Using metabolic energy as "currency" to measure the "cost" of locomotion – that is, the amount of energy that must be spent to move from one place to another – we can compare the costs of different types of locomotion. Terrestrial locomotion – walking or running – is the most expensive form of locomotion. Given that humans are naturally terrestrial, many people may be surprised to learn that walking is so costly. The cost per kilogram of locomotion for human running is about five times higher than for the flight of a typical bird, and ten times more expensive than for fish swimming. Just why is locomotion so cheap for a fish? The main reason is that the water supports most of the body weight of such a swimmer, so all the animal needs to do to swim is to produce enough force to overcome the drag of its own body. Most aquatic animals have nearly the same density as the water in which they swim, so they do almost no work to support their weight against gravity. However, swimming is cheap only for those animals well adapted to swimming completely submerged. When animals such as ducks and muskrat swim on the surface, they use two or three times more energy to swim on the surface than when submerged, and as much as twenty times more energy than fish of a similar size. This is because of what is called the "bow wave" any object moving on the surface of water pushes up a bow wave at the front, which streams alongside and trails back. Boat designers have long known that the bigger the bow wave, the harder it is to push a boat through the water. The bow wave produces extra drag on any body moving on the surface of water. An animal swimming on the surface of the water uses extra energy in order to overcome drag. Thus, for our purposes, efficient "swimming" means underwater locomotion by animals with streamlined bodies, not the exhausting, inefficient locomotion of humans in swimming pools. Flying animals move through air that is less dense and less viscous than water, so why does flying cost more than swimming? First, most flying animals move much faster than a swimmer in order to produce enough lift (the upward force necessary to overcome gravity). This higher speed increases the drag that a flyer must overcome. Furthermore, a flyer has an extra source of drag that a swimmer does not have: the extra drag that comes from lift production. In a way, the extra drag represents the cost of supporting the flyer's weight in air. Walking (or running or galloping) is so costly because it involves at least three processes that require muscular work. The first is simply supporting the body's weight. The second is overcoming the friction in joints and muscles, and the third is constantly producing accelerations (speeding up) and decelerations (slowing down). The exact proportion of muscular effort that goes into these three processes depends on the anatomy of a given animal, but the third process probably accounts for most of the energy used by the muscles. When a person takes a step, first one foot pushes off, which accelerates the body. Then the other foot swings forward and hits the ground, and as the weight shifts onto that foot, the body decelerates. Some of the leg muscles actively tense to act as shock absorbers during this deceleration. Momentum carries the body over the grounded foot, at which time that foot pushes off to accelerate the body, and the cycle repeats. In terms of energy, walking is inefficient because of the acceleration and deceleration required with every step. Both the decelerations and accelerations need muscular effort and thus energy use. In swimming and flying, animals accelerate and decelerate relatively little over the course of a tail stroke or a wingbeat, so less energy is consumed by this process. As an analogy, consider riding a bicycle. When a person rides a bicycle, the bicycle does not accelerate or decelerate much with each turn of the petal. Thus, a person can ride a bicycle much faster than he or she could run using the same amount of effort.