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Running is a means for an animal to move on foot. It is defined in sporting terms as a gait in which at some point all feet are off the ground at the same time. This is in contrast to walking, where one foot is always in contact with the ground, the legs are kept mostly straight and the center of gravity rides along fairly smoothly on top of the legs. The term running can refer to any of a variety of speeds ranging from jogging to sprinting.
Humans leap from one leg to the other while running. Each leap raises the center of gravity during take-off and lowers it on landing as the knee bends to absorb the shock. At mid arc, both feet are momentarily off the ground. This continual rise and fall of bodyweight expends energy opposing gravity and absorbing shock during take-off and landing. Running uses more energy than walking to travel the same distance. Therefore, running is less efficient than walking in terms of calories expended per unit distance, though it is faster.
In 2004, scientists at the University of Utah and Harvard University hypothesized that the ability of humans to sustain long-distance endurance running may have been instrumental in the evolution of the human form.
Lower body motion
Running is executed as a sequence of strides, which alternate between the two legs. Each leg's stride can be roughly divided into three phases: support, drive, and recovery. Support and drive occur when the foot is in contact with the ground. Recovery occurs when the foot is off the ground. Since only one foot is on the ground at a time in running, one leg is always in recovery, while the other goes through support and drive. Then, briefly, as the runner leaps through the air, both legs are in recovery. These phases are described in detail below.
SupportDuring the support phase, the foot is in contact with the ground and supports the body against gravity. The body's centre of mass is typically somewhere in the lower abdominal area between the hips. The supporting foot touches the ground slightly ahead of the point that lies directly below the body's centre of mass. The knee joint is at its greatest extension just prior to the support phase. When contact is made with the ground, the knee joint begins to flex, and the extent it flexes varies with running style. Stiff-legged running styles reduce knee flexion, and looser, or more dynamic, running styles increase it. As the supporting leg bends at the knee, the pelvis dips down on the opposite side. These motions absorb shock and are opposed by the coordinated action of several muscles. The pelvic dip is opposed by the tensor fasciae lataeilio-tibial band of the supporting leg, the hip abductor, and the abdominals and lower back muscles. The knee flexion is opposed by the Muscle contraction eccentric contraction of the quadriceps muscle. The supporting hip continues to extend, and the body's centre of mass passes over the supporting leg. The knee then begins to extend, and the opposite hip rises from its brief dip. The support phase begins to transition into drive.
The support phase quickly transitions into the drive phase. The drive leg extends at the knee joint, and at the hips, such that the toe maintains contact with the ground as that leg trails behind the body. The foot pushes backward and also down, creating a diagonal force vector, which, in an efficient running style, is aimed squarely at the runner's centre of mass. Since the diagonal vector has a vertical component, the drive phase continues to provide some support against gravity and can be regarded as an extension of the support phase. During the drive, the foot may extend also, by a flexing of the soleus and gastrocnemius muscle in the calf. In some running styles, notably long-distance "shuffles" which keep the feet close to the ground, the ankle remains more or less rigid during drive. Because the knee joint straightens, though not completely, much of the power of the drive comes from the quadriceps muscle group, and in some running styles, additional power comes from the calves as they extend the foot for a longer drive. This motion is most exhibited in sprinting.
When the driving toe loses contact with the ground, the recovery phase begins. During recovery, the hip flexes, which rapidly drives the knee forward. Much of the motion of the lower leg is driven by the forces transferred from the upper leg rather than by the action of the muscles. As the knee kicks forward, it exerts torque against the lower leg through the knee joint, causing the leg to snap upward. The degree of leg lift can be consciously adjusted by the runner, with additional muscle power. During the last stage of recovery, the hip achieves maximal flexion, and, as the lower leg rapidly unfolds, which it does in a passive way, the knee joint also reaches its greatest, though not full, extension. During this extension of the leg and flexion of the hip, the hamstring and gluteal muscles are required to stretch rapidly. Muscles which are stretched respond by contracting by a reflex action. Recovery ends when the foot comes into contact with the ground, transitioning again into the support phase.
Upper body motion
The motions of the upper body are essential to maintaining balance, and a forward motion for optimal running. They compensate for the motions of the lower body, keeping the body in rotational balance. A leg's recovery is matched by a forward drive of the opposite arm, and a leg's support and drive motions are balanced by backward movement of the opposite arm. The shoulders and torso are also involved. Because the leg drive is slower than the kick of recovery, the arm thrusting backward is slower also. The forward arm drive is more forceful and rapid.
The more force exerted by the lower body, the more exaggerated the upper body motions have to be to absorb the momentum. While it is possible to run without movements of the arms, the spine and shoulders will generally still be recruited. Using the arms to absorb the forces aids in maintaining balance at higher speed. Otherwise, optimal force would be hard to attain for fear of falling over.
Most of the energy expended in running goes to the compensating motions, and so considerable gains in running speed as well as economy can be made by eliminating wasteful or incorrect motions. For instance, if the force vector in the drive phase is aimed too far away from the centre of mass of the body, it will transfer an angular momentum to the body which has to be absorbed.
The faster the running, the more energy has to be dissipated through compensating motions throughout the entire body. This is why elite sprinters have powerful upper body physiques. As the competitive distance increases, there is a rapid drop in the upper body and overall muscle mass typically exhibited by the people who compete at a high level in each respective event. Long distance runners typically have lean muscles.
It should also be noted that the upper body also helps to propel the body up hills. The faster one swings their arms up a hill, the faster the person will move up the hill because the legs will match the speed at which the arms are moving.
Elements of good running technique
Upright posture and a slight forward lean
Leaning forward places a runner's center of mass on the front part of the foot, which avoids landing on the heel and facilitates the use of the spring mechanism of the foot. It also makes it easier for the runner to avoid landing the foot in front of the center of mass and the resultant braking effect. While upright posture is essential, a runner should maintain a relaxed frame and use his/her core to keep posture upright and stable. This helps prevent injury as long as the body is neither rigid or tense. The most common running mistakes are tilting the chin up and scrunching shoulders. 
Stride rate and types
Exercise physiologists have found that the stride rates are extremely consistent across professional runners, between 185 and 200 steps per minute. The main difference between long- and short-distance runners is the length of stride rather than the rate of stride.
During running, the speed at which the runner moves may be calculated by multiplying the cadence (steps per second) by the stride length. Running is often measured in terms of pace in minutes per mile or kilometer. Fast stride rates coincide with the rate one pumps their arms. The faster one's arms move up and down, parallel with the body, the faster the rate of stride. Different types of stride are necessary for different types of running. When sprinting, runners stay on their toes bringing their legs up, using shorter and faster strides. Long distance runners tend to have more relaxed strides that vary.
Because of its high-impact nature, many injuries are associated with running. They include "runner's knee" (pain in the knee), shin splints, pulled muscles (especially the hamstring), twisted ankles, iliotibial band syndrome, plantar fasciitis, and Achilles tendinitis. Stress fractures are also fairly common in runners training at a high volume or intensity. Repetitive stress on the same tissues without enough time for recovery or running with improper form can lead to many of the above. Generally these injuries can be minimized by warming up beforehand, improving running form, performing strength training exercises, eating a well balanced diet, getting enough rest, and "icing" (applying ice to sore muscles or taking an ice bath). 
Foot blisters are also common among runners. Specialized socks help to prevent blisters greatly. For existing cases, lancing the blister with a sterile needle and applying a cyanoacrylate glue (such as Superglue or Krazy Glue) may help to protect the wound and enable further running. This is common practice among hardened endurance athletes.
Another common, running-related injury is chafing, caused by repetitive rubbing of one piece of skin against another, or against an article of clothing. One common location for chafe to occur is the runner's upper thighs. The skin feels coarse and develops a rash-like look. It can be prevented by either rubbing deodorant or special anti-chafing creams (sold in sticks that look like deodorant) to the area of the skin that rubs together. Chafe is also likely to occur on the nipple. A common solution is to affix a piece of medical tape over each nipple before running.
A cold bath is a popular treatment of subacute injuries or inflammation, muscular strains, and overall muscular soreness, but which efficacy is controversial. Some claim that for runners in particular, ice baths offer two distinct improvements over traditional techniques. First, immersion allows controlled, even constriction around all muscles, effectively closing microscopic damage that cannot be felt and numbing the pain that can. One may step into the tub to relieve sore calves, quads, hams, and connective tissues from hips to toes will gain the same benefits, making hydrotherapy an attractive preventive regimen. Saint Andrew’s cross-country coach John O’Connell, a 2:48 masters marathoner, will hit the ice baths before the ibuprofen. "Pain relievers can disguise injury," he warns. "Ice baths treat both injury and soreness." The second advantage involves a physiological reaction provoked by the large amount of muscle submerged. Assuming one has overcome the mind’s initial flight response in those first torturous minutes, the body fights back by invoking a "blood rush". This rapid transmission circulation flushes the damage-inflicting waste from the system, while the cold water on the outside preserves contraction. Like an oil change or a fluid dump, the blood rush revitalizes the very areas that demand fresh nutrients.
Some runners may experience injuries when running on concrete surfaces. The problem with running on concrete is that the body adjusts to this flat surface running and some of the muscles will become weaker, along with the added impact of running on a harder surface. Therefore it is advised to change terrain occasionally – such as trail, beach, or grass running. This is more unstable ground and allows the legs to strengthen different muscles. Runners should be wary of twisting their ankles on such terrain. Running downhill also increases knee stress and should therefore be avoided. Reducing the frequency and duration can also prevent injury; three 20–30 minute sessions a week should suffice.
A runner who finds himself injured should not continue to run because continuing could further damage the injury and prolong the recovery. A common acronym used to help the recovery process is RICE: Rest, Ice, Compression, and Elevation.
Another injury prevention method common in the running community is stretching. Stretching is often recommended as a requirement to avoid running injuries, and it is almost uniformly performed by competitive runners of any level. Recent medical literature, however, finds mixed effects of stretching prior to running. One study found insufficient evidence to support the claim that stretching prior to running was effective in injury prevention or soreness reduction. Another, however, has demonstrated that stretching prior to running increases injuries, while stretching afterwards actually decreases them. The American College of Sports Medicine recommends that all stretching be done after exercise because this is when the muscles are most warmed up and capable of increasing flexibility. Recent studies have also shown that stretching will reduce the amount of strength the muscle can produce during that training session.
Recently, some runners have concluded that barefoot running reduces running related injuries. "Some experts now believe that most athletic shoes, with their inflexible soles, structured sides and super-cushioned inserts keep feet so restricted that they may actually be making feet lazy, weak and more prone to injury. As a result, barefoot training is gaining more attention among coaches, personal trainers and runners." "Research has shown that wearing shoes to exercise takes more energy, and that barefoot runners use about 4 percent less oxygen than shoe runners. Other studies suggest barefoot athletes naturally compensate for the lack of cushioning and land more softly than runners in shoes, putting less shock and strain on the rest of the body. Barefoot runners also tend to land in the middle of their foot, which can improve running form and reduce injury." However, this position on barefoot running remains controversial and a majority of professionals advocate the wearing of appropriate shoes as the best method for avoiding injury.
Additionally, there have also been claims that improved posture reduces injuries and helps to cope with existing injuries. For example, one 2004 study showed that improved running form can significantly reduce eccentric loading of the knee.
Recent studies have shown that runners do not have more osteoarthritis than people who do not run.
Although it is not an injury, people with asthma suffer sometimes from running, especially if they have exercise-induced asthma. Asthma becomes more a problem with colder weather, increased speed, and uphills.
Benefits of running
While there is the potential for injury in running (just as there is in any sport), there are many benefits. Some of these benefits include potential weight loss, improved cardiovascular health, increased muscle mass, increased bone density, and an improved emotional state. Following a consistent routine of running can increase HDL levels, reducing the risks of cardiovascular disease. Running, like all forms of regular exercise, can effectively slow or reverse the effects of aging.
Running can assist people in losing weight and staying in shape. Different speeds and distances are appropriate for different individual health and fitness levels. For new runners, it takes time to get into shape. The key is consistency and a slow increase in speed and distance. While running, it is best to pay attention to how one's body feels. If a runner is gasping for breath or feels exhausted while running, it may be beneficial to slow down or try a shorter distance for a few weeks. If a runner feels that the pace or distance is no longer challenging, then the runner may want to speed up or run farther.
Running can also have psychological benefits, as many participants in the sport report feeling an elated, euphoric state, often referred to as a "runner's high". Running is the usual recommended therapy to treat people with clinical depression and people coping with addiction.
Running as a sport
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Running is both a competition and a type of training for sports that have running or endurance components. As a sport, it is split into events divided by distance and sometimes includes permutations such as the obstacles in steeplechase and hurdles. Running races are contests to determine which of the competitors is able to run a certain distance in the shortest time. Today, competitive running events make up the core of the sport of athletics. Events are usually grouped into several classes, each requiring substantially different athletic strengths and involving different tactics, training methods, and types of competitors.
Running competitions have probably existed for most of humanity's history and were a key part of the ancient Olympic Games as well as the modern Olympics. Today, road racing is a popular sport among non-professional athletes, who included over 7.7 million people in America alone in 2002.
Limits of speed
Footspeed, or sprint speed, is the maximum speed at which a human can run. It is affected by many factors, varies greatly throughout the population, and is important in athletics and many sports.
The highest human footspeed ever recorded is 48 km/h (29.8 mph), seen during a 100 meter sprint by Usain Bolt.
Types of running events
- Track running
- Road running
- Cross country running
- Trail running
- Fell running
- Relay race
- Recreational running
- Talus Running (running across the rock debris at the bottom of mountains)