Exercise

I

Introduction

Exercise, activity that results in contraction of skeletal muscle. 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 mental health along with their general physical well-being.

Today there is an increasing emphasis on preventive medicine, or maintaining health, partly as a result of the increasing costs of healthcare and our greater awareness of the effects of lifestyle on health and longevity.

II

Physiology of Exercise

Contraction of skeletal muscles, the muscles under conscious control, is the primary physiological event during exercise. Because skeletal muscles can actively contract, but are not designed to actively lengthen, they are arranged as opposing pairs. As one muscle shortens, another is stretched. An example of such a pair of muscles can be observed in the upper arm, where the biceps and triceps have opposite actions. To flex the arm at the elbow, the biceps contract, while the triceps stretch. To extend the forearm, the triceps contract, while the biceps lengthen.

III

Cellular Changes and Exercise

At the molecular level, muscle contraction occurs when large proteins called actin and myosin slide together to shorten muscle fibers. The energy for contraction and relaxation of skeletal muscle is provided by a molecule called adenosine triphosphate (ATP). ATP is a high-energy molecule formed during the breakdown of glucose (a kind of sugar) or fats. Glucose can be stored in muscle as glycogen, and enters exercising muscle from the blood.

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The metabolism of glycogen or glucose to provide energy for exercise occurs in one of two ways, depending on the presence of available oxygen to the muscle, which in turn depends on the type of exercise being performed. If oxygen is not available (anaerobic activity), glycogen or glucose will be broken down by the anaerobic pathway (glycolysis). If oxygen is available (aerobic activity), it will be metabolized by the aerobic pathway (known as the Citric Acid Cycle). When oxygen is readily available, glucose reacts completely with the oxygen to produce water and carbon dioxide. A portion of the energy released from one molecule of glucose is utilized to produce ATP.

A

Anaerobic and Aerobic Exercise

During anaerobic metabolism, the breakdown of glucose stops at an early point, producing lactic acid and two molecules of ATP. This anaerobic metabolism produces a so-called oxygen debt, which is repaid later when oxygen becomes available. When a skeletal muscle is heavily worked, the acute soreness that results is due partly to a buildup of lactic acid. The presence of lactic acid can also be felt during exercise as a burning sensation in the muscles.

Anaerobic exercise involves heavy work by a limited number of muscles, for example during weight lifting. These types of activities are maintained only for short intervals, and the supply of oxygen is insufficient for aerobic metabolism, resulting in a substantial oxygen debt and anaerobic metabolism within those muscles. Another example is sprinting, in which the exercise is high in intensity but short in duration, resulting in substantial oxygen debt. Weight lifting and other types of anaerobic exercise increase strength and muscle mass, but are of limited benefit to cardiovascular, or circulatory, health.

Unlike anaerobic exercise, 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 uses several major muscle groups throughout the body, resulting in greater demands on the circulatory and respiratory systems to supply oxygen to the working muscles. Aerobic exercise includes walking, running, and swimming, and is the form recommended for reducing the risk of heart disease and increasing endurance.

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