See historical study by J. Whorton (1982).
Physical fitness is used in two close meanings: general fitness (a state of health and well-being) and specific fitness (a task-oriented definition based on the ability to perform specific aspects of sports or occupations).
Physical fitness is the functioning of the heart, blood vessels, lungs, and muscles to function at optimum efficiency. In previous years, fitness was defined as the capacity to carry out the day’s activities without undue fatigue. Automation increased leisure time, and changes in lifestyles following the industrial revolution meant this criterion was no longer sufficient. Optimum efficiency is the key. Physical fitness is now defined as the body’s ability to function efficiently and effectively in work and leisure activities, to be healthy, to resist hypokinetic diseases, and to meet emergency situations. Fitness can also be divided into five categories: aerobic fitness, muscular strength, muscular endurance, flexibility, and body composition.
Many sources also cite mental and emotional health as an important part of overall fitness. This is often presented in textbooks as a triangle made up of three sub-sections, which represent physical, emotional, and mental fitness. Hence, one may be physically fit but may still suffer from a mental illness or have emotional problems. The "ideal triangle" is balanced in all areas. Physical fitness can also prevent or treat many chronic health conditions brought on by unhealthy lifestyle or aging, among other implications. Working out can also help to sleep better. To stay healthy it's important to participate in physical activity.
Fire fighters and police officers must undergo regular Fitness testing to determine if they are capable of the physically demanding tasks required for the job before they are employed.