Physical fitness is the body's capacity to carry out work and protect itself
against disease, infection, and the effects of physical stresses like heat
or cold. The level of fitness needed is related to the level of stress
the body must overcome.
Specific types of physical fitness are needed for every person's body to meet special demands. For instance, if a job requires lifting of very heavy loads, additional strength must be developed in specific muscles. Through working or exercising, muscles in the body develop strength. Coordination in nerve muscles is also improved. The body's ability to change posture instantly requires orthostatic fitness. Orthostatic fitness can be determined by measuring how well the blood circulation can adapt to an instant change of posture, like standing up after lying down.
Bursts of physical activity of greatest effort lasting less than 10 seconds require anaerobic fitness. This is the ability of cells to perform without oxygen. Anaerobic activity includes sudden jolting movements, such as sprinting to catch a bus or an extra burst of speed needed to make a touchdown. While doing anaerobic exercises, rigorous muscle activity is needed. This activity exceeds the capacity of the heart and the lungs to get oxygen to the cells.
When ending an anaerobic activity, the person is left gasping for air while the heart and lungs are working hard to supply oxygen to reverse this oxygen loss condition. Any sport or activity that occasionally requires short bursts of speed followed by long pauses is considered an anaerobic activity.
Rigorous efforts of a longer duration require aerobic fitness. This way of exercising is distinguished by the body's ability to transport and consume oxygen effectively. Running, swimming, bicycling, and cross-country skiing are examples of aerobic exercises. Aerobic exercise is classified by the continuing, moderately rigorous effort that happens at a pace enabling the heart and lungs to provide the oxygen required by the muscles.