Competitor emerging from the swim portion of a triathlon
Physical exercise is any bodily activity that enhances or maintains physical fitness and overall health or wellness. It is performed for various reasons. These include strengthening muscles and the cardiovascular system, honing athletic skills, weight loss or maintenance and for enjoyment. Frequent and regular physical exercise boosts the immune system, and helps prevent the "diseases of affluence" such as heart disease, cardiovascular disease, Type 2 diabetes and obesity. It also improves mental health, helps prevent depression, helps to promote or maintain positive self-esteem, and can even augment an individual's sex appeal or body image, which again is also linked with higher levels of self-esteem. Childhood obesity is a growing global concern and physical exercise may help decrease the effects of childhood obesity in developed countries.
 Types of exercise
Exercises are generally grouped into three types depending on the overall effect they have on the human body:
- Flexibility exercises, such as stretching, improve the range of motion of muscles and joints.
- Aerobic exercises, such as cycling, swimming, walking, skipping rope, rowing, running, hiking or playing tennis, focus on increasing cardiovascular endurance.
- Anaerobic exercises, such as weight training, functional training or sprinting, increase short-term muscle strength.
 Categories of physical exercise
Sometimes the terms 'dynamic' and 'static' are used. 'Dynamic' exercises such as steady running, tend to produce a lowering of the diastolic blood pressure during exercise, due to the improved blood flow. Conversely, static exercise (such as weight-lifting) can cause the systolic pressure to rise significantly (during the exercise).
 Categories of general physical skills
Physical exercise is used to improve physical skills.
Physical skills fall into the following general categories, per CrossFit: 
Physical exercise is important for maintaining physical fitness and can contribute positively to maintaining a healthy weight, building and maintaining healthy bone density, muscle strength, and joint mobility, promoting physiological well-being, reducing surgical risks, and strengthening the immune system.
Exercise also reduces levels of cortisol. Cortisol is a stress hormone that builds fat in the abdominal region, making weight loss difficult. Cortisol causes many health problems, both physical and mental.
Frequent and regular aerobic exercise has been shown to help prevent or treat serious and life-threatening chronic conditions such as high blood pressure, obesity, heart disease, Type 2 diabetes, insomnia, and depression. Endurance exercise before meals lowers blood glucose more than the same exercise after meals. According to the World Health Organization, lack of physical activity contributes to approximately 17% of heart disease and diabetes, 12% of falls in the elderly, and 10% of breast cancer and colon cancer.
There is some evidence that vigorous exercise (90'95% of VO2 Max) is more beneficial than moderate exercise (40 to 70% of VO2 Max). Some studies have shown that vigorous exercise executed by healthy individuals can increase opioid peptides (a.k.a. endorphins, naturally occurring opioids that in conjunction with other neurotransmitters are responsible for exercise-induced euphoria and have been shown to be addictive), increase testosterone and growth hormone, effects that are not as fully realized with moderate exercise. More recent research indicates that anandamide may play a greater role than endorphins in "runner's high".
Both aerobic and anaerobic exercise also work to increase the mechanical efficiency of the heart by increasing cardiac volume (aerobic exercise), or myocardial thickness (strength training). Such changes are generally beneficial and healthy if they occur in response to exercise.
Not everyone benefits equally from exercise. There is tremendous variation in individual response to training: where most people will see a moderate increase in endurance from aerobic exercise, some individuals will as much as double their oxygen uptake, while others can never augment endurance. Similarly, only a minority of people will show significant muscle growth after prolonged weight training, while a larger fraction experience improvements in strength. This genetic variation in improvement from training is one of the key physiological differences between elite athletes and the larger population. Studies have shown that exercising in middle age leads to better physical ability later in life.
 Effect on the cardiovascular system
The effect of exercise on the cardiovascular system is well documented.
There is a direct relation between physical inactivity and cardiovascular mortality, and physical inactivity is an independent risk factor for the development of coronary artery disease. There is a dose-response relation between the amount of exercise performed from approximately 700 to 2000 kcal of energy expenditure per week and all-cause mortality and cardiovascular disease mortality in middle-aged and elderly populations. The greatest potential for reduced mortality is in the sedentary who become moderately active. Most beneficial effects of physical activity on cardiovascular disease mortality can be attained through moderate-intensity activity (40% to 60% of maximal oxygen uptake, depending on age). ... persons who modify their behavior after myocardial infarction to include regular exercise have improved rates of survival. ... Persons who remain sedentary have the highest risk for all-cause and cardiovascular disease mortality. 
 Effect on the immune system
Although there have been hundreds of studies on exercise and the immune system, there is little direct evidence on its connection to illness. Epidemiological evidence suggests that moderate exercise has a beneficial effect on the human immune system while extreme exercise impairs it, an effect which is modeled in a J curve. Moderate exercise has been associated with a 29% decreased incidence of upper respiratory tract infections (URTI), but studies of marathon runners found that their prolonged high-intensity exercise was associated with an increased risk of an infection, although another study did not find the effect. Immune cell functions are impaired following acute sessions of prolonged, high-intensity exercise, and some studies have found that athletes are at a higher risk for infections. The immune systems of athletes and nonathletes are generally similar. Athletes may have slightly elevated natural killer cell count and cytolytic action, but these are unlikely to be clinically significant.
Vitamin C supplementation has been associated with lower URTIs in marathon runners.
Biomarkers of inflammation such as C-reactive protein, which are associated with chronic diseases, are reduced in active individuals relative to sedentary individuals, and the positive effects of exercise may be due to its anti-inflammatory effects. The depression in the immune system following acute bouts of exercise may be one of the mechanisms for this anti-inflammatory effect.
 Effects on brain function
A 2008 review of cognitive enrichment therapies (strategies to slow or reverse cognitive decline) concluded that "physical activity, and aerobic exercise in particular, enhances older adults' cognitive function".
In mice, exercise improves cognitive functioning via improvement of hippocampus-dependent spatial learning, and enhancement of synaptic plasticity and neurogenesis. In addition, physical activity has been shown to be neuroprotective in many neurodegenerative and neuromuscular diseases. For instance, it reduces the risk of developing dementia. Furthermore, anecdotal evidence suggests that frequent exercise may reverse alcohol-induced brain damage.
There are several possibilities for why exercise is good for the brain:
Physical activity is thought to have other beneficial effects related to cognition as it increases levels of nerve growth factors, which support the survival and growth of a number of neuronal cells.
 Effects on depression
A number of factors may contribute to depression including being overweight, low self-esteem, stress and anxiety. Endorphins act as a natural pain reliever and antidepressant in the body. Endorphins have long been regarded as responsible for what is known as "runner's high", a euphoric feeling a person receives from intense physical exertion. However, recent research indicates that anandamide may possibly play a greater role than endorphins in "runner's high". When a person exercises, levels of both circulating serotonin and endorphins are increased. These levels are known to stay elevated even several days after exercise is discontinued, possibly contributing to improvement in mood, increased self-esteem, and weight management. Exercise alone is a potential prevention method and/or treatment for mild forms of depression.
Exercise also affects the sleep that a person will receive at night. When the body is physically exhausted it will slip into Rapid Eye Movement (REM) sleep easier and for a longer period.
 Public health measures
Signs that encourage the use of stairs, as well as community campaigns, have been shown to be effective in increasing exercise in a population. The city of Bogot�¡, Colombia, for example, blocks off 113 kilometers (70 miles) of roads on Sundays and holidays to make it easier for its citizens to get exercise. These pedestrian zones are part of an effort to combat chronic diseases, including obesity.
 Common misconceptions
Many myths have arisen surrounding exercise, some of which have a basis in reality, and some which are completely false. Myths include:
- That excessive exercise can cause immediate death. Death by exercise has some small basis in fact. Water intoxication can result from prolific sweating (producing electrolyte losses) combined with consumption of large amounts of plain water and insufficient replenishment of electrolytes, especially salt and potassium (e.g. when running a marathon). It is also possible to die from a heart attack or similar affliction if overly intense exercise is performed by someone who is not at an appropriate level of fitness for that particular activity or has an undiagnosed rare condition like hypertrophic cardiomyopathy. A doctor should always be consulted before any radical changes are made to a person's current exercise routine. Rhabdomyolysis is also a risk. Other common dangers may occur from extreme overheating or aggravation of a physical defect, such as a thrombosis or aneurysm.
- That weightlifting makes you short or stops growth. One confirmed danger is that heavy weight training in adolescents (ages 11'16) can damage the epiphyseal plate of long bones and can stunt joints. It is still commonly suggested that adolescents refrain from any kind of "strenuous" weight-lifting. However, weight training is a healthy form of exercise for adolescents and can be perfectly safe if performed under the supervision of a trained professional.
 Targeted fat reduction
Spot reduction is a myth claiming that exercising and training a particular body part will preferentially shed the fat on that part. For example, doing sit-ups is not the best way to reduce subcutaneous abdominal obesity. One cannot reduce fat from one area of the body to the exclusion of others. Most of the energy derived from fat gets to the muscle through the bloodstream and reduces stored fat in the entire body, from the last place where fat was deposited. Sit-ups may improve the size and shape of abdominal muscles but will not specifically target belly fat for loss. Such exercise might help reduce overall body fat percentage and shrink the size of fat cells.
Stored fat is generally liberated and metabolised by the body in a LIFO order.
 Muscle and fat tissue
One misconception is that muscle tissue will turn into fat tissue once a person stops exercising. This is not literally true'fat tissue and muscle tissue are fundamentally different'but the common expression that "muscle will turn to fat" is truthful in the sense that catabolism of muscle fibers for energy can result in excess glucose being stored as fat. Moreover, the composition of a body part can change toward less muscle and more fat, so that a cross-section of the upper-arm for example, will have a greater area corresponding to fat and a smaller area corresponding to muscle. This is not muscle "turning into fat" however'it is simply a combination of muscle atrophy and increased fat storage in different tissues of the same body part. The muscle is being replaced by fat. Another element of increased fatty deposits is that of diet, as most trainees will not significantly reduce their diet in order to compensate for the lack of exercise/activity.
 Excessive exercise
Exercise is a stressor and the stresses of exercise have a catabolic effect on the body'contractile proteins within muscles are consumed for energy, carbohydrates and fats are similarly consumed and connective tissues are stressed and can form micro-tears. However, given adequate nutrition and sufficient rest to avoid overtraining, the body's reaction to this stimulus is to adapt and replete tissues at a higher level than that existing before exercising. The results are all the training effects of regular exercise: increased muscular strength, endurance, bone density, and connective tissue toughness.
Too much exercise can be harmful. Without proper rest, the chance of stroke or other circulation problems increases, and muscle tissue may develop slowly.
Inappropriate exercise can do more harm than good, with the definition of "inappropriate" varying according to the individual. For many activities, especially running, there are significant injuries that occur with poorly regimented exercise schedules. In extreme instances, over-exercising induces serious performance loss. Unaccustomed overexertion of muscles leads to rhabdomyolysis (damage to muscle) most often seen in new army recruits. Another danger is overtraining in which the intensity or volume of training exceeds the body's capacity to recover between bouts.
Stopping excessive exercise suddenly can also create a change in mood. Feelings of depression and agitation can occur when withdrawal from the natural endorphins produced by exercise occurs. Exercise should be controlled by each body's inherent limitations. While one set of joints and muscles may have the tolerance to withstand multiple marathons, another body may be damaged by 20 minutes of light jogging. This must be determined for each individual.
Too much exercise can also cause a female to miss her period, a symptom known as amenorrhea.
 Exercise trends
Worldwide there has been a large shift towards less physically demanding work. This has been accompanied by increasing use of mechanized transportation, a greater prevalence of labor saving technology in the home, and less active recreational pursuits.
 Nutrition and recovery
Proper nutrition is as important to health as exercise. When exercising, it becomes even more important to have a good diet to ensure that the body has the correct ratio of macronutrients whilst providing ample micronutrients, in order to aid the body with the recovery process following strenuous exercise.
Proper rest and recovery are also as important to health as exercise; otherwise the body exists in a permanently injured state and will not improve or adapt adequately to the exercise. It is necessary to refill the glycogen stores in the skeletal muscles and liver. One sports nutritionist recommends drinking an "engineered recovery beverage" or chocolate milk (which combines protein, carbohydrates, and water) within 30 minutes of exercise. Branched-chain amino acids are also recommended for exercise recovery.
Nutrition and recovery can be compromised by psychological compulsions (eating disorders such as exercise bulimia, anorexia, and other bulimias), misinformation, a lack of organization, or a lack of motivation.
Delayed onset muscle soreness can occur after any kind of exercise, particularly if the body is in an unconditioned state relative to that exercise.
The benefits of exercise have been known since antiquity. Marcus Cicero, around 65 BC, stated: "It is exercise alone that supports the spirits, and keeps the mind in vigor." However, the link between physical health and exercise (or lack of it) was only discovered in 1949 and reported in 1953 by a team led by Jerry Morris. Dr. Morris noted that men of similar social class and occupation (bus conductors versus bus drivers) had markedly different rates of heart attacks, depending on the level of exercise they got: bus drivers had a sedentary occupation and a higher incidence of heart disease, while bus conductors were forced to move continually and had a lower incidence heart disease. This link had not previously been noted and was later confirmed by other researchers.
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