Physical Fitness is the ability of the human body to function with vigor and alertness, without undue fatigue, and with ample energy to engage in leisure activities, and to meet physical stresses. Muscular strength and endurance, cardio respiratory integrity, and general alertness are the overt signs of physical fitness.

Physical fitness is usually measured in relation to functional expectations—that is, typically, by periodic tests measuring strength, endurance, agility, coordination, and flexibility. In addition, stress testing, which ascertains the body's accommodation to powerful, sustained physical stimuli, is used to analyze fitness. If individuals are able to accommodate to the stressors, they are assumed to be fit.

The level of physical fitness can be influenced by regular, systematic exercise. Moderate activity will maintain the individual at a level that is usually adequate to handle ordinary stress. If the fitness level is to be improved, however, it is necessary to participate in more intensive exercise that overloads the physiological systems and thus promotes change.

Exercises that demand total body involvement improve and maintain fitness most effectively—for example, jogging, running, swimming, vigorous dancing, cycling, and brisk walking. Organized games and sports have long rest periods within the play design have only minimal influence on fitness. Programs especially planned to help individuals attain fitness are offered in numerous places: schools and gymnasiums, private clubs and studios, and special, professionally organized clinics that are attentive to problems of cardiac and pulmonary function. The individual must be careful in choosing an exercise program and should ensure it is staffed by experts in physical education or medicine.

Normal, healthy individuals may plan their own exercise programs. The general rule is to exercise only until discomfort is experienced—that is, until breathing becomes labored, circulation seems inadequate, or fatigue influences performance. People with health problems caused by heart attacks, strokes, and illness should see a physician before devising an exercise program.

Proper nutrition is important to physical fitness, because energy expenditure depends on nutrition. If diet is inadequate, the fitness level will drop. Overweight, underweight, and weak individuals will have below average fitness levels

Generally, longer periods of exercise can reduce the risk of heart disease for the individual. Yet many individuals and organizations promote fitness through extreme means. Such programs should be approached with skepticism and caution. The attainment and maintenance of physical fitness, coupled with nutritious diet and reasonable rest patterns, require dedication to a long-term, systematic investment in an active life.