimulate walking as a means of transportation in cities can be found at Eltis, Europe's portal for local transport.
The development of specific rights of way with appropriate infrastructure can promote increased participation and enjoyment of walking. Examples of types of investment include pedestrian malls, and foreshoreways such as oceanways and also riverwalks.
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The first successful attempts at walking robots tended to have six legs. The number of legs was reduced as microprocessor technology advanced, and there are now a number of robots that can walk on two legs. One, for example, is ASIMO. Although robots have taken great strides in advancement, they still don't walk nearly as well as human beings as they often need to keep their knees bent permanently in order to improve stability.
Nicholson, Geoff, The Lost Art of Walking, Harbour Books, 2010. ISBN 978-1-905128-15-0
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