Fitness is defined as the quality of being suitable to perform a particular task. Around 1950, perhaps consistent with the industrial revolution and the treatise of WWII, the term fitness increased in western vernacular by a factor of ten.  Modern definition of fitness describe either a person or machine’s ability to perform a specific function or a holistic definition of human adaptability to cope with various situations.
This has lead to an interrelation of human fitness and attractiveness which has mobilized global fitness and fitness equipment industries. Regarding specific function, fitness is attributed to personnel who possess significant aerobic or anaerobic ability, i. e. strength or endurance. A holistic definition of fitness is described by Greg Glassman in the CrossFitt journal as an increased work capacity across broad times and model domains; mastery of several attributes of fitness including strength, endurance, power, speed, balance and coordination and being able to improve the amount of work done in a given time with any of these domains. 
A well rounded fitness program will improve a person in all aspects of fitness, rather than one, such as onlycardio/respiratory endurance or only weight training. A U. S. Marine performing a fitness routine. A comprehensive fitness program tailored to an individual typically focuses on one or more specific skills, and on age- or health-related needs such as bone health.  Many sources also cite mental, social and emotional health as an important part of overall fitness. This is often presented in textbooks as a triangle made up of three points, which represent physical, emotional, and mental fitness.
Physical fitness can also prevent or treat many chronic health conditions brought on by unhealthy lifestyle or aging.  Working out can also help people sleep better and have a better mood. To stay healthy it is important to engage in physical activity.  The U. S. Centers for Disease Control and Prevention encourages the adult public, ages 18 to 64, to engage each week in at least one and a quarter hours of vigorous-intensity aerobic activity or two and a half hours of moderate-intensity aerobic activity; that time can be met in any increments.
 Diet is an important component to overall health that works best in combination with exercise.  A balanced diet and exercising regularly are important for maintaining good health. Obesity is defined as body mass index, a measure of weight in relationship to height (Blair, 1993). With obesity on the rise, the U. S. has implemented more exercise and diet plans. Exercise and diet will help prevent type 2 diabetes, which is more common with obesity. There are millions of programs, websites, television shows, magazines, and movies regarding health and fitness. Recently, the trends of diets and lifestyle habits have become more and more encouraged.
Understanding the importance of the health benefits resulted from diet and exercise will help decrease the amount of obesity in this country. Physical activity and exercise is defined in terms of type, intensity, duration and frequency (Blair, 1993).  Developing research has demonstrated that many of the benefits of exercise are mediated through the role of skeletal muscle as an endocrine organ. That is, contracting muscles release multiple substances known as myokines which promote the growth of new tissue, tissue repair, and various anti-inflammatory functions, which in turn reduce the risk of developing various inflammatory diseases.  Training
Specific or task-oriented fitness is a person’s ability to perform in a specific activity with a reasonable efficiency: for example, sports or military service. Specific training prepares athletes to perform well in their sports. Examples are: ?100 m sprint: in a sprint the athlete must be trained to work anaerobically throughout the race, an example of how to do this would be interval training.
Marathon: in this case the athlete must be trained to work aerobically and their endurance must be built-up to a maximum. ?Many fire fighters and police officers undergo regular fitness testing to determine if they are capable of the physically demanding tasks required of the job.  ?Members of armed forces will often be required to pass a formal fitness test – for example soldiers of the US Army must be able to pass the Army Physical Fitness Test(APFT).  ?Hill sprints: requires a level of fitness to begin with, the exercise is particularly good for the leg muscles. The army often trains doing mountain climbing and races.