Physical exercise is any bodily activity that enhances or maintains physical fitness and overall health. It is performed for many different 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.  Childhood obesity is a growing global concern and physical exercise may help decrease the effects of childhood obesity in developed countries. 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, 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 •Strength training •Agility training 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). Benefits 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.  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 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.