The term is usually used in reference to any activity that promotes physical fitness. Although muscle contraction is the common element of all forms of exercise, many other organs and systems are affected, for example, the heart and lungs. Many people also find that regular exercise enhances their sense of mental well-being along with their general physical health.
Today there is an increasing emphasis on preventive medicine, or maintaining health, partly as a result of the increasing costs of health care and our greater awareness of the effects of lifestyle on health and longevity. Over one-quarter of Americans (three-quarters by some standards) are significantly overweight and are at risk for a wide variety of health problems.
Aerobic exercise uses oxygen to keep large muscle groups moving continuously at an intensity that can be maintained for at least 20 minutes. Aerobic exercise includes walking, jogging, and swimming, and is the form recommended for reducing the risk of heart disease and increasing endurance.
Heart, Respiration and Exercise(体育锻炼和心脏呼吸系统的关系)
The heart pumps increased volumes of blood to supply oxygen and nutrients and remove carbon dioxide and metabolic wastes; the respiratory system handles an increased workload, exchanging oxygen and carbon dioxide between the blood and the atmosphere. The nervous system and various hormones have important roles as well, integrating the bodys response to exercise and regulating the metabolic changes that occur in muscle and other tissues.